Nuclear and Chemical Dating Techniques - ACS Publications

11 Dating Groundwater A

Short R e v i e w

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STANLEY N . DAVIS and HAROLD W. BENTLEY University of Arizona, Department of Hydrology and Water Resources, Tucson, A R 85721

The age of groundwater is the length of time the water has been isolated from the atmosphere. Theoretically, ages can be estimated by (1) the travel time of groundwater from the point of recharge to the subsurface point of interest as calculated by Darcy's law combined with an equation of continuity, (2) the decay of radionuclides which have entered the water from contact with the atmosphere, (3) the accumulation of products of radioactive reactions in the subsurface, (4) the degree of disequilibrium between radionuclides and their radioactive daughter products, (5) the time-dependent changes in the molecular structure of compounds dissolved in water, (6) the presence of man-made materials in groundwater, (7) the correlation of paleoclimatic indicators in the water with the known chronology of past climates, and (8) the presence or absence of ions which can be related to past geologic events that have been previously dated. Owing to uncertainties in each of the methods, as many methods as possible should be used in every field situation. Because hydrodynamic dispersion and molecular diffusion always take place, a single precise age for a given groundwater sample does not exist. "Dating" the sample by various methods, however, will help determine the extent of dispersion and diffusion as well as mixing of water from various aquifers which takes place within many wells. I f dispersion, diffusion, and cross-mixing are minimal, then under ideal conditions ages can be determined for waters less than about 30,000 years old, and rough approximations of ages up to about one million years appear possible.

The age of groundwater is the length of time the water has been isolated from the atmosphere. Although this definition i s useful for many purposes, i t does not reflect the true complexity 0097-6156/82/0176-0187$09.00/0 © 1982 American Chemical Society In Nuclear and Chemical Dating Techniques; Currie, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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N U C L E A R A N D C H E M I C A L DATING T E C H N I Q U E S

of most c i r c u l a t i o n systems of groundwater. Only simple systems which can be compared w i t h flow through a s i n g l e long pipe can y i e l d n e a r l y homogeneous dates f o r water sampled from the same general p a r t o f an a q u i f e r . Few n a t u r a l systems approach t h i s type of simple l i n e a r , non-mixing or " p i s t o n " , flow. In a d d i t i o n , most groundwater i s taken from w e l l s which tap more than one r e s t r i c t e d water-bearing zone. Consequently, a sample of water w i l l be a mixture of waters of d i f f e r e n t ages even i f the ground water flow were t o approach the i d e a l i z e d p i s t o n flow. Natural s p r i n g s a l s o are commonly connected i n a complicated way t o v a r i o u s water-bearing zones so t h a t samples of s p r i n g water may represent mixtures of waters of v a s t l y d i f f e r e n t ages. Because of the complex h i s t o r y of most groundwater samples, the n e c e s s i t y o f combining m u l t i p l e d a t i n g methods w i t h thorough r e g i o n a l and l o c a l hydrogeologic s t u d i e s cannot be emphasized too strongly. O b v i o u s l y , the l a r g e r the number of d a t i n g methods which are used, the more i n f o r m a t i o n can be obtained concerning the n a t u r a l systems. The purpose of t h i s present review i s t o emphasize the a v a i l a b i l i t y of many independent d a t i n g methods and to i n d i c a t e the s t a t e of development o f the methods. Although the best-known methods w i l l be mentioned, an emphasis w i l l be placed i n t h i s paper on some of the newer methods which are under development. DARCY'S LAW The o l d e s t and most w i d e l y used method o f e s t i m a t i n g water age i s the c a l c u l a t i o n o f t r a v e l times using Darcy's law combined w i t h an e x p r e s s i o n of c o n t i n u i t y . I f a f i e l d of s t e a d y - s t a t e , groundwater flow i s subdivided i n t o a two-dimensional flow net ( f i g u r e 1 ) , then Darcy's law can be w r i t t e n as: Q = K m Aw

(1)

i n which Q i s the d i s c h a r g e per u n i t time, Κ i s the h y d r a u l i c c o n d u c t i v i t y , assumed t o be i s o tropic, m i s the t h i c k n e s s of the f l o w f i e l d normal t o the plane of the flow net, Aw i s the width o f the stream tube, and Ah i s the change i n h y d r a u l i c head over the incremental f l o w p a t h , AL. I f the groundwater and a q u i f e r are assumed t o be i n c o m p r e s s i b l e , the c o n t i n u i t y equation f o r water flow i s : η m Aw A t "e Λ

In Nuclear and Chemical Dating Techniques; Currie, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

(2)

11.

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Dating Groundwater

189

i n which n

e

i s the e f f e c t i v e p o r o s i t y o f the a q u i f e r ,

At i s the time taken by the water t o t r a v e r s e the d i s tance AL, and other symbols are as given above. Combining equations 1 and 2 and s o l v i n g f o r At y i e l d s : _ n (AL)

2

e


A t

kAh

-

(3)

The age o f t h e groundwater i s obtained by the summation o f a l l At values along the flow path from the s u r f a c e i n t a k e area t o the p o i n t o f i n t e r e s t i n the subsurface. This has been done f o r the e n t i r e f i e l d o f f l o w shown i n f i g u r e 1. The r e s u l t i n g i s o chronal l i n e s are shown i n f i g u r e 2. Although f i g u r e 2 represents an a q u i f e r o f i n f i n i t e depth and, t h e r e f o r e , does not represent a n a t u r a l system, t h e f a c t t h a t o l d e r water tends t o r i s e along t h e axes o f v a l l e y s c o n t a i n i n g p e r e n n i a l streams i s w i d e l y recognized and has been documented by C a r l s t o n and others [ I ] . Figure 2 f u r t h e r emphasizes the f a c t t h a t w e l l s which a r e screened a t v a r i o u s depths w i l l produce water o f mixed ages. A 200 m w e l l near t h e r i v e r c o u l d have a mixture o f water ranging i n age from modern to 4,000 years o l d . Figure 2 was c o n s t r u c t e d by c o n t o u r i n g o f values c a l c u l a t e d from t h e f l o w net o f f i g u r e 1. Much more complex diagrams a r e p o s s i b l e provided s u f f i c i e n t i n f o r m a t i o n concerning t h e a q u i f e r and f l u i d - f l o w c o n d i t i o n s can be obtained. For simple boundary c o n d i t i o n s and homogeneous a q u i f e r s , a d i r e c t a n a l y t i c a l s o l u t i o n for isochronal surfaces i s a v a i l a b l e [2]. U n f o r t u n a t e l y , d e s p i t e t h e continued e v o l u t i o n o f s o p h i s t i cated numerical techniques which can be used t o estimate water ages i n i d e a l i z e d systems, the i n a b i l i t y t o d e f i n e a l l the c r i t i cal hydrogeologic d e t a i l s o f a q u i f e r s w i l l probably always leave l a r g e u n c e r t a i n t i e s i n the e s t i m a t i o n o f groundwater ages by p u r e l y hydrodynamic methods. As Theis [ 3 ] has p o i n t e d o u t , d e t a i l e d c o r i n g o f many f l u i d - b e a r i n g zones which appear homoge neous has y i e l d e d samples w i t h p e r m e a b i l i t i e s ranging through a t l e a s t two orders o f magnitude. This v a r i a t i o n o f p e r m e a b i l i t y w i l l g i v e r i s e t o the phenomenon o f megadispersion which, o f course, g i v e s r i s e i n t u r n t o water o f mixed ages i n any given zone i n an a q u i f e r . In f a c t , i n many recent s t u d i e s , r a d i o m e t r i c methods o f d a t i n g water have been used t o help understand the p o s s i b l e extent of water mixing i n a q u i f e r s [4-6]. Besides t h e problem o f d e f i n i n g g e o l o g i c d e t a i l s and t h e a s s o c i a t e d problems o f megadispersion, t h e problem o f d e f i n i n g AL and Ah values f o r past f l o w c o n d i t i o n s should not be ignored. As 1

f i g u r e s i n brackets i n d i c a t e the l i t e r a t u r e references a t the end of t h i s paper.


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UNIFORM

Figure 1.

TECHNIQUES

RECHARGE

Flow net representing groundwater circulation near a river which inter cepts homogeneous and isotropic aquifer of infinite thickness.

Figure 2. Age of groundwater circulating in the flow system shown in Figure 1. Isochronal numbers represent years. Ne is the effective porosity (Ne = 0.25; re charge (infiltration) = 12 cm/year), and Κ is the hydraulic conductivity (K = 2.50 cm/year = 8 X 10 cm/s (roughly equivalent to silt)). 6


DAVIS AND

11.

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Dating Groundwater

BENTLEY


can be seen from equation 3, the age c a l c u l a t i o n s are p a r t i c u l a r l y s e n s i t i v e to the determination of AL because i t i s squared. Changes i n past c l i m a t e s , t e c t o n i c movements, e r o s i o n of a q u i f e r s , pressures induced by c o n t i n e n t a l g l a c i e r s , and changes i n sea l e v e l may a l l p o s s i b l y produce t r a n s i e n t h y d r a u l i c heads which p e r s i s t f o r thousands of years. I f long-term t r a n s i e n t s e x i s t as p o s t u l a t e d by Toth [7] and K a f r i and Arad [ 8 ] , then the determina t i o n s of long-term average values of AL and Ah f o r the purpose of water d a t i n g by hydrodynamic equations become very d i f f i c u l t indeed. RADIONUCLIDES OF ATMOSPHERIC ORIGIN A l a r g e number of r a d i o n u c l i d e s are produced c o n t i n u o u s l y i n the upper atmosphere through v a r i o u s i n t e r a c t i o n s between gases and cosmic r a d i a t i o n [9,10]. Some of these r a d i o n u c l i d e s are produced a l s o i n the s o i l and bodies o f surface water by cosmic r a d i a t i o n t h a t penetrates the earth's atmosphere t o i n t e r a c t w i t h m a t e r i a l s a t the surface of the e a r t h . Radionuclides which are of h y d r o l o g i e i n t e r e s t and which are a l s o produced p r i m a r i l y i n the atmosphere ( p r i o r t o 1945) are l i s t e d i n Table 1. Table 1.

Nuclide 8 5

Kr 3

3 9

H Ar

32 . s

14

36

a

Half-life (years)

l e s s than 10"

12.26

3.6

270 330

Kr C1

a

Use

P o s s i b l e I n i t i a l Concentration i n Rain Water ( l a r g e l y from Oeschger, [10]) (dpm/liter)

10.7

5,730

C

8 1

Radionuclides of Atmospheric O r i g i n and of P o t e n t i a l i n Dating Ground Water.

4 χ 10"

5

1 χ 10"

3

2 χ 10"

0.7 χ 10"

301,000

1 χ 10" 3 2

dpm

(before

1950)

(before

1954)

1

210,000

S e e d i s c u s s i o n of h a l f - l i f e under

8

5

Si

8 to 2 χ

10"

4

l a t e r i n manuscript. 3

During the past 35 y e a r s , n a t u r a l c o n c e n t r a t i o n s of H and K r , and to a much l e s s e r extent C , have been masked by t h e i r man-made e q u i v a l e n t s . In f a c t , n a t u r a l c o n c e n t r a t i o n s of K r are completely masked a t present by the v a s t amount of K r from artificial f i s s i o n reactions. Owing to the r e l a t i v e l y s h o r t 8 5

14

8 5

8 5


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N U C L E A R AND

C H E M I C A L DATING T E C H N I Q U E S 8 5

h a l f - l i f e and the very low n a t u r a l p r o d u c t i o n r a t e of K r , the n a t u r a l c o n c e n t r a t i o n s of K r i n water which o r i g i n a t e d p r i o r t o 1945 w i l l probably never be measured. Natural H c o n c e n t r a t i o n s were much l a r g e r than those of K r , so, a t l e a s t t h e o r e t i c a l l y , t r a c e s of n a t u r a l H should s t i l l be b a r e l y d e t e c t a b l e i n ground water between about 35 and 60 years o l d . On the other hand, because of i t s longer h a l f - l i f e , C c o n c e n t r a t i o n s from water o l d e r than 35 years can be detected without d i f f i c u l t y . The other r a d i o n u c l i d e s given i n Table 1, namely A r , Si, K r , and C 1 , are not being produced a t present i n l a r g e amounts by a r t i f i c i a l means. However, nuclear detonations i n s a l t and i n or near s a l t water have from time t o time produced l a r g e amounts of C 1 d u r i n g the past three decades. A l s o , Dansgaard and others [11] have presented evidence f o r a s i g n i f i c a n t but short-term pulse of bomb-produced S i f a l l o u t f o l l o w i n g the weapons t e s t i n g of the e a r l y 1960's. The p r o d u c t i o n of A r and K r by a r t i f i c i a l means i s probably small i n comparison t o n a t u r a l p r o d u c t i o n (Oeschger, personal communication, 1978). A number of general reviews o f the use of a t m o s p h e r i c a l l y produced r a d i o n u c l i d e s f o r d a t i n g groundwater have been w r i t t e n [12-16]. Most of these reviews c e n t e r on the use of H and C. A fundamental assumption made f o r most d a t i n g w i t h atmo s p h e r i c r a d i o n u c l i d e s i s t h a t the cosmic r a d i a t i o n f l u x and hence, the n a t u r a l p r o d u c t i o n of the r a d i o n u c l i d e s has been constant w i t h time. Various s t u d i e s of t h i s problem using C and t r e e - r i n g c a l i b r a t i o n have been made. I s o t o p i c s t u d i e s of meteorites have a l s o been u s e f u l [17]. C o n s i d e r i n g the probable l a c k of b a s i c accuracy of d a t i n g water, the problem of changes i n cosmic ray f l u x i s not s e r i o u s . 8 5

3

8 5

3

14

3 9


8 1

3 2

3 6

36

3 2

3 9

8 1

3

14

14

Carbon-14 K. 0. Mlinnich [18] p u b l i s h e d the f i r s t d e s c r i p t i o n of the use of C to date groundwater. Since t h i s pioneer paper, c o u n t l e s s s t u d i e s have been made u t i l i z i n g C i n c o n j u n c t i o n w i t h conven t i o n a l hydrogeologic i n v e s t i g a t i o n s i n almost a l l p a r t s of the world [4,13,19-22]. One of the most e x t e n s i v e of these s t u d i e s was by Pearson who sampled the C a r r i z o a q u i f e r i n Texas and was able t o show a reasonable r e l a t i o n s h i p between hydrodynamic and C ages of water over a wide region [23]. Despite the v a s t amount of work on C d a t i n g which has a l r e a d y been accomplished and d e s p i t e the f a c t t h a t i t i s the best developed method a v a i l a b l e today, numerous d i f f i c u l t i e s still e x i s t with i t s application. F i r s t , carbonate geochemistry which helped c o n t r o l C c o n c e n t r a t i o n s i n the past i s not simple t o r e c o n s t r u c t . Carbonate minerals are commonly i n a s t a t e of near e q u i l i b r i u m w i t h groundwater, and o n l y s l i g h t changes i n water temperature or chemistry w i l l promote e i t h e r d i s s o l u t i o n or pre c i p i t a t i o n of carbonate i o n s . In t h i s way, the p r o p o r t i o n of modern carbon i n the water can be changed and some i s o t o p e 14

14

14

14

14


11.

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AND

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193

Dating Groundwater

f r a c t i o n a t i o n c o u l d take p l a c e . Second, l a r g e amounts o f organ i c a l l y d e r i v e d carbon from a n c i e n t c o a l and l i g n i t e can e n t e r i n t o the groundwater by way of bicarbonate ions and t h i s dead carbon can predominate even i n r e l a t i v e l y young groundwater [24]. T h i r d , a minute but s i g n i f i c a n t amount of C i s probably produced i n the subsurface. Although not important i n d a t i n g water l e s s than a few thousand years o l d , C produced i n the subsurface may l i m i t accurate d a t i n g t o water which i s 50,000 t o 80,000 y e a r s o l d or l e s s [25]. Under c e r t a i n circumstances, the extent o f d i s s o l u t i o n of marine carbonate rocks can be estimated by u s i n g C / C r a t i o s which are much l a r g e r than carbon from t e r r e s t r i a l p l a n t s . Very roughly, b i o l o g i c a l l y d e r i v e d C0 i n s o i l of the middle l a t i t u d e s has a 6 C value of -25 and marine carbonate rocks have ô C values c l o s e t o 0.0 [26]. Because most C e n t e r s groundwater through the d i s s o l u t i o n of C0 which i s , i n t u r n , d e r i v e d from t e r r e s t r i a l p l a n t s , a subsurface i n c r e a s e i n C / C r a t i o s should r e f l e c t the d i s s o l u t i o n of marine carbonate rocks which are assumed t o be devoid of C . The v a r i o u s steps f o r i s o t o p i c and geochemical c o r r e c t i o n s of C dates have been reviewed by Fontes and G a m i e r [27]. They gave s e v e r a l examples of t h e i r method of c o r r e c t i n g dates and compare t h e i r method w i t h methods of s e v e r a l other authors. They p o i n t e d out t h a t i f the c o r r e c t geochemical adjustments are not made, r e s u l t i n g dates can vary by more than 100 percent, t h u s , underscoring the n e c e s s i t y of using geochemi cal l y sound models t o i n t e r p r e t the C data. Although not thoroughly documented, our judgment i s t h a t many C dates of water should be considered only as order-of-magnitude estimates r a t h e r than "dates" i n the usual meaning of the i s o t o p e geochemist. Where chemical c o m p l i c a t i o n s are minimal, dates w i t h ±20 percent accuracy may be p o s s i b l e . However, other p u b l i s h e d "dates" may be e a s i l y i n e r r o r by more than ±100 percent. To the h y d r o g e o l o g i s t , n e v e r t h e l e s s , even an order-of-magnitude estimate of water age can be of great p r a c t i c a l value i n t r y i n g t o decipher complex groundwater systems. 14

14


1 3

1 2

2

13

13

14

2

1 3

1 2

14

14

14

14

Hydrogen-3 3

P r i o r t o 1952, most n a t u r a l H , or t r i t i u m , was d e r i v e d from cosmic r a d i a t i o n i n t e r a c t i n g w i t h the atmosphere. H i s t o r i c a l con c e n t r a t i o n s i n r a i n water i n the middle l a t i t u d e s p r i o r t o t h i s time were on the order of 10 t r i t i u m u n i t s (TU), one t r i t i u m u n i t being equal t o one H atom per 1 0 atoms of s t a b l e hydrogen. The manufacture and t e s t i n g of f u s i o n devices have i n j e c t e d l a r g e amounts o f t r i t i u m i n t o the atmosphere d u r i n g the past 28 years. Peak c o n c e n t r a t i o n s of more than 10,000 TU were measured i n r a i n over Canada f o l l o w i n g massive weapons t e s t s i n the U.S.S.R. i n the mid-I960's. Owing t o the nature of atmospheric c i r c u l a t i o n p a t t e r n s and the predominance of ocean s u r f a c e which a c t s as a H s i n k , p r e c i p i t a t i o n i n the southern hemisphere has roughly 3

1 8

3



194

N U C L E A R AND

C H E M I C A L DATING T E C H N I Q U E S

one-tenth the t r i t i u m c o n c e n t r a t i o n of p r e c i p i t a t i o n i n the northern hemisphere. Regional d i f f e r e n c e s of t r i t i u m concen t r a t i o n s r e l a t e d to the d i s t a n c e from the coast and l o c a l c l i m a t i c c o n t r o l s on r a i n f a l l are a l s o s i g n i f i c a n t . Seasonal v a r i a t i o n s are, i n a d d i t i o n , l a r g e . In the northern hemisphere during the 1960's, summer maxima of t r i t i u m concentrations were ten times the w i n t e r minima. I f t r i t i u m were evenly d i s t r i b u t e d i n space and time w i t h i n the atmosphere, i t would make an almost i d e a l r a d i o n u c l i d e w i t h which t o date very young groundwater [1,28]. U n f o r t u n a t e l y , an accurate h i s t o r i c a l r e c o n s t r u c t i o n of the e f f e c t i v e t r i t i u m con c e n t r a t i o n i n past recharge water f o r a given a q u i f e r i s a d i f f i c u l t task. Not only are o r i g i n a l c o n c e n t r a t i o n s of t r i tium i n p r e c i p i t a t i o n a t a given l o c a t i o n p o o r l y known, but évapotranspiration and other n a t u r a l phenomena r e l a t e d t o l o c a l weather, v e g e t a t i o n , and geology which w i l l a f f e c t the t r i t i u m concentrations i n groundwater are l a r g e l y unstudied. For example, Ehhalt [29] has shown t h a t microorganisms i n the s o i l are able to o x i d i z e t r i t i a t e d molecular hydrogen d i r e c t l y from the atmo sphere. Inasmuch as the t r i t i u m content of the atmosphere may reach 10 to 10 times the r e l a t i v e t r i t i u m c o n c e n t r a t i o n s of r a i n water, the d i r e c t c o n t r i b u t i o n of t r i t i u m to the groundwater through s o i l b a c t e r i a may be as important under some circumstances as t r i t i u m c o n t r i b u t e d from p r e c i p i t a t i o n [29]. Owing to the complex problem of d e f i n i n g t r i t i u m concentra t i o n s a t the time of groundwater recharge, most s t u d i e s make only a q u a l i t a t i v e judgment of groundwater age based on t r i t i u m concen t r a t i o n s [5,30,31]. The Isotope Hydrology S e c t i o n of IAEA [15] recommended the f o l l o w i n g t h r e e - f o l d d i v i s i o n of " t r i t i u m - a g e s " : 1. Water w i t h concentrations l e s s than 3 TU i n d i c a t e s ground water ages i n excess of 20 years. 2. Water w i t h concentrations between 3 and 20 TU i n d i c a t e s the presence of some t r i t i u m from t e s t i n g of f u s i o n devices and the water probably dates from the f i r s t t e s t i n g p e r i o d , t h a t i s between 1953 and 1961. 3. Water w i t h c o n c e n t r a t i o n s i n excess of 20 TU would suggest water o r i g i n a t i n g s i n c e 1961. The r e l a t i v e l y s h o r t h a l f - l i f e of t r i t i u m (12.26 y e a r s ) r e q u i r e s an a p p r o p r i a t e m o d i f i c a t i o n of the above c r i t e r i a f o r t r i t i u m s t u d i e s made a t a date l a t e r than the date of p u b l i c a t i o n (1973). A l s o , the c r i t e r i a are developed f o r the m i d - l a t i t u d e s i n the northern hemisphere and should not be a p p l i e d elsewhere. T r i t i u m e x t r a c t e d from s o i l moisture i n the unsaturated zone a t various depths below the surface has been used to i n f e r the progress of recharge of u n d e r l y i n g a q u i f e r s . Studies of recharge i n a r i d and s e m i a r i d zones where water moves very s l o w l y i n a downward d i r e c t i o n have been p a r t i c u l a r l y instructive [20, 32-35]. The s h o r t h a l f - l i f e of t r i t i u m imposes a time l i m i t on the usefulness of t r i t i u m d a t i n g . However, because t r i t i u m decays to 3

4


11.

DAVIS

AND

Dating Groundwater

BENTLEY

195

3

the s t a b l e helium i s o t o p e , He, and because n a t u r a l background c o n c e n t r a t i o n s of He i n water are so low, the o r i g i n a l t r i t i u m c o n c e n t r a t i o n of groundwater can be determined t h e o r e t i c a l l y by measuring the excess He present [36,37]. Then, i f the o r i g i n a l tritium concentrations i n the groundwater recharge can be determined as a f u n c t i o n of time, d a t i n g of the water may be possible. To be u s e f u l f o r p r e c i s e d a t i n g , the H- He method would need to assume, f i r s t , t h a t l a r g e seasonal f l u c t u a t i o n s of o r i g i n a l H are "averaged" by some mixing process i n the sub surface and, second, t h a t n o n - t r i t i u m sources of anomalously l a r g e He c o n c e n t r a t i o n s are not present. Such sources, f o r t u n a t e l y , are probably c o n f i n e d p r i m a r i l y to areas where deep thermal waters r i s e to the s u r f a c e and would not be present i n normal, near s u r f a c e , groundwater. The matter of He c o n c e n t r a t i o n s i n water, n e v e r t h e l e s s , needs f u r t h e r study. As i s mentioned below, anomalous He values could p o s s i b l y be found a s s o c i a t e d w i t h natural c o n c e n t r a t i o n s of l i t h i u m . The t h e o r e t i c a l aspects of subsurface production of t r i t i u m have been i n v e s t i g a t e d . Normal a q u i f e r s should not have more than about 0.5 TU which o r i g i n a t e i n the subsurface p r i m a r i l y by natural f i s s i o n of U and by capture of thermal neutrons by L i w i t h a subsequent r e l e a s e of an alpha p a r t i c l e [38]. Unusually high c o n c e n t r a t i o n s of uranium and l i t h i u m , however, c o u l d g i v e r i s e to perhaps as much as 1.5 TU through subsurface production. Such small c o n c e n t r a t i o n s are of l i t t l e d i r e c t importance t o normal t r i t i u m d a t i n g because the usual p r e c i s i o n of t r i t i u m analyses i s commonly about the same as the p o s t u l a t e d background values produced by n a t u r a l subsurface nuclear r e a c t i o n s . Never t h e l e s s , i n view of the above d i s c u s s i o n , t r a c e amounts of t r i t i u m found i n o l d groundwater should not be e x p l a i n e d on the b a s i s of sample contamination nor the mixing of small amounts of modern groundwater w i t h predominantly o l d water unless the e n t i r e matter has r e c e i v e d c a r e f u l study. 3

3

3

3

3


3

3

3

2 3 8

6

Chlorine-36 36

The f i r s t analyses of C 1 i n n a t u r a l waters were reported by S c h a e f f e r and others [39]. Based on t h i s work, Davis suggested [40] t h a t C 1 would be u s e f u l t o date o l d groundwater because i t s h a l f - l i f e of 3.01 χ 10 years i s i d e a l f o r the range of 5 χ 10 to 1 χ 10 years which i s beyond the normal range of C d a t i n g . In a d d i t i o n , c h l o r i d e i n groundwater i s n e i t h e r d e r i v e d normally from, nor r e a c t s w i t h , the s o l i d matrix of the a q u i f e r . Thus, the problems of geochemical i n t e r p r e t a t i o n are not as formidable w i t h C 1 as w i t h C . Tamers and Ronzani [41] were the f i r s t to actually i n v e s t i g a t e d a t i n g of groundwater using C1. U n f o r t u n a t e l y , they considered only cosmogenic C 1 production a t the earth's s u r f a c e and ignored the component of atmospheric o r i g i n , shown by Bentley [42] t o be much more important. 36

s

4

6

36

14

14

36

36


196


3 6

Moreover, t h e i r conventional counting o f the very low C 1 a c t i v i t y r e q u i r e d h e r o i c a n a l y t i c a l methods. The development o f mass s p e c t r o m e t r i c techniques f o r n u c l i d e i d e n t i f i c a t i o n using a tandem Van de G r a a f f a c c e l e r a t o r a t the U n i v e r s i t y o f Rochester Nuclear S t r u c t u r e Laboratory by H. Gove, K. Purser, A. L i t h e r l a n d , and numerous a s s o c i a t e s has provided an e x c e l l e n t means f o r the p r e c i s e measurement o f C 1 c o n c e n t r a t i o n s i n n a t u r a l water [43]. Thus f a r , about 40 groundwater r e l a t e d samples which have been c o l l e c t e d and p u r i f i e d c h e m i c a l l y by H. Bentley have been analyzed f o r C 1 by D. Elmore, H. B e n t l e y , and others using the U n i v e r s i t y o f Rochester machine. Some o f these samples are l i s t e d i n Table 2. 3 6


3 6

Table 2.

Analyses o f Chlorine-36 Using A c c e l e r a t o r a t t h e Univer s i t y o f Rochester Nuclear S t r u c t u r e Research Laboratory. 3 6

(Analyses are given as the r a t i o o f C 1 n u c l e i t o 1 0 ~ times the t o t a l number o f c h l o r i n e n u c l e i )

36

Sample Number

C1/C1

1 5

(xlO )

Samples o f water l e s s than 20,000 years o l d Tucson, A r i z o n a , a l l u v i a l a q u i f e r 1.

C i t y w e l l #B-18

365 ±

18

2.

C i t y w e l l #C-13

379 ±

22

Madrid, Spain, T e r t i a r y a l l u v i u m 3. 4.

Well 535-7-b Well 535-7-a

231 ± 21 295 ± 12

5.

Well 535-5-c

235 ±

7

Southern, Texas, C a r r i z o Sandstone 6.

Well

32 ±

3

7.

Well

64 ±

6

258 ±

13

North Dakota, Fox H i l l s Sandstone 8.

W e l l , Bowman 131-102-14AAB s

7

Samples o f water between 1 0 and 1 0 years o l d North Dakota, Fox H i l l s Sandstone 9. Stanton, Well #400, 144-085-03DCD . . . . 7.1 ± 2.3 10. Mandan, Well #139-081-09 AAA1 10.2 ± 2 In Nuclear and Chemical Dating Techniques; Currie, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

1 5

11.

DAVIS

AND

Dating Groundwater

BENTLEY

197

Table 2 continued 36

Sample Number

1 5

C1/C1

(xlO )


South C a r o l i n a , Metamorphic rocks below Coastal P l a i n sediments 11.

Savannah R i v e r P l a n t , Test w e l l #DRB6 . .

12.

Savannah R i v e r P l a n t , Test w e l l #DRB11

Samples o f water l i k e l y t o c o n t a i n Tucson, A r i z o n a , a l l u v i a l 13.

3 6

9.5 ± 1.0

. 6.9 ±

.7

C 1 o f bomb o r i g i n

aquifer

Well-Campbell Farms

1950 ± 150

Southern Texas, C a r r i z o Sandstone 14. Well Al-68-51-803 Ocean Water 15.

A t l a n t i c Ocean, surface water, AII85

. .

90 ±

9

2 ±

2

177 ± 1 ± 256 ±

10 2 17

Samples o f s o l i d m a t e r i a l 16. 17. 18.

Modern s a l t c r u s t , W i l l cox P l a y a , Arizona C l e a r Fork, Texas, s a l t from s a l t dome Thorium ore 3 6

.

1 5

Variations i n the C 1 / C l x l 0 r a t i o s o f samples 1 through 12 shown i n Table 2 are a f u n c t i o n o f numerous f a c t o r s among which l a t i t u d e , p r o x i m i t y t o the c o a s t , and age appear t o be most important. Inasmuch as the average age o f c h l o r i d e i n t h e oceans i s probably i n excess o f Ί 0 y e a r s , t h e c o n c e n t r a t i o n o f C 1 per t o t a l c h l o r i d e atoms i s very low i n ocean water (sample 15, Table 2). The e f f e c t o f "dead" marine c h l o r i d e near t h e coast i s seen c l e a r l y i n t h e c o n t r a s t between young samples from Texas (Samples 6 and 7, Table 2) as compared w i t h young samples from Madrid (Samples 3, 4, and 5 ) , Tucson (Samples 1 and 2 ) , and North Dakota (Sample 8 ) . The C 1 / C l x l O r a t i o s i n a l l the young samples can be p r e d i c t e d [42] by c o n s i d e r i n g t h e atmospheric cosmogenic C1 which depends on l a t i t u d e [44] and the average annual d e l i v e r y o f c h l o r i d e t o the sample area [45] which i s l a r g e l y a f u n c t i o n o f the d i s t a n c e t o the coast. Time, o f course, a l l o w s d i s i n t e g r a t i o n o f the C 1 once water enters the subsurface. Ideally, C 1 c o n c e n t r a t i o n s should decrease r e g u l a r l y downdip i n an a q u i f e r as water c a r r i e s t h e c h l o r i d e deeper i n t o an a q u i f e r . In t h e a q u i f e r s t u d i e d i n most 8

3 6

3 6

1 5

36

3 6

3 6


198

N U C L E A R AND


d e t a i l thus f a r , the Fox H i l l s Sandstone of North Dakota, a reasonable c o r r e l a t i o n e x i s t s between hydrodynamic ages and C1 ages [46; B e n t l e y , manuscript i n p r e p a r a t i o n ] . In the C a r r i z o Sandstone, however, the data obtained thus f a r (most of which are not shown i n Table 2) are not as e a s i l y i n t e r p r e t e d because there i s an i n i t i a l i n c r e a s e i n C 1 c o n c e n t r a t i o n s as w e l l as C1/C1 r a t i o s i n the uppermost p a r t of the a q u i f e r . Pearson's data on C ages of C a r r i z o groundwater [23] i n d i c a t e t h a t these anomalous samples are a l l l e s s than 30,000 years o l d . Induced upward m i g r a t i o n of o l d water c o n t a i n i n g dead c h l o r i d e from lower a q u i f e r s may accompany the development of some of the w e l l s near the outcrop, thus lowering the C1/C1 r a t i o s and causing water from these w e l l s t o appear o l d e r than the down-gradient waters where groundwater e x t r a c t i o n i s minimal. A l s o , lowering of the ocean l e v e l s w i t h a r e t r e a t of the s h o r e l i n e d u r i n g the P l e i s t o cene may have caused a r e d u c t i o n i n dead c h l o r i d e i n the recharge areas of the C a r r i z o Sandstone d u r i n g the p e r i o d of approximately 15,000 t o 70,000 years ago. This would make the P l e i s t o c e n e waters which are now down-dip i n the a q u i f e r , appear younger than modern groundwater near the outcrop of the a q u i f e r . Concentration increases of C 1 may have been caused by higher évapotranspira t i o n of the o l d e r waters due to a more a r i d c l i m a t e i n the past. Another p o s s i b l e mechanism f o r C 1 c o n c e n t r a t i o n i n c r e a s e i s i o n c o n c e n t r a t i o n due to membrane e f f e c t s of the c l a y s and shales which c o n f i n e the C a r r i z o a q u i f e r . Complications i n C 1 d a t i n g of groundwater, which are a l s o a p p l i c a b l e to d a t i n g w i t h other atmospheric r a d i o n u c l i d e s , are (1) p o s s i b l e isotope f r a c t i o n a t i o n due to membrane e f f e c t s of groundwater passing through s i l t and c l a y beds, (2) c r o s s formational flow of groundwater i n a q u i f e r s which appear to be i s o l a t e d by nonpermeable beds but a c t u a l l y are not i s o l a t e d , (3) p o s s i b l e d i f f u s i o n of dead c h l o r i n e from f l u i d i n c l u s i o n s i n minerals w i t h i n c r y s t a l l i n e r o c k s , and (4) subsurface production of C 1 by the n a t u r a l subsurface neutron f l u x . The a c t u a l e f f e c t s of the items l i s t e d above are q u i t e s i t e s p e c i f i c and, t h e r e f o r e , need to be i n v e s t i g a t e d on a case by case b a s i s . Not w i t h s t a n d i n g t h i s s i t e - s p e c i f i c nature, c o n s i d e r a b l e general research i s needed t o help bound the problems. For example, Bentley [42] has c a l c u l a t e d the normal ranges to be expected from subsurface production of C 1 and has concluded t h a t i t becomes s i g n i f i c a n t a f t e r about two h a l f - l i v e s and may dominate the C1 c o n c e n t r a t i o n s a f t e r f o u r h a l f - l i v e s . The u s e f u l range of d a t i n g by C 1 i s l i m i t e d on the upper extreme a t about one m i l l i o n years by e f f e c t s of subsurface production. In the absence of s i g n i f i c a n t neutron production i n the subsurface, as would be expected i n e x c e p t i o n a l l y pure d e p o s i t s of limestone and h a l i t e , subsurface production should be very s l i g h t (Sample 17, Table 2). On the other hand, uranium or thorium ore should have a maximum subsurface production which would roughly be e q u i v a l e n t to the r a t e of atmospheric production (Sample 18, Table 2). 36

36

36


14

36

36

36

36

36

36

36

36


11.

Dating Groundwater

DAVIS A N D B E N T L E Y

199

Whether o r not cosmic r a d i a t i o n o f the s o i l s u r f a c e increases the a v a i l a b l e C 1 s i g n i f i c a n t l y i s an open question. C e r t a i n l y some C 1 i s produced, but the e f f e c t s a r e probably not domi nant. Sample 16 (Table 2) i s from a p l a y a which has a t h i n s a l t c r u s t . Even though the water seeping i n t o the p l a y a and the s a l t a l r e a d y there a r e both young and should c o n t a i n r e l a t i v e l y l a r g e amounts o f C 1 , t h e C1/C1 r a t i o i s s t i l l below t h a t o f shallow groundwater i n the r e g i o n (Samples 1 and 2). The extent o f C 1 d e r i v e d from cosmic r a d i a t i o n o f s o i l i s not p a r t i c u l a r l y impor t a n t t o know, however, f o r most water d a t i n g p r o j e c t s . As we v i s u a l i z e the d a t i n g method, the i n i t i a l C 1 c o n c e n t r a t i o n s w i l l be e s t a b l i s h e d by sampling water which i s near the a q u i f e r i n t a k e area but which i s s t i l l a few hundred t o a few thousand years old. C 1 d e r i v e d from r a i n , d r y f a l l o u t , and s o i l l e a c h i n g should be mixed together and roughly averaged over p e r i o d s o f several c e n t u r i e s . Because t h e C 1 method o f d a t i n g may be s e n s i t i v e t o P l e i s t o c e n e c l i m a t i c f l u c t u a t i o n s , e x t e n s i v e sampling i n t h e a q u i f e r o f i n t e r e s t combined w i t h other s t u d i e s o f r a d i o n u c l i d e s and paleotemperature i n d i c a t o r s i s advised. 3 6

3 6

3 6

36

3 6


3 6

3 6

3 6

Silicon-32 3 2

3

The f a c t t h a t S i has a h a l f - l i f e intermediate between H and C , the two r a d i o n u c l i d e s most commonly used f o r d a t i n g water, suggestes t h a t i t could be important f o r d a t i n g water which i s g e n e r a l l y between 50 and 1,000 years o l d [47]. L a i and his co-workers i n I n d i a have been the most a c t i v e i n i n v e s t i g a t i n g the hydrogeologic a p p l i c a t i o n s o f S i d a t i n g [48]. Most S i dates appear t o be much younger than C dates o f the same water. This discordance may be e x p l a i n e d i n p a r t by hydrodynamic mixing o f waters i n shallow a q u i f e r s [49]. A recent determination o f t h e h a l f - l i f e o f S i , however, has suggested t h a t the discordance i n d a t i n g i s even l a r g e r than formerly believed. Published values o f the h a l f - l i f e vary from 101 years to 710 years. The s m a l l e s t value i s the l a t e s t determination [50,51] which, i f adopted, w i l l r e p l a c e the p r e v i o u s l y accepted value o f 330 years. Ages c a l c u l a t e d using the 101-year h a l f - l i f e w i l l be s i g n i f i c a n t l y s m a l l e r than any p r e v i o u s l y p u b l i s h e d dates. In a d d i t i o n t o t h e questions r e l a t e d t o the h a l f - l i f e o f S i , many questions e x i s t as t o the d e t a i l s o f the near-surface r a d i o c h e m i s t r y and geochemistry o f s i l i c a . As already mentioned, there probably e x i s t s a s i g n i f i c a n t but p o o r l y known c o n t r i b u t i o n of S i from f a l l o u t from t e s t i n g nuclear bombs [11]. The f i r s t few centimeters o f s o i l a r e s u b j e c t t o nuclear r e a c t i o n s produced by cosmic r a d i a t i o n such as the p o s s i b l e s p a l l a t i o n o f C 1 t o produce S i . The extent o f near-surface production i s unknown. Even more s e r i o u s i s t h e very complex nature o f s i l i c a geochemis t r y , p a r t i c u l a r l y i n t h e s o i l h o r i z o n . S i g n i f i c a n t amounts o f s i l i c a accumulate i n growing p l a n t m a t e r i a l s [52] and under some circumstances can a l s o accumulate as p h y t o l i t h s o f opal [53] and 1 4

3 2

3 2

1 4

3 2

3 2

3 2

3 5

3 2


200


p o s s i b l y other more complex compounds i n the upper p a r t of the s o i l horizon. Inasmuch as most of the s i l i c a which i s mixed w i t h organic m a t e r i a l i n the s o i l i s probably i n a r e l a t i v e l y s o l u b l e form or i n a form which i s e a s i l y desorbed, water which e v e n t u a l l y becomes groundwater recharge could have i n i t i a l S i concentra t i o n s somewhat above those of the o r i g i n a l p r e c i p i t a t i o n . At the present stage of development, d a t i n g w i t h Si is best a p p l i e d t o the establishment of r e l a t i v e ages of water i n a s i n g l e a q u i f e r . The method i s probably not a r e l i a b l e means of e s t a b l i s h i n g an a b s o l u t e age. 3 2


3 2

Argon-39 3 9

The h a l f - l i f e of 270 years of A r makes i t u s e f u l f o r d a t i n g m a t e r i a l s i n the 50 t o 2,000-year range [ 5 4 ] , which i s i n the age range between d a t i n g by H and C . P r e l i m i n a r y s t u d i e s by L o o s l i and Oeschger [55] suggest t h a t , indeed, A r w i l l g i v e good r e l a t i v e dates f o r d i f f e r e n t samples o f groundwater. However, l i k e S i dates, the A r dates are g e n e r a l l y much younger than dates of the same water obtained by using C . L o o s l i and Oeschger [55] considered t h r e e primary explana t i o n s f o r the A r - C d a t i n g d i s c r e p a n c i e s . F i r s t , and perhaps most important, the l a r g e natural abundance of potassium would make the r e a c t i o n K ( n , p ) A r q u i t e important. Thus, subsurface production of A r could produce apparent ages which are f a r too young. L o o s l i and Oeschger [55] as w e l l as Z i t o (1980, personal communication) have estimated t h a t , under c e r t a i n circumstances, the subsurface p r o d u c t i o n of A r could exceed the atmospheric production. However, data are l a c k i n g f o r r e l i a b l e c a l c u l a t i o n s of both the K cross s e c t i o n f o r the capture of thermal neutrons and the r a t e of t r a n s f e r t o the water of the A r which i s generated i n s o l i d s . A second p o s s i b l e e x p l a n a t i o n f o r the d i s crepancies i s t h a t the C dates are f a r too l a r g e because o f dead carbon e n t e r i n g the system or a "chromatographic s e p a r a t i o n " of d i s s o l v e d species c o n t a i n i n g C takes p l a c e as water flows downg r a d i e n t i n the a q u i f e r . F i n a l l y , L o o s l i e and Oeschger [55] con s i d e r e d t h a t a subsurface mixture of o l d and young waters can account f o r some of the d i f f e r e n t dates which are obtained by v a r i o u s methods. 3

14

3 9

3 2

3 9

14

3 9

1 4

3 9

3 9

3 9

3 9

3 9

3 9

14

14

Krypton-81 8 1

Dating groundwater w i t h K r , i f i t ever proves f e a s i b l e , would have several advantages. F i r s t , the long h a l f - l i f e of 210,000 years should a l l o w d a t i n g of water beyond the range of C. Second, the gas i s i n e r t which would s i m p l i f y the problems of geochemical i n t e r p r e t a t i o n of a n a l y t i c a l r e s u l t s . T h i r d , the production of s i g n i f i c a n t amounts of K r i s probably c o n f i n e d t o n a t u r a l n u c l e a r r e a c t i o n s i n the atmosphere and s h a l l o w s o i l horizon induced by cosmic radiation. Natural subsurface 14

8 1


11.

DAVIS

AND

Dating Groundwater

BENTLEY

201

p r o d u c t i o n and a r t i f i c i a l n u c l e a r r e a c t i o n s should not complicate the i n t e r p r e t a t i o n o f the r e s u l t s . U n f o r t u n a t e l y , n a t u r a l concen t r a t i o n s are very small and water sample s i z e s o f more than 1 0 l i t e r s may be r e q u i r e d t o o b t a i n measurable amounts o f K r [ 1 0 ] . Such l a r g e sample s i z e s w i l l r e q u i r e some type o f gas s e p a r a t i o n system w i t h a l a r g e through f l o w i n g system of water from the w e l l or s p r i n g which i s being s t u d i e d . The p o t e n t i a l problems o f sample contamination w i t h such a system w i l l be d i f f i c u l t t o handle. Thus f a r , water d a t i n g by K r has not been accomplished. 6

8 1

8 1


Krypton-85 8 5

The present atmospheric c o n c e n t r a t i o n s o f K r are almost e n t i r e l y from a r t i f i c i a l n u c l e a r f i s s i o n . I t s h a l f - l i f e o f 10.7 years i s n e a r l y the same as H , so the useful range o f d a t i n g i s similar. U n l i k e H , however, c o n c e n t r a t i o n s o f K r have been i n c r e a s i n g a t a more-or-less steady r a t e ( f i g u r e 3) f o r the past 35 y e a r s . T h e r e f o r e , the i n p u t f u n c t i o n f o r K r i n groundwater i s much s i m p l e r than H and, t h e o r e t i c a l l y , the r e s u l t i n g dates should be more accurate. The primary d i f f i c u l t y w i t h using Kr f o r d a t i n g purposes i s the f a c t t h a t , even f o r modern water, the c o n c e n t r a t i o n s are very low, n e c e s s i t a t i n g the s e p a r a t i o n o f krypton gas from r e l a t i v e l y l a r g e samples (120 t o 360 l i t e r s ) o f water [ 5 6 ] . Although not w i d e l y used a t p r e s e n t , K r d a t i n g could be s u p e r i o r t o H d a t i n g f o r groundwaters l e s s than 30 years o l d . 3

3

8 5

8 5

3

8 5

8 5

3

ACCUMULATION OF PRODUCTS OF RADIOACTIVE DECAY Introduction As time passes, the d i r e c t o r i n d i r e c t products o f v a r i o u s r a d i o a c t i v e decay processes may accumulate. I f these products tend t o be formed i n groundwater o r tend t o migrate i n t o the groundwater and i f the products move w i t h a known r e l a t i o n s h i p t o the movement o f the groundwater, then the c o n c e n t r a t i o n o f the products i n the water may i n d i c a t e water age. A fundamental advantage o f using decay products as a b a s i s f o r d a t i n g i s the f a c t t h a t as time p r o g r e s s e s , more products w i l l be present and the a n a l y t i c a l aspects o f d a t i n g w i l l become e a s i e r . T h i s i s i n c o n t r a s t w i t h the use of atmospheric r a d i o n u c l i d e s which w i l l become more d i f f i c u l t t o d e t e c t as the age o f the groundwater increases. A number o f decay products may be o f i n t e r e s t u l t i m a t e l y as a b a s i s o f d a t i n g groundwater. At p r e s e n t , however, the accumula t i o n o f i n e r t gases appears t o o f f e r the most s i g n i f i c a n t p o s s i b i l i t i e s f o r d a t i n g [19,36,58-60]. Some candidate gases are given i n Table 3. Of those l i s t e d , He w i l l probably be the most u s e f u l because o f i t s r e l a t i v e l y r a p i d r a t e o f production. As a l r e a d y mentioned, because i t i s the decay product o f t r i t i u m , the other 4



202


Figure 3.

TECHNIQUES

85

Concentration of Kr measured in northern hemisphere air samples (57).


11.

Dating Groundwater


Table 3.

203

Noble Gases o f P o s s i b l e Use f o r Dating Groundwater. P o s s i b l e Range o f

Nuclide 'He

Origin

Ages f o r D a t i n g

5

S t a b l e end product o f the decay o f H.

5 t o 50 years

N e u t r a l i z a t i o n o f alpha p a r t i c l e s .

10

4

t o 1 0 years

3


"He 21 40

7

Ne

Capture o f alpha p a r t i c l e s by 0 .

10

6

t o 1 0 years

Ar

Decay product o f K . (10.7% o f K decays t o A r , the r e s t decays t o C a ) .

4 0

10

5

t o 1 0 years

Kr

Cosmic r a d i a t i o n i n t e r a c t i n g w i t h the atmosphere.

10

4

6 t o 10 years

Kr

F i s s i o n o f U f o r natural systems. F i s s i o n o f U and P u f o r a r t i f i c i a l systems. Only anthropogenic K r i s q u a n t i t a t i v e l y important.

1 8

4 0

7

7

4 0

4 0

81

85

2 3 8

2 3 5

1 t o 40 years

2 3 9

8 5

130 136 222

Xe

Natural f i s s i o n o f

2 3 8

U.

10

6

t o 1 0 years

Xe

Natural f i s s i o n o f

2 3 8

U.

10

6

t o 1 0 years

Rn

Decay product o f decay s e r i e s .

2 2 6

R a i n the

2 3 8

U

7

7

0.5 t o 10 days

t h e o r e t i c a l l y , t h e accumulation o f noble gases c o u l d be used t o date water o l d e r than 1 0 y e a r s ; however, under most s i t u a t i o n s g e o l o g i c c o n s i d e r a t i o n s would c a s t doubts on the s i g n i f i c a n c e o f such dates. For example, i n the United S t a t e s , more than h a l f of t h e groundwater which i s pumped comes from a q u i f e r s which d i d not e x i s t 1 0 years ago. 7

7

3

isotope o f helium, He, may be a l s o useful t o date very young groundwater. The p o t e n t i a l usefulness o f o n l y He and A r w i l l be d i s c u s s e d i n t h i s s e c t i o n . Not enough i s known about the pro d u c t i o n r a t e s and/or r e l e a s e r a t e s t o groundwater o f the other gases. 4

4 0

Helium-4 Accumulation 4

The subsurface accumulation o f He i s l a r g e l y from t h e n e u t r a l i z a t i o n o f alpha p a r t i c l e s . Although alpha p a r t i c l e s can o r i g i n a t e i n a v a r i e t y o f ways, most w i l l come from the decay o f


204


2 3 2

146

1 9 0

1 7 4

heavy r a d i o n u c l i d e s such as T h , Sm, P t , and H f . Of the heavy r a d i o n u c l i d e s , U i s the most important source o f alpha p a r t i c l e s under normal circumstances, and T h i s next i n importance. The t h i r d i n importance i s U . Sources o f He other than uranium and thorium probably account f o r l e s s than one percent o f the t o t a l . Owing t o t h e f a c t t h a t most U and v i r t u a l l y a l l t h e T h n u c l i d e s a r e bound i n the s t r u c t u r e o f minerals and a l s o t o the f a c t t h a t t h e mean path l e n g t h o f alpha r a d i a t i o n i s very s h o r t , most newly formed He r e s i d e s o r i g i n a l l y i n s o l i d m a t e r i a l . The r a t e s w i t h which He d i f f u s e s from t h e s o l i d rock matrix i n t o adjacent w a t e r - f i l l e d pores i s l a r g e l y unknown. The f a c t t h a t t h e He i s not r e t a i n e d p e r f e c t l y i n t h e l a t t i c e o f the rock-forming minerals i s well-known because o f the e a r l y f a i l u r e s t o date min e r a l s by t h e "helium c l o c k " . The exact r a t e o f escape o f He i s undoubtedly a complex f u n c t i o n o f temperature, c o n c e n t r a t i o n g r a d i e n t s o f H e , d i s t r i b u t i o n o f alpha-generating n u c l i d e s w i t h respect t o rock pores, o r i g i n a l energy o f t h e alpha p a r t i c l e s , and types o f minerals i n c o n t a c t w i t h t h e groundwater. As a f i r s t approximation, t h e r e l e a s e o f He from s o l i d m a t e r i a l i n t o t h e groundwater system might be considered as a s t e a d y - s t a t e process [60]. I f so, then p e r f e c t l y s t a t i c groundwater would experience a n e a r l y l i n e a r i n c r e a s e i n He w i t h time. T h i s l i n e a r r e l a t i o n i s t r u e because t h e h a l f - l i f e o f most alpha-producing r a d i o n u c l i d e s i s more than two orders o f magnitude l a r g e r than t h e age o f the o l d e s t groundwater which might be dated. E a r l y suggestions t o use helium t o date groundwater were made by Savchemko i n 1936 as quoted by S p i r i d o n o v and others [58] and Davis [ 4 0 ] . Serious attempts t o date groundwater by helium accumulation measurements have been made r e c e n t l y by S p i r i d o n o v and others [ 5 8 ] , Marine [ 6 0 ] , F r i t z and others [ 4 ] , and Bath and others [19]. Of the v a r i o u s s t u d i e s , those made by Bath and co-workers [19] have been most e l a b o r a t e and shown most promise. Although C and He d a t i n g o f t h e same groundwater samples y i e l d s a d i s t i n c t c o r r e l a t i o n between t h e dates d e r i v e d by t h e two separate methods, t h e c o r r e l a t i o n i s not a d i r e c t one-to-one r e l a t i o n s h i p . Almost a l l He dates a r e d i s t i n c t l y o l d e r , some by a f a c t o r o f 3 o r more, than corresponding C dates [19,4]. Some r e p r e s e n t a t i v e values from the l i t e r a t u r e are given i n Table 4. 2 3 8

2 3 2

2 3 5

4

2 3 8

2 3 2

4


4

4

4

4

4

4

1 4

4

4

1 4



F r i t z and o t h e r s , 1979 [4] Marine, 1976 [60]

Bath and o t h e r s , 1979 [19]

900,000 (assuming 1 ppm U and 4 ppm Th) 840,000

27,000

10,600

17,600

42,100

65,600

30,630 (uncorrected) Less than 20,000 P o s s i b l e sample contamination. Between 1,500 and 4,300 years BP

Between 7,300 and 10,600 years BP Between 4,000 and 7,300 years BP Between 33,000 and 36,100 years BP Between 24,900 and 28,200 y e a r s BP

Savannah R i v e r , South C a r o l i n a

Eastern England, Hal am

Grove

Egmanton

Ramptoη

Gainsborough

Reference

He Age ( y e a r s )

S t r i p a , Sweden

14C Age ( y e a r s )

Comparison o f Carbon-14 and Helium-4 Dates o f Groundwater.

L o c a t i o n o f Sample

Table 4.


206

N U C L E A R A N D C H E M I C A L DATING TECHNIQUES 4

The g r e a t e s t obvious weakness i n the He method o f d a t i n g i s i n the assumptions necessary t o c a l c u l a t e the f l u x o f He i n t o the groundwater. The exact d i s t r i b u t i o n of U and Th i n the rock mass i s not g e n e r a l l y measured but i s , n e v e r t h e l e s s , o f c r i t i c a l importance. In p a r t i c u l a r , U i s r e l a t i v e l y mobile i n water under o x i d i z i n g c o n d i t i o n s and w i l l commonly form f i n e - g r a i n e d mineral coatings along w a t e r - f i l l e d f r a c t u r e s i n otherwise s o l i d rock. The o p p o r t u n i t y f o r d i r e c t He r e l e a s e t o groundwater under these c o n d i t i o n s i s almost i n f i n i t e l y g r e a t e r than He r e l e a s e from l a r g e r , s o l i d mineral g r a i n s imbedded i n v i r t u a l l y nonpermeable rocks. The e f f e c t s of m i c r o f r a c t u r e s might a l s o be important as avenues of He m i g r a t i o n . The apertures of the m i c r o f r a c t u r e s w i l l change w i t h the s t r e s s c o n d i t i o n s i n the rock and w i l l open or c l o s e i n response t o changes i n surface loads imposed by g l a c i a l i c e and f l u c t u a t i o n s of nearby bodies of water. Thus, c o n f l i c t i n g C and He dates from water from the S t r i p a Mine i n Sweden [ 4 ] , f o r example, may be caused by d i l a t i o n of micro f r a c t u r e s w i t h accompanying r e l e a s e of accumulated He. This might have taken p l a c e d u r i n g the l a s t p e r i o d of déglaciation approximately 10,000 years ago. Heaton and Vogel [61] have p o s t u l a t e d t h a t the m i g r a t i o n of methane could a l s o a c t as a c a r r i e r g a s , f o r the m i g r a t i o n of He. The matter of the f l u x of helium from intermediate and deep g e o l o g i c sources i s , moreover, an u n s e t t l e d question. R e l a t i v e l y l a r g e helium f l u x e s have been measured f o r many years i n areas of recent volcanism and geothermal a c t i v i t y [62]. A l s o , l a r g e helium c o n c e n t r a t i o n s have been found i n young, nonthermal groundwater [55] and surface water [63]. The p o s s i b i l i t y e x i s t s t h a t helium from deeper p a r t s of the earth's c r u s t or from the mantle can seep upward i n s i g n i f i c a n t q u a n t i t i e s i n areas o u t s i d e of obvious geothermal a c t i v i t y . An index of t h i s helium f l u x from deep sources might be the He/ He r a t i o which would be much higher than values c a l c u l a t e d from p u r e l y 20 g e n e r a t i o n of helium. The mere presence of He, however, does not a u t o m a t i c a l l y i n d i c a t e deep sources of the helium because of the s i g n i f i c a n t p r o d u c t i o n of He from n a t u r a l r a d i o a c t i v i t y [38]. 4

4


4

4

14

4

4

3

S l t u

4

3

3

Argon-40 4 0

4 0

Owing t o the d i s i n t e g r a t i o n of K , the amount of Ar present i n groundwater should i n c r e a s e s l o w l y w i t h time. Despite the n a t u r a l abundance o f K , however, the volume o f A r which i s produced i n the average rock i s l e s s per u n i t time than the volume of He. I f one kg o f average igneous rock i s assumed t o have 1 mg U, 4 mg Th, and 25 g K, the p r o d u c t i o n of gas a t STP per kg of rock w i l l be: 4 0

4 0

4

2 3 8

1 0

4

2 3 5

11

4

from U , 1.160 χ 1 0 ' mL/yr. of He; from U , 4.73 χ 1 0 " mL/yr. of He; from T h , 1.139 χ 1 0 ~ mL/yr. of He; and from K , 9.49 χ ΙΟ"" mL/yr. of A r . 2 3 2

1 0

4 0

4

11

4 0


11.

DAVIS

AND

207

Dating Groundwater

BENTLEY

4 0

Thus, under the assumed c o n d i t i o n s which were chosen t o f a v o r A r p r o d u c t i o n , the A r p r o d u c t i o n w i l l be l e s s than one h a l f o f the He production. Furthermore, the d i f f u s i o n o f A r out o f the production s i t e s i n the m i n e r a l s w i l l be l e s s e f f e c t i v e than the d i f f u s i o n o f He. Even i f a l l the l o c a l l y generated A r were t o enter the groundwater, the A r would be d i f f i c u l t , n e v e r t h e l e s s , to d i f f e r e n t i a t e from a t m o s p h e r i c a l l y d e r i v e d A r . The A r / A r r a t i o i n groundwater should increase w i t h time from the o r i g i n a l atmospheric r a t i o o f 295.5. However, the i n c r e a s e w i l l be very slow. I f one percent p o r o s i t y and 2.8 g/mL d e n s i t y are assumed f o r the t h e o r e t i c a l igneous rock d i s c u s s e d above and i f the rock i s s a t u r a t e d w i t h water which has e q u i l i b r a t e d w i t h the atmosphere at 5 °C, then the A r / A r r a t i o w i l l o n l y increase t o 297 a f t e r 10 years p r o v i d e d a l l the A r which has been generated i n the rock migrates i n t o the water. A f t e r 1 0 y e a r s , the r a t i o w i l l only increase t o 313. Given c e r t a i n e r r o r s r e l a t e d t o sampling and a n a l y s e s , i t i s doubtful t h a t water l e s s than 1 0 years can ever be dated by A r ; t h i s i s p a r t i c u l a r l y t r u e i f one considers the f a c t t h a t most o f the A r generated i n the rock w i l l probably be r e t a i n e d by m i n e r a l s i n the rock. Normal c o n c e n t r a t i o n s o f K d i s s o l v e d i n groundwater are a t l e a s t an order o f magnitude lower, and commonly four orders o f magnitude lower, than K i n the s o l i d rock, so d i r e c t g e n e r a t i o n o f A r i n the water would not p r o v i d e s i g n i f i c a n t amounts o f A r f o r d a t i n g u n t i l a f t e r several m i l l i o n y e a r s . 4 0

4

4 0

4

4 0

4 0


4 0

4 0

4 0

3 6

3 6

s

4 0

6

5

4 0

4 0

4 0

4 0

4 0

4 0

URANIUM DISEQUILIBRIUM I f species w i t h s h o r t h a l f - l i v e s are o m i t t e d , the f i r s t por t i o n o f the U decay s e r i e s can be w r i t t e n as 2 3 8

238u 226

9

4.47 χ 1 0 y

s

r >

2.45 χ 1 0 y r ^

2 3 4 u

2 3

o

4

T h

7.7 χ 1 0 y r ^

3

R a

1.60 χ 1 0 y r ^

2

2

2

R

n

Numbers o f years r e f e r t o h a l f - l i v e s o f t h e r e a c t i o n s . Because the h a l f - l i f e o f U i s much longer than the other r a d i o n u c l i d e s i n the s e r i e s , the a c t i v i t i e s o f a l l the r a d i o n u c l i d e s w i l l be roughly equal a f t e r about 2 χ 1 0 years provided the parent and daughter products are i n a c l o s e d system. In n a t u r a l systems which are exposed t o c i r c u l a t i n g groundwater, however, t h e activity ratios o f these species are r a r e l y equal [ 6 4 ] . T h e o r e t i c a l l y , t h i s d i s e q u i l i b r i u m o f r a d i o n u c l i d e s i n the water can be used t o date the water p r o v i d e d enough i s known about the l o c a l geochemistry o f the d i s s o l u t i o n , s o r p t i o n , and p r e c i p i t a t i o n processes a f f e c t i n g the r a d i o n u c l i d e s . In p r a c t i c e , however, such d e t a i l e d i n f o r m a t i o n normally cannot be o b t a i n e d , so major s i m p l i f y i n g assumptions a r e commonly made. For some models, the geochemical system i s assumed t o be i n a s t e a d y - s t a t e , thorium i s assumed t o be e n t i r e l y i n the s o l i d form, uranium i s assumed t o 2 3 8

6



208


go i n t o s o l u t i o n i n the near-surface p o r t i o n s o f a q u i f e r s where o x i d i z i n g c o n d i t i o n s p r e v a i l , uranium i s assumed t o p r e c i p i t a t e i n the s o l i d form p r i m a r i l y i n deeper p o r t i o n s o f t h e a q u i f e r where o x i d i z i n g c o n d i t i o n s change t o reducing c o n d i t i o n s ( f i g u r e 4 ) , and l a s t l y , deep w i t h i n the a q u i f e r where c o n d i t i o n s a r e r e d u c i n g , t h e uranium a l r e a d y i n s o l u t i o n i s assumed t o remain i n s o l u t i o n without s i g n i f i c a n t e f f e c t s o f s o r p t i o n on s o l i d m a t e r i a l s . In d a t i n g groundwater w i t h the 234u/238y a c t i v i t y r a t i o , an i n i t i a l r a t i o must be estimated from f i e l d data. Most commonly, the i n i t i a l r a t i o s i n water from s h a l l o w a q u i f e r s are g r e a t e r than 1.0 w i t h values higher than 10.0 encountered i n some regions. High values are caused by d i r e c t r e c o i l o f a l p h a - e m i t t i n g n u c l e i and the s e l e c t i v e d i s s o l u t i o n o f U from s i t e s where the mineral s t r u c t u r e has been damaged by alpha d i s i n t e g r a t i o n o f U [65]. I f the only source o f U were d i r e c t alpha r e c o i l o f t h e n u c l i d e i n t o t h e water, the b u i l d u p o f U would be r e l a t e d p r i m a r i l y t o time and t o the m i c r o d i s t r i b u t i o n o f the uranium i n r e l a t i o n t o t h e w a t e r - f i l l e d pores. I f the geochemistry o f t h e s o l i d m a t e r i a l s i s uniform throughout t h e a q u i f e r , then i n c r e a s e s in U might be used t o deduce water ages up t o about 1.5 χ 1 0 years. However, s e l e c t i v e d i s s o l u t i o n o f m i n e r a l s along alphar e c o i l t r a c k s w i l l a l s o introduce U i n t o t h e water [ 6 5 ] , so the buildup o f U i s , i n a d d i t i o n , some complex f u n c t i o n o f t h e hydrochemistry o f the mineral-water system. C l e a r l y , t h e b u i l d u p curves f o r 234y/238y interpreted with caution. Another p o s s i b i l i t y f o r u t i l i z i n g 234y/238y f dating i s to measure t h e decrease o f the 2 3 4 y / 2 3 8 y t i o i n the deeper p a r t o f the reducing zone o f t h e a q u i f e r as an index o f r e l a t i v e water age ( f i g u r e s 4 and 5 ) . T h e o r e t i c a l l y , as t h e water changes from an o x i d i z i n g environment t o a reducing environment, most o f t h e uranium w i l l p r e c i p i t a t e i n t h i s t r a n s i t i o n zone [12,66]. Ground water f l o w i n g through the t r a n s i t i o n zone and c o n t i n u i n g downg r a d i e n t should e n t e r i n t o a zone t h a t i s geochemical l y q u i t e uniform and i s q u i t e c l o s e t o being i n chemical e q u i l i b r i u m . I t has been p o s t u l a t e d [12] t h a t i n t h i s downgradient r e g i o n t h e uranium which has entered i n t o s o l u t i o n w i l l tend t o remain i n s o l u t i o n and t h a t a d d i t i o n a l uranium w i l l not be d i s s o l v e d . Thus, o n l y r a d i o a c t i v e decay w i l l a f f e c t the 2 3 4 y / 2 3 8 y t i o s which then can be used d i r e c t l y f o r d a t i n g . Most researchers studying uranium isotopes i n water agree t h a t only q u a l i t a t i v e d a t i n g i s p o s s i b l e a t present. However, w i t h more i n f o r m a t i o n concerning the e n t i r e geochemical system, the c a l c u l a t i o n o f a c t u a l groundwater ages may be p o s s i b l e . Recent s t u d i e s o f uranium i n ore d e p o s i t s [ 6 7 ] and f r a c t u r e d source rocks as w e l l as water-deposited c a l c i t e i n caves ( s p e l e othems) have added u s e f u l background m a t e r i a l [64, 68-71]. For example, under some circumstances the 2 3 4 y / 2 3 8 y r a t i o s i n speleothems appear t o be r e l a t i v e l y constant over p e r i o d s o f several thousand years suggesting an average s t a b i l i t y i n t h a t p a r t o f t h e geochemical system a f f e c t i n g uranium d i s s o l u t i o n . 2 3 4

2 3 8

2 3 4

2 3 4

2 3 4

6

2 3 4

2 3 4

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u

s

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r a

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Figure 4. Simplified geochemical explanation of U migration in an aquifer.

Figure 5. Changes in ^XJ/^V ratios in water as a function of time in the aquifer shown in Figure 4. Velocity of the water is assumed to be constant, so distance of travel is directly proportional to time (12).


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This i n t u r n would lend c r e d i b i l i t y t o the method of d a t i n g water by uranium d i s e q u i l i b r i u m . A d d i t i o n a l l y , the use of Th/ U r a t i o s t o date the s o l i d p a r t s of a q u i f e r s would g i v e i n f o r m a t i o n on an upper age l i m i t f o r the water s a t u r a t i n g the a q u i f e r . For example, i f T h / U dates i n d i c a t e t h a t minerals were being deposited i n the a q u i f e r 20,000 years ago, then the present water s a t u r a t i n g the a q u i f e r i s probably l e s s than 20,000 y e a r s . 2 3 0

2 3 0

2 3 4

2 3 4


CHEMICAL DISEQUILIBRIA Under i d e a l circumstances, c e r t a i n chemical processes which are r e l a t i v e l y s l u g g i s h may p o s s i b l y be used f o r water d a t i n g . Near-surface water which i s low i n d i s s o l v e d s i l i c a , f o r example, might be undersaturated w i t h r e s p e c t t o s i l i c a which i n t u r n would suggest t h a t the water i s l e s s than 10 years o l d and probably l e s s than a few months o l d . U n f o r t u n a t e l y , the l a r g e number of v a r i ables which c o n t r o l d i s s o l u t i o n or p r e c i p i t a t i o n of m i n e r a l s i n natural systems probably can never be d e f i n e d w i t h s u f f i c i e n t p r e c i s i o n t o enable more than the most g e n e r a l , q u a l i t a t i v e dating. Rather than c o n s i d e r i n g mineral-water r e a c t i o n s as a b a s i s f o r d a t i n g , the presence of c e r t a i n metastable molecules which a l t e r spontaneously w i t h time may o f f e r b e t t e r o p p o r t u n i t i e s f o r d a t i n g water. Some a t t e n t i o n has been given t o t h i s p o s s i b i l i t y [12], but, t o date, f i e l d - o r i e n t e d data are l a c k i n g . The o n l y chemical group mentioned thus f a r i n the l i t e r a t u r e i n connection w i t h water d a t i n g has been the amino a c i d s which undergo sponta neous changes w i t h time. These changes, termed r a c e m i z a t i o n by organic geochemists, are a l s o temperature dependent [72,73]. To be useful f o r d a t i n g water, amino a c i d s o r i g i n a t i n g only a t the surface must be i d e n t i f i e d , t h e i r r a t e of racemization determined, and the thermal h i s t o r y of the water a f t e r d i s s o l u t i o n of the amino a c i d s must be estimated. The presence of a n c i e n t o r g a n i c m a t e r i a l s , p a r t i c u l a r l y b u r i e d s o i l s , i n the subsurface and the present-day a c t i v i t y of c e r t a i n b a c t e r i a i n the subsurface make the t a s k of i d e n t i f y i n g unique amino a c i d s f o r d a t i n g d i f f i c u l t . Moreover, many groundwater systems i n c l u d e deep c i r c u l a t i o n where temperatures may be more than 20 °C warmer than a t the s u r f a c e , so s i g n i f i c a n t temperature-dependent e f f e c t s could be encountered [73]. Although modeling the flow system may a l l o w meaningful estimates of the temperature e f f e c t s , d a t i n g w i t h amino a c i d s w i l l probably always y i e l d only q u a l i t a t i v e r e s u l t s . ANTHROPOGENIC CONSTITUENTS Several anthropogenic c o n s t i t u e n t s which are present i n the atmosphere are p o t e n t i a l l y u s e f u l as an index of water age. Two r a d i o a c t i v e gases from n u c l e a r weapons and from power r e a c t o r s , H and K r , have been d i s c u s s e d already. Several other r a d i o n u c l i d e s of man-made o r i g i n are present i n the atmosphere and i n 3

8 5


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9

the s o i l i n s o l u b l e form. Of these r a d i o n u c l i d e s , S r , I , T c , and R u would have the longest h a l f - l i v e s combined w i t h l e s s e r tendency t o be absorbed on s o l i d s than other man-man r a d i o n u c l i d e s such as C s . Even w i t h the extensive t e s t i n g o f nuclear e x p l o s i v e s i n the atmosphere during the 1960's, e n v i r o n mental concentrations of most a r t i f i c i a l r a d i o n u c l i d e s are a t such low l e v e l s t h a t a n a l y t i c a l d e t e c t i o n o f t h e i r presence i n recent groundwater would be d i f f i c u l t . T r i t i u m and p o s s i b l y K r w i l l probably remain the only convenient r a d i o n u c l i d e s having man-made o r i g i n s which can be used world-wide. A f a m i l y o f anthropogenic chemicals c a l l e d halocarbons o f f e r some i n t e r e s t i n g p o s s i b i l i t i e s f o r d a t i n g water which i s l e s s than about 40 years o l d [74,75]. Many o f the compounds are e a s i l y detected i n very small c o n c e n t r a t i o n s w i t h a gas chromatograph having an e l e c t r o n capture d e t e c t o r . Thus f a r , t r i c h l o r o f l u o r o methane (Freon-11) and d i c h l o r o d i f l u o r o m e t h a n e (Freon-12) have been i n v e s t i g a t e d w i t h some success. Two d a t i n g methods a r e used. One method i s based on t h e f a c t t h a t t h e atmospheric inventory o f the compounds has increased d r a m a t i c a l l y during the past 40 years. Concentrations i n rainwater and t h e r e s u l t i n g groundwater recharge water, consequently, a l s o have increased s y s t e m a t i c a l l y during t h i s p e r i o d . Because both compounds (Freon 11 and 12) are s t a b l e a t ambient temperatures and n e i t h e r compound adsorbs s t r o n g l y on normal a q u i f e r m a t e r i a l s , the c o n c e n t r a t i o n s of the compounds i n groundwater should c o r r e l a t e w i t h water ages back t o 30 o r 40 years before the present [75]. The other method uses the f a c t t h a t the two compounds have been introduced i n t o the atmosphere a t d i f f e r e n t r a t e s [76,77]. Because o f i t s e a r l y widespread use i n r e f r i g e r a t i o n , Freon 12 c o n c e n t r a t i o n s increased i n t h e atmosphere f i r s t . Later use o f Freon 11 together w i t h Freon 12 as aerosol p r o p e l l e n t s , foaming agents, c l e a n e r s , e t c . has produced a changing Freon 11/Freon 12 r a t i o w i t h time i n the atmosphere as w e l l as i n newly recharged groundwater. This r a t i o , t h e r e f o r e , may be c o r r e l a t e d w i t h the age of the water. Freon d a t i n g has advantages over t r i t i u m d a t i n g o f lower p o t e n t i a l c o s t and g r e a t e r p r e c i s i o n . The g r e a t e r p r e c i s i o n comes from a b e t t e r knowledge o f t h e i n i t i a l input concentrations because the i n t r o d u c t i o n o f Freons i n t o the atmosphere has been much more uniform than t r i t i u m i n both space and time. P o t e n t i a l disadvantages come p r i m a r i l y from p o s s i b l e s o r p t i o n o f Freons, p a r t i c u l a r l y on organic m a t e r i a l s . A l s o , c o n s i d e r a b l e development work w i t h Freon d a t i n g i s needed before c o n c e n t r a t i o n s i n recent water can be c o r r e l a t e d w i t h confidence w i t h l o c a l c l i m a t o l o g i c a l and geographic f a c t o r s even though the general world-wide buildup of the compounds i n the atmosphere can be r e c o n s t r u c t e d w i t h some confidence. 9 9


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MATCHING PALEOCLIMATIC INDICATORS WITH WATER AGES Several c o n s t i t u e n t s i n groundwater g i v e e i t h e r d i r e c t or i n d i r e c t i n d i c a t i o n s of o r i g i n a l water temperatures or general c l i m a t i c c o n d i t i o n s during the time when the water i n f i l t r a t e d i n t o the subsurface. At l e a s t three general approaches can be used to r e c o n s t r u c t these past c l i m a t e s . The most common method uses the H/ H and 0 / 0 r a t i o s i n water t o i n f e r storm pat t e r n s , évapotranspiration, and past temperatures [78,79]. A second method uses the c o n c e n t r a t i o n s of noble gases i n water a l s o to i n f e r paleotemperatures. A t h i r d method uses the c h l o r i d e content of groundwater t o i n t e r p r e t a n c i e n t r a t e s of évapotrans p i r a t i o n and/or p o s i t i o n s of a n c i e n t s h o r e l i n e s . Once c l i m a t i c trends as i n t e r p r e t e d from the groundwater data are e s t a b l i s h e d , they can be matched w i t h the known chronology of climatic f l u c t u a t i o n s back t o about 200,000 years before present. This method, of course, i s not p r e c i s e and may even be misleading unless abundant r e g i o n a l data are a v a i l a b l e . Notwith standing many shortcomings of the method, the simple f a c t t h a t groundwater may have recharged a t a time when average surface temeratures were d i s t i n c t l y d i f f e r e n t than a t present i s a v a l u a b l e b i t of i n f o r m a t i o n which, when combined w i t h other d a t i n g methods, may provide i n f o r m a t i o n on groundwater ages.


2

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Hydrogen-2/0xygen-18 R a t i o The s t a b l e isotopes of oxygen and of hydrogen f r a c t i o n a t e i n the atmosphere and a t the earth's surface. In g e n e r a l , the l i g h t e r isotopes are a s s o c i a t e d w i t h p r e c i p i t a t i o n i n c o o l e r weather, a t higher e l e v a t i o n s , and a t a great d i s t a n c e from the sea [80]. Because of s e v e r a l f a v o r a b l e geographic and c l i m a t o l o g i c a l f a c t o r s i n Greenland, Dansgaard [81] was able t o c o r r e l a t e c o n c e n t r a t i o n s of the s t a b l e i s o t o p e s of oxygen and hydrogen i n i c e w i t h average temperatures on the i c e cap. Other workers have attempted t o extend the work of Dansgaard t o the i n t e r p r e t a t i o n of paleotemperatures i n o l d groundwater [78]. Nevertheless, t h i s extension i s open to question. Several p h y s i c a l f a c t o r s must be, on the average, constant enough so t h a t the temperature i m p r i n t on the i s o t o p e r a t i o s can be detected. Some of the most important of these f a c t o r s are: "I·

2.

Topography. The l o c a l e l e v a t i o n a t the a q u i f e r i n t a k e area as w e l l as the c o n f i g u r a t i o n and e l e v a t i o n s of surrounding mountains must be r e l a t i v e l y constant. Recharge inducing storms. The general nature of the storms and t h e i r t r a j e c t o r i e s over the a q u i f e r i n t a k e area must be, on the average, constant. A change, f o r example, from dominantly l o c a l convective summer storms to dominantly f r o n t a l w i n t e r storms could account f o r changes i n i s o t o p i c r a t i o s i n the water which n e i t h e r r e f l e c t changes i n average temperatures nor t o t a l annual p r e c i p i t a t i o n .


11.

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E v a p o t r a n s p i r a t i o n . The t o t a l évapotranspiration which takes p l a c e p r i o r t o groundwater recharge should be constant. P o s i t i o n o f the c o a s t l i n e . The p o s i t i o n o f t h e c o a s t l i n e w i t h reference t o the p o s i t i o n o f the a q u i f e r i n t a k e area should remain constant. T h i s i s p a r t i c u l a r l y c r i t i c a l along c o a s t l i n e s w i t h g e n t l y s l o p i n g topography and s h a l l o w water on the c o n t i n e n t a l s h e l f . In such areas, l a t e r a l s h i f t s o f the s h o r e l i n e of more than 200 km have been common during the past 20,000 y e a r s . Subsurface r e a c t i o n s . The geochemical modifications o f groundwater, p a r t i c u l a r l y i o n f i l t r a t i o n and hydrothermal r e a c t i o n s , must not be o f the type which would f r a c t i o n a t e the isotopes being s t u d i e d .

F o r t u n a t e l y , i f enough samples are a v a i l a b l e from a given a q u i f e r , the v a r i a t i o n s of H and 0 can be compared w i t h C r a i g ' s empirical r e l a t i o n s h i p , which f o r normal s u r f a c e waters i s δ Η = 8 δ 0 + 10°/oo. Large departures from C r a i g ' s c o r r e l a t i o n l i n e c o u l d i n d i c a t e t h e e f f e c t s o f évapotranspiration o r o f hydrothermal r e a c t i o n s . At p r e s e n t , the viewpoints o f hydrogeologists concerning the u t i l i t y o f H and 0 analyses t o give p a l e o c l i m a t i c i n f o r m a t i o n vary from almost b l i n d acceptance t o r a t h e r general s k e p t i c i s m [82]. 2

2

1 8

1 8

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Noble Gases The c o n c e n t r a t i o n s o f noble gases i n groundwater should r e f l e c t t h e surface temperature a t t h e time o f groundwater recharge, provided the recharge i s r a p i d and goes d i r e c t l y i n t o the a q u i f e r [83-85]. The s o l u b i l i t y of each noble gas i n water i s d i f f e r e n t , each w i t h a unique r e l a t i o n s h i p w i t h temperature. A t a f i x e d temperature, the h e a v i e r gases are more s o l u b l e , Xe being roughly 400 times more s o l u b l e (on the b a s i s of mass r a t i o s ) than He a t 20 °C. More i m p o r t a n t l y , the s o l u b i l i t i e s o f the h e a v i e r gases are f a r more temperature s e n s i t i v e than the l i g h t e r gases. For example, the s o l u b i l i t y o f He v a r i e s o n l y s i x percent due t o a temperature change from 5° t o 20 °C; whereas, the same temperature change causes a 40 percent change i n t h e s o l u b i l i t y o f Xe. Therefore, w i t h a g i v e n sample o f groundwater t h r e e independent paleotemperatures can be c a l c u l a t e d , one f o r each o f the h e a v i e r noble gases ( A r , Kr, and Xe). Paleotemperatures d e r i v e d from noble gas analyses are poten t i a l l y more meaningful than those from oxygen-deuterium analyses because the noble gas content i s a d i r e c t measure o f the tempera t u r e o f the water a t t h e time o f i n f i l t r a t i o n r a t h e r than a complex f u n c t i o n o f geographic and meteorological f a c t o r s as i s the case w i t h H and 0 . Despite t h i s p o t e n t i a l s u p e r i o r i t y , few noble gas s t u d i e s o f water paleotemperatures have been published. S p e c i f i c a l l y , questions need t o be answered r e l a t i v e 2

1 8


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to p o s s i b l e subsurface changes due t o r e - e q u i l i b r i u m w i t h s o i l gases o r gases generated i n the a q u i f e r such as methane. A l s o , improvements a r e needed i n techniques o f f i e l d c o l l e c t i o n and l a b o r a t o r y analyses.


Chloride The c h l o r i d e content o f groundwater may be a s e n s i t i v e i n d i c a t o r o f e i t h e r the d i s t a n c e between the i n t a k e area o f the a q u i f e r and c o a s t o r t h e amount o f évapotranspiration p r i o r t o groundwater recharge. Because c h l o r i d e i s not normally d e r i v e d from d i s s o l u t i o n o f s o l i d a q u i f e r m a t e r i a l s and i t does not e n t e r i n t o i o n exchange r e a c t i o n s t o any great e x t e n t , the c h l o r i d e content i n shallow a q u i f e r s and a q u i f e r s i s o l a t e d from sources of connate water should r e f l e c t some o f t h e o r i g i n a l e n v i r o n mental f a c t o r s o f the outcrop area [19,86]. W i t h i n about 500 km o f c o a s t a l areas, t h e c h l o r i d e content of p r e c i p i t a t i o n i s s t r o n g l y r e l a t e d t o the p r o x i m i t y o f the shoreline. The ocean-derived c h l o r i d e i n the p r e c i p i t a t i o n may commonly vary from 10 t o 20 mg/L a t the coast t o l e s s than 1 mg/L a t a d i s t a n c e o f 200 km from the coast. P r e c i s e amounts are r e l a t e d c l o s e l y t o c l i m a t o l o g i c a l f a c t o r s such as p r e v a i l i n g winds and t o t a l p r e c i p i t a t i o n . Local v e g e t a t i o n cover and topographic e f f e c t s may a l s o be important, p a r t i c u l a r l y i n c o n t r o l l i n g d r y f a l l o u t o f sea-spray p a r t i c l e s w i t h i n a few k i l i m e t e r s o f the coast. I f most o f t h e c l i m a t o l o g i c a l and topographic f a c t o r s are r e l a t i v e l y constant w i t h time, as may have been t r u e along the G u l f Coast o f Texas, then groundwater which i s a few thousand years o l d may have the e f f e c t s o f a f l u c t u a t i n g c o a s t l i n e pre served i n the form o f bands o f groundwater having d i f f e r e n c e s i n t h e i r c h l o r i d e content. Such appears t o be the case i n the C a r r i z o a q u i f e r o f southern Texas. The c h l o r i d e content o f groundwater i n i n l a n d regions should be more s e n s i t i v e t o c o n c e n t r a t i o n by évapotranspiration than t o f l u c t u a t i o n s o f the p o s i t i o n o f d i s t a n t s h o r e l i n e s . A band o f lower c h l o r i d e water i n the Great A u s t r a l i a n A r t e s i a n Basin has been i n t e r p r e t e d as groundwater o r i g i n a t i n g d u r i n g a p e r i o d o f greater r a i n f a l l and/or l e s s évapotranspiration [ 8 6 ] . T h i s i n t e r p r e t a t i o n i s supported by independent c a l c u l a t i o n s o f the hydrodynamic age o f t h e water which suggest recharge d u r i n g the l a t e s t episode o f g l a c i a t i o n . GEOLOGIC RECONSTRUCTIONS Standard methods o f r e c o n s t r u c t i n g g e o l o g i c h i s t o r y a r e e s s e n t i a l t o check other methods o f water d a t i n g . Geologic h i s t o r y i s h i g h l y s p e c i f i c t o each s i t e o f i n t e r e s t , so u s e f u l g e n e r a l i z a t i o n s a r e d i f f i c u l t t o make. N e v e r t h e l e s s , the impor tance o f general g e o l o g i c reasoning cannot be emphasized too



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strongly. For example, groundwater i n a recharge zone composed of t h e l a t e s t P l e i s t o c e n e outwash cannot be o l d e r than about 20,000 years which would represent the maximum age o f the outwash. Another common s i t u a t i o n would be water which has a chemical i m p r i n t o f some known g e o l o g i c event such as a v o l c a n i c e r u p t i o n o r an i n v a s i o n o f marine water along c o a s t a l areas o f the world [87]. As important as such i n f o r m a t i o n may be, n e v e r t h e l e s s , water ages i n t h e normal sense can r a r e l y be obtained. However, knowing upper o r lower l i m i t s t o water ages i s a check o f great importance on dates obtained by methods which y i e l d s p e c i f i c numbers and appear t o be more p r e c i s e but commonly are not. CURRENT PROBLEMS Three primary problem areas e x i s t i n d a t i n g groundwater. These are: (1) Formulation o f r e a l i s t i c geochemical-hydrodynamic models needed t o i n t e r p r e t data which are generated by f i e l d and l a b o r a t o r y measurements, (2) development o f s e n s i t i v e and a c c u r a t e a n a l y t i c a l methods needed t o measure t r a c e amounts o f v a r i o u s s t a b l e and unstable n u c l i d e s , and ( 3 ) t h e o r e t i c a l and f i e l d o r i e n t e d s t u d i e s t o determine w i t h g r e a t e r accuracy the e x t e n t and d i s t r i b u t i o n o f t h e subsurface production o f r a d i o n u c l i d e s which are commonly assumed t o o r i g i n a t e only i n the atmosphere. Each d a t i n g method r e q u i r e s some type of model t o a i d i n the i n t e r p r e t a t i o n o f the data. The models may vary from s o - c a l l e d conceptual models which a r e u n i v e r s a l l y r e q u i r e d i n g e o l o g i c a l i n v e s t i g a t i o n s t o very i n t r i c a t e , coupled geochemical-hydrodynamic models which are f o r m a l i z e d i n exceedingly complex computer pro grams. I n g e n e r a l , the g e o l o g i c a l l y o r i e n t e d conceptual models are s i t e s p e c i f i c and r e q u i r e e x t e n s i v e f i e l d work by experienced geologists. Such work i s s o p h i s t i c a t e d a t present and w i l l increase i n complexity w i t h t h e continued development o f t h e science as a whole. The development of p u r e l y hydrodynamic models i s q u i t e advanced. These models a r e g e n e r a l l y adequate f o r hydrodynamic d a t i n g o f water samples from sedimentary a q u i f e r s . For fractured igneous and metamorphic aquifers, however, hydrodynamic models which a r e s u f f i c i e n t l y r e a l i s t i c f o r water d a t i n g are not a v a i l a b l e . Coupled geochemical-hydrodynamic models are i n t h e i r infancy. Considerable development work i s needed, although elementary chromatographic-like and m i x i n g - c e l l models have been used w i t h apparent success. The e x t e n t t o which molecular d i f f u s i o n a f f e c t s d a t i n g o f f r a c t u r e d rock has y e t t o be evaluated thoroughly w i t h proper models. Although d i f f u s i o n i s a slow process i n dense c r y s t a l l i n e r o c k s , i t c o u l d s t i l l have an important i n f l u e n c e on dates of very o l d groundwater. With a t m o s p h e r i c a l l y d e r i v e d r a d i o n u c l i d e s , dates o f water a f f e c t e d by t h i s slow d i f f u s i o n should appear too o l d . On the o t h e r hand, d a t i n g o f water based on the accumulation o f helium which d i f f u s e s out o f s o l i d rock i n t o


216


fissures should yield dates which are too young. Thus far, however, helium dates of water from fractured rocks appear to be older than C dates of the same water, results that are opposite to those expected from dates influenced by slow diffusion. Methods of sample preparation and nuclide analyses need to be improved materially i f certain nuclides are to be used as a basis of dating water. Of particular interest is K r which has a low natural abundance and a low specific activity. Because i t has a long half-life, is inert, and is probably produced exclu sively in the atmosphere, i t would be an ideal radionuclide for dating old groundwater. Other radionuclides of possible interest for dating water which might need the development of special analytical methods are Ca and Se. As yet, these radio nuclides have not been reported from analyses of groundwater. Theoretical studies [25,42] have shown that significant amounts of a number of radionuclides usually assumed to be derived only from the atmosphere may actually be produced in the subsurface, largely through interactions with secondary neutrons produced by alpha capture reactions. The alpha particles are derived mostly from normal decay of natural U and Th. Whether or not subsurface production of radionuclides can indeed influence dating has yet to be demonstrated by field and laboratory tests. The matter needs further study, particularly in relation to C dating of water which is more than 40,000 years old. 14


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Eugene S. Simpson, Glenn M. Thompson, Anthony Muller, Richard Zito, Juan Carlos Lerman and other associates at the University of Arizona have been most generous with their time and ideas. Many of the researchers cited in our review have also contributed to our study in ways too numerous to mention, to these and especially to Professor Hans Oeschger and Dr. David Elmore we owe our gratitude. The present study was funded by U. S. Nuclear Regulatory Commission Contract NRC-04-78-272. References [1] Carlston, C. W., Thatcher, L. L., Rhodehamel, E. C., Tritium as a hydrologic tool, the Wharton Tract study, Internat. Assoc. Sci. Hydrol. Publ. No. 52, 503-512 (1960). [2] Nelson, R. W., Reisenauer, Α. Ε., Application of radioactive tracers in scientific groundwater hydrology, Radioisotopes in Hydrology, Tokyo Symposium 1963, p. 207-230, Inter. Atomic Energy Agency, Vienna. [3] Theis, C. V., Hydrologic phenomena affecting the use of tracers in timing groundwater flow, Radioisotopes in Hydrol ogy, Tokyo Symposium 1963, p. 193-206, Inter. Atomic Energy Agency, Vienna.



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[4] Fritz, P., Barker, J. F., Gale, J. Ε., Geochemistry and iso tope hydrology of groundwater in the Stripa granite, results and preliminary interpretation, Lawrence Berkeley Laboratory Report LBL-8285, Berkeley, California, 135 p., 1979. [5] Hobba, W. Α., Jr., Fisher, D. W., Pearson, F. J., J r . , Chemerys, J. C., Hydrology and geochemistry of thermal springs of the Appalachians, U. S. Geol. Survey Prof. Paper 1044E, 36 p., 1979. [6] Osmond, J. Κ., Kaufman, M. I., Cowart, J. Β., Mixing volume calculations, sources and aging trends of Floridan aquifer water by uranium isotopic methods, Geochim. et Cosmochim. Acta, 38, 1083-1100 (1974). [7] Tóth, J., Gravity-induced cross-formational flow of forma tion fluids, red earth region, Alberta Canada: Analysis, patterns, and evolution, Water Resour. Res., 14(5), 805-844 (1978). [8] Kafri, U., Arad, Α., Paleohydrology and migration of the groundwater divide in regions of tectonic instability in Israel, Geol. Soc. of America Bull., 89, 1723-1732 (1978). [9] Lal, D., Peters, Β., Cosmic-ray produced isotopes and their applications to problems in geophysics, In: Progress in Elementary Particle and Cosmic Ray Physics, North Holland Publishing Co., Amsterdam, Vol. 6, p. 1-74, 1962. [10] Oeschger, Η., Some cosmic ray produced radionuclides of interest in dating old groundwater, In: Davis, S. Ν., ed., Workshop on dating old ground water, Dept. Hydrology and Water Resources, University of Arizona, report on contract to Union Carbide Corp. in Oak Ridge (Y/OWI/SUB-78/55412), p. 129, 1978. [11] Dansgaard, W., Clausen, Η. Β., Aarkrog, Α., Evidence for bomb-produced silicon-32, J. Geophys. Res., 71(22), 5474-5477 (1966). [12] Davis, S. Ν., ed., Workshop on dating old ground water, Subcontract 19Y-55412v, Report to Union Carbide Corp., Nuclear Division by Dept. of Hydrology and Water Resources, University of Arizona, Tucson, 138 p., 1978. [13] Freeze, R. Α., Cherry, J. Α., Groundwater, Prentice-Hall, Inc., Englewood C l i f f s , NJ, p. 134-139, 290-295, 1979. [14] Gaspar, Ε., Onescu, Μ., Radioactive tracers in hydrology, Amsterdam, Elsevier Publishing Co., 342 p., 1972. [15] Isotope Hydrology Section, International Atomic Energy Agency, Nuclear techniques in ground-water hydrology, In: Ground-water studies, UNESCO, Paris, Sections 10.1-10.4, 38 p., 1973. [16] Plata Bedmar, Α., Isotopos en Hidrología, Editorial Alhambra, S. Α., Madrid, 328 p., 1972. [17] Oeschger, Η., Houtermans, J., Loosli, Η., Wahlen, Μ., The constancy of cosmic radiation from isotope studies in meteorites and on the Earth, Nobel Symposium, Vol. 12, p. 471-498, 1970.


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[18] Münnich, K. O., Messungen des C Gehaltes vom hartem Grundwasser, Naturwiss., 44, 32 (1957). [19] Bath, Α. Η., Edmunds, W. Μ., Andrews, J. Ν., Palaeoclimatic trends deduced from the hydrochemistry of a Triassic sand stone aquifer, United Kingdom, In: Isotope Hydrology 1978, Internat. Atomic Energy Agency, Vienna, Vol. 2, p. 545-568, 1979. [20] Bergstrom, R. Ε., Aten, R. Ε., Natural recharge and locali zation of fresh water in Kuwait, J. Hydrology, 2(3), 213231 (1965). [21] Calf, G. Ε., The isotope hydrology of the Mereenie Sandstone aquifer, Alice Springs, Northern Territory, Australia, J. Hydrology, 38, 343-355 (1978). [22] Grove, D. Β., Rubin, Μ., Hanshaw, Β. Β., Beetem, W. Α., Carbon-14 dates of ground water from a Paleozoic carbonate aquifer, southcentral Nevada, U. S. Geol. Survey Prof. Paper 650-C, p. 215-218, 1969. [23] Pearson, F. J., Jr., White, D. Ε., Carbon-14 ages and flow rates of water in Carrizo Sand, Atascosa County, Texas, Water Resources Research, 3(1), 251-261 (1967). [24] Winograd, I., and Farlekas, Problems in C dating of water from aquifers of deltaic origin, Internat. Atomic Energy Agency, Vienna, Isotope Techniques in Groundwater Hydrology, Vol. II, p. 69-93, 1974. [25] Zito, R., Donahue, D. J., Davis, S. Ν., Bentley, H. W., Fritz, P., Possible subsurface production of carbon-14, Geophys. Research Lett., 7(4), 235-238 (1980). [26] Rightmire, C. T., Hanshaw, Β. Β., Relationship between the carbon isotope composition of soil CO and dissolved carbo nate species in groundwater, Water Resour. Research, 9(4), 958-567 (1973). [27] Fontes, J. -C., Garnier, J. Μ., Determination of the initial C activity of the total dissolved carbon, A review of the existing models and a new approach, Water Resour. Research, 15(2), 399-413 (1979). [28] Libby, W. F., Tritium Geophysics, J. Geophys. Research, 66, 3767-3782 (1961). [29] Ehhalt, D., On the uptake of tritium by soil water and groundwater, Water Resour. Research, 9(4), 1073-1074 (1973). [30] Hufen, T. H., Buddemeier, R. W., Lau, L. S., Isotopic and chemical characteristics of high-level groundwaters on Oahu, Hawaii, Water Resour. Research, 10, 366-370 (1974). [31] Poland, J. F., Stewart, G. T., New tritium data on movement of groundwater in western Fresno County, California, Water Resour. Research,11,716-724 (1975). [32] Allison, G. Β., Hughes, M. W., The use of environmental tritium to estimate recharge to a South-Australian aquifer, J. Hydrology, 26(3)(4), 245-254 (1975). 14

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[33] Dincer, T., Al-Mugrin, Α., Zimmermann, U., Study of the infiltration and recharge through the sand dunes in arid zones with special reference to the stable isotopes and thermonuclear tritium, J. Hydrology, 23, 79-109 (1974). [34] Smith, D. Β., Wearn, P. L., Richards, H. J., Rowe, P. C., Water movement in the unsaturated zone of high and low permeability strata by measuring natural lithium, In: Isotope Hydrology 1970, Vienna, Interna. Atomic Energy Assoc., p. 73-87, 1970. [35] Vogel, J. C., Thilo, L., Van Dijken, Μ., Determination of groundwater recharge with tritium, J. Hydrology, 23, 131-140 (1974). [36] Tolstikhin, I. Ν., Kamenskii, I. L., Determination of ground water ages by the T- He method, Geochem. Int., 6, 810-811 (1969). [37] Torgersen, T., Clarke, W. Β., Jenkins, W. J., The tritium/ helium-3 method in hydrology, In: Isotope Hydrology 1978, Interna. Atomic Energy Agency, Vienna, Vol. 2, p. 917-929, 1979. [38] Zito, R., Davis, S. Ν., 1980 Subsurface production of the mirror isotopes H and He, unpublished manuscript, University of Arizona, Department of Hydrology and Water Resources, 24 p. [39] Schaeffer, O. Α., Thompson, S. O., Lark, N. L., Chlorine-36 radioactivity in rain, J. Geophys. Research, 65, 4013-4016 (1960). [40] Davis, S. Ν., DeWeist, R. J. Μ., Hydrogeology, John Wiley & Sons, New York, 463 p., 1966. [41] Tamers, Μ. Α., Ronzani, C., Scharpenseel, H. W., Naturally occurring chlorine-36, Atompraxis, 15, 433-437 (1969). [42] Bentley, H. W., Some comments on the use of chlorine-36 for dating very old ground water, In: Workshop on dating old ground water, S. N. Davis, ed., Subcontract 19Y-55412v, report to Union Carbide Corp., Nuclear Division, by Department of Hydrology and Water Resources, University of Arizona, 138 p., 1978. [43] Elmore, D., Fulton, B. R., Clover, M. R., Marsden, J. R., Gove, Η. Ε., Naylor, Η., Purser, Κ. Η., Kilius, L. R., Beukens, R. P., Litherland, Α. Ε., Analysis of 36Cl in environmental water samples using an electrostatic accelerator, Nature, 277, 22-25 (1979). [44] Lal, D., Peters, Β., Cosmic ray produced radioactivity on the earth, Handbuch der Physik, XLVI/2, 551-612 (1967). [45] Eriksson, Ε., The yearly circulation of chloride and sulfur in nature, meteorological, geochemical, and pedological implications, Part II, Tell us, 12(1), 63-109 (1959). [46] Bentley, H. W., Davis, S. Ν., Feasibility of Cl-dating of very old ground water, EOS, American Geophysical Union Transactions, 61(17), 230 (1980). 3

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[47] Kater, R., Development of a method for measuring natural Si activities and aspects of its use in hydrogeological researches, Neue Bergbautechnik, 5, 941-943 (1975). [48] Lal, D., Nijampurkar, V. Ν., Rama, S., Silicon-32 hydrology, Isotope Hydrology 1970, Inter. Atomic Energy Assoc., Vienna, p. 847-868 (1970). [49] Gupta, S. Κ., Lal, D., Silicon-32, In: Workshop on dating old ground water, S. Davis, ed., Subcontract 19Y-55412v, report to Union Carbide Corp., Nuclear Division, by Depart ment Hydrology and Water Resources, University of Arizona, Tucson, 131-138, 1978. [50] Elmore, D., Anantaraman, Ν., Fulbright, H. W., Gove, Η. Ε., Hans, H. S., Nishiizumi, Κ., Murrell, M. T., Honda, Μ., Half-life of Si using tandem accelerator mass spectrometry, Nuclear Structure Research Laboratory, University of Rochester, NY, Publication UR-NSRL-220, 11 p., 1980. [51] Kutschera, W., Henning, W., Paul, Μ., Smither, R. Κ., Stephenson, E. J., Yntema, J. L., Alburger, D. Ε., Cumming, J. Β., Harbottle, G., Physical Review Lett., 45(8), 592-593 (1980). [52] Lovering, T. S., Significance of accumulator plants in rock weathering, Geol. Soc. America Bull., 70, 781-800 (1959). [53] Riquier, J., Les phytolithes de certains sols tropicaux el des podzols, Trans. 7th Internat. Congress Soil Sci., Madison, WI, Vol. 4, 1960. [54] Oeschger, Η., Gugelmann, L. Η., Schotterer, U., Siegenfhaler, U., Wiest, A., Ar dating of groundwater, In: Isotope Tech niques in Groundwater Hydrology 1974, Inter. Atomic Energy Agency, Vienna, p. 179-189, 1974. [55] Loosli, Η. Η., Oeschger, Η., Argon-39, carbon-14 and krypton85 measurements in groundwater samples, In: Isotope Hydrol ogy 1978, Internat. Atomic Energy Agency, Vienna, Vol. 2, p. 931-945, 1978. [56] Rózański, Κ., Florkowski, T., Krypton-85 dating of ground water, In: Isotope Hydrology 1978, Internat. Atomic Energy Agency, Vienna, Vol. 2, p. 949-959, 1979.


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[57] National Council on Radiation Protection and Measurements, Krypton-85 in the atmosphere--accumulation, biological significance, and control technology, Nat. Council Rad. Protection and Meas., Washington, D.C., NCRP Report No. 44, 79 p., 1975. [58] Spiridonov, A. I., Sultankhodzhayer, Α. Ν., Beder, Β. Α., Taneyev, Α. Ν., Tyminskiy, V. G., Some problems in the computation of the age of ground waters, Soviet Hydrology, selected papers, Issue No. 3, p. 265-267, 1973. [59] Teitsma, Α., Clarke, W. Β., Fission xenon isotope dating, J. Geophys. Res., 83, 5443-5453 (1978). [60] Marine, I. W., Geochemistry of ground water at the Savannah River Plant, Savannah River Laboratory Report DP-1356, Aiken, South Carolina, 102 p., 1976.



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[61] Heaton, T. H. E., Vogel, J. C., Gas concentrations and ages of groundwaters in Beaufort Group Sediments, South Africa, Water S. Α., (South Africa), 5(4), 160-170 (1979). [62] Naughton, J. J., Lee, J. Η., Keeling, D., Finlayson, J. Β., Dority, Helium flux from the earth's mantle as estimated from Hawaiian fumarolic degassing, Science, 180, 55-57 (1973). [63] Torgersen, T., Clarke, W. Β., Excess helium-4 in Teggar Lake, Possibilities for a uranium ore body, Science, 199, 769-771 (1978). [64] Rosholt, J. Ν., Doe, B. R., Tatsumoto, Μ., Evolution of the isotopic composition of uranium and thorium in soil profiles, Geol. Soc. America Bull., 77, 987-1004 (1966). [65] Fleischer, R. L., Isotopic disequilibrium of uranium, alpharecoil damage and preferential solution effects, Science, 207, 979-981 (1980). [66] Cowart, J. Β., Osmond, J. Κ., Oxidation/reduction in the Edwards Limestone aquifer as indicated by dissolved uranium isotopes, Geol. Soc. America, Abstracts with Programs, 9(7), 938 (1977). [67] Airey, P. L., Australian Atomic Energy Research Establish ment, personal communication, 1980. [68] Harmon, R. S., Lively, R. S., Th/ U dating of Quaternary uranium deposits, Geol. Soc. America, Abstracts with Programs, 9, 1005 (1977). [69] Lively, R. S., Alexander, E. C., Jr., Th- U ages of speleothems from Mystery Cave, Minnesota; Abstract, National Speleological Society, Quarterly Journal, 42(2), 34 (1980). [70] Thompson, P., Schwarcz, H. P., Ford, D. C., Continental Pleistocene climatic variations from speleothem age and isotopic data, Science, 184, 893-895 (1974). [71] Thompson, G. Μ., Lumsden, D. Ν., Walker, R. L., Carter, J. A., Uranium series dating of stalagmites from Blanchard Springs Caverns, USA, Geochimica et Cosmochimica Acta, 39, 1211-1218 (1975). [72] Bada, J. L., Shou, Μ., Effects of various environmental parameters on amino acid racemization rates in fossil bones, Geological Soc. America, Abstracts with Programs, 8, 762-763 (1976). [73] Hare, P. Ε., Amino acid dating, limitations, and potential, Geological Soc. America, Abstracts with Programs, 9, 10041005 (1977). [74] Davis, S. Ν., Thompson, G. Μ., Bentley, H. W., Stiles, G. Κ., Ground-water tracers--a short review, Ground Water, 18(1), 14-23 (1980). [75] Thompson, G. Μ., Hayes, J. Μ., Trichlorofluoromethane in groundwater--A possible tracer and indicator of groundwater age, Water Resour Research, 15(3), 546-554 (1979). 230

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[76] Hayes, J. Μ., Davis, S. Ν., Chlorofluoromethanes in ground water, unpublished research proposal to U. S. Office Water Research and Tech., Indiana University, 19 p., February 21, 1975. [77] Randall, J. H., Schultz, T. R., Davis, S. Ν., Suitability of fluorocarbons as tracers in ground water resource evaluation, unpublished Project Completion Report OWRT No. A-063, University of Arizona, 37 p., November, 1977. [78] Gat, J. R., Comments on the stable isotope method in regional groundwater investigations, Water Resour. Research, 7, 980-993 (1971). [79] Sonntag, C., Klitzsch, Ε., Löhnert, E. P., El-Shazly, Ε. Μ., Münich, K. O., Junghans, Ch., Thorweike, U., Weistroffer, K., Swailem, F. Μ., Palaeoclimatic information from deuterium and oxygen-18 in carbon-14 dated north Saharian groundwaters, In: Isotope Hydrology 1978, Inter. Atomic Energy Agency, Vienna, Vol. 2, p. 569-580, 1979. [80] Dansgaard, W., Stable isotopes in precipitations, Tellus, 16 436-468 (1964). [81] Dansgaard, W., Johnsen, S. J., Møller, J., Langway, C. C., One thousand centuries of climatic record from Camp Century on the Greenland ice sheet, Science, 166, 377-381 (1969). [82] Hanshaw, Β. Β., verbal presentation, University of Arizona, 1978. [83] Sugisaki, R., Measurement of effective flow velocity of ground water by means of dissolved gases, American J. Sci., 259, 144-153 (1961). [84] Andrews, J. Η., Lee, D. J., Inert gases in groundwater from the Bunter Sandstone of England as indicators of age and paleoclimatic trends, J. Hydrology, 41, 233-252 (1979). [85] Mazor, Ε., Paleotemperatures and other hydrological param eters deduced from noble gases dissolved in groundwaters, Jordan Rift Valley, Israel, Geochim. Cosmochim. Acta, 36, 1321- (1972). [86] Airey, P. L., Calf, G. Ε., Campbell, B. L., Hartley, P. Ε., Roman, D., Habermehl, Μ. Α., Aspects of the isotope hydrol ogy of the Great Artesian Basin, Australia, In: Isotope Hydrology 1978, Interna. Atomic Energy Agency, Vienna, Vol. 1, p. 205-217, 1979. [87] Caswell, W. Β., Maine's ground-water situation, Groundwater, 17(3), 235-243 (1979). RECEIVED

July

23,

1981.


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