12 The Use of Natural Pb-210 as a Heavy Metal Tracer
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in the River-Estuarine System LARRY K. BENNINGER, DALE M. LEWIS, and KARL K. TUREKIAN Department of Geology and Geophysics, Yale University, New Haven, Conn. 06520
The naturally occurring radioactive isotope of lead, Pb-210, provides a valuable tracer for the behavior of heavy metals in the soil-stream-estuary system. Since i t is continuously produced only as a member of the U-238 decay series, i t is free from the problems of environmental or analytical contamination so often encountered in stable heavy metal studies. In addition, because of its half-life of about 22 years i t is useful not only as a tracer but also as a dating tool to monitor events of the past 100 years in various repositories. Lead-210 can be supplied to the soi 1-stream-estuary system through two pathways: (1) atmospheric--Rn-222 released from soils to the atmosphere decays to Pb-210 which then follows the fate of aerosols, ultimately to be returned to the Earth's surface via atmospheric precipitation; (2) terrigenous--Ra-226 in soils, rocks, streams and groundwater generates Pb-210 which is in some degree subject to mobilization. By following the pathways of Pb-210 from both these sources we can make predictions about the expected behavior of common lead at the Earth's surface. Further, by analogy with lead we can get an idea of the behavior of other trace metals in the Earth's aqueous reservoirs. The Fate of the Atmospherically-Supplied Pb-210 Precipitation, as rain, snow or dry fallout, continually strips the atmosphere of aerosols, including the Pb-210 produced there from Rn-222 decay. It has been estimated that tropospheric aerosols associated with Pb-210 have a mean residence time of a week or less (1). As Pb-210 is deposited i t can fall either on standing bodies of water, the ocean or lakes, or on land. There is sufficient evidence now to indicate that in lakes i t is rapidly removed from the dissolved state to the lake sediments (2, 3^ l i l t is also being removed from the ocean surface. We neecT to know the fate of Pb-210 (and presumably other metals) when i t encounters soil. One of the ways of determining this is to see how the atmospherically-derived Pb-210 is distributed in the soil profile 202
Church; Marine Chemistry in the Coastal Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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12.
BENNINGER E T A L .
Pb-210 as a Heavy Metal Tracer
203
and how w e l l the t o t a l i n v e n t o r y of a t m o s p h e r i c a l l y - d e r i v e d Pb-210 i n the s o i l p r o f i l e r e f l e c t s the expected f l u x . I f the s o i l c o n t a i n s a l l the Pb-210 t h a t i s d e l i v e r e d t o i t by p r e c i p i t a t i o n , we can be sure t h a t the p r o p e r t i e s o f the s o i l s make them e f f i c i e n t scavengers f o r l e a d and o t h e r t r a c e metals s i m i l a r t o lead i n chemical behavior. Continuous c o l l e c t i o n o f p r e c i p i t a t i o n and dry f a l l o u t and monthly a n a l y s i s f o r Pb-210 content have been c a r r i e d out a t the K l i n e Geology Laboratory a t Y a l e f o r the past y e a r - a n d - a - h a l f . These i n d i c a t e a f l u x o f 1 dpm Pb-210/cm /y i n the New Haven a r e a . I f we assume t h a t t h i s i s an adequate measure o f the Pb210 f l u x f o r the northeastern United S t a t e s , then by assaying s o i l i n v e n t o r i e s we can a r r i v e a t the extent of m o b i l i z a t i o n o f Pb-210 from the s o i l . I f a s o i l i s t o be used i n such a study i t must be i n steady s t a t e w i t h r e s p e c t t o the i n p u t o f Pb-210 from atmospheric s o u r c e s ; t h i s r e q u i r e s t h a t the s o i l p r o f i l e be undisturbed by man s a c t i v i t i e s f o r 5 h a l f - l i v e s o f Pb-210 o r around 100 y e a r s . Then the f l u x o f a t m o s p h e r i c a l l y - d e r i v e d Pb-210 to the s o i l i s given by the standing crop o f excess Pb-210 d i v i d e d by the mean l i f e of Pb-210, i f there i s no l o s s from the s o i l p r o f i l e by leaching. Several s o i l p r o f i l e s from the Northeast have now been analysed t o t e s t t h i s . We have s t u d i e d the Cook F o r e s t S t a t e Park i n north c e n t r a l P e n n s y l v a n i a , where there i s a v i r g i n stand o f timber up t o 300 years o l d , and two s i t e s i n c o a s t a l C o n n e c t i c u t , the Farm R i v e r s a l t marsh and an a s s o c i a t e d upland i s l a n d . In a d d i t i o n to our d a t a , Fisenne (5) has reported Ra-226 and Pb-210 data f o r an undisturbed s o i T p r o f i l e from a tobacco-growing r e g i o n of Maryland. Table I shows the i n t e g r a t e d f l u x t o these sample s i t e s compared t o the r a i n f a l l d a t a . W i t h i n the u n c e r t a i n t i e s , s o i l s and s a l t marshes r e t a i n v i r t u a l l y a l l o f the Pb-210 s u p p l i e d from the atmosphere. 2
1
Table I Flux o f Pb-210 to the Eastern U.S.A. location
flux dpm/cnr/yr
New Haven, Conn.
1
p r e c i p . & dry f a l l o u t
East Haven, Conn.
1
s a l t marsh p r o f i l e (6)
East Haven, Conn.
0.8
Cook Forest S t . P a r k , Pa. Maryland soil.
1 1.2
type o f measurement
s o i l p r o f i l e (6) soil
profile
s o i l p r o f i l e (5)
The Pb-210 i s a s s o c i a t e d w i t h the o r g a n i c f r a c t i o n of the T h i s m a t e r i a l has the c a p a c i t y to sequester o t h e r metals
Church; Marine Chemistry in the Coastal Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
204
MARINE CHEMISTRY
as w e l l , as the data from a s o i l p r o f i l e o f the Hubbard Brook Experimental F o r e s t i n New Hampshire c l e a r l y show (Table I I ) . Thus we conclude t h a t metal s s u p p l i e d t o a s o i l p r o f i l e have a low m o b i l i t y i n the d i s s o l v e d phase probably because o f the strong sequestering properties of s o i l organics.
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Table
II
Hubbard Brook, N.H. s o i l p r o f i l e (7) soil horizon A
l
Cu PPm
Zn PPm
Cd PPm
Pb PPm
280
56
88
11
802
73
1.5
0.9
1.1
2.1
0
0
A,
organic content
Pb-210 excess dpm/gm
Lead-210 i n Ground Waters There a r e three p o s s i b l e sources o f Pb-210 i n ground water: (1) A t m o s p h e r i c a l l y - d e r i v e d Pb-210 which i n f i l t r a t e s the s o i l and e n t e r s the ground water r e s e r v o i r s . We know from the s o i l data t h a t t h i s cannot be a l a r g e f r a c t i o n o f the amount s u p p l i e d t o the s o i l . In most o f the e a s t e r n United S t a t e s the mean annual p r e c i p i t a t i o n i s ^100 cm. However the process o f é v a p o t r a n s p i r a t i o n r e t u r n s about h a l f t h i s water t o the atmosphere, r e s u l t i n g i n a net a d d i t i o n o f 50 cm o f water per y e a r o r about 0.05 A/cm /y. S i n c e the f l u x o f Pb-210 i s 1 dpm/cm2/yr, the c o n c e n t r a t i o n o f Pb-210 i n a n n u a l l y charged a q u i f e r s would then be about 20 dpm/ί, i f no removal occurred d u r i n g i n f i l t r a t i o n . (2) Ra-226 and Rn-222 i n s o l u t i o n i n the ground w a t e r , d e r i v e d from l e a c h i n g o r r e c o i l from r o c k s . Ra-226 i n ground water ranges from 0.2 dpm/jt t o 40 dpm/fc but i n general i s around 10 dpm/i ( 8 ) . (3) Pb-210 d i r e c t l y leached from rock and s o i l during weathering. Holtzman's (8) study o f w e l l waters i n I l l i n o i s shows t h a t the a c t i v i t y o f Pb-210 i s always lower than the Ra-226 a c t i v i t y . The s h a l l o w w e l l s c o n t a i n low amounts o f Ra-226 from rock weathering and the h i g h e s t Pb-210 c o n c e n t r a t i o n s o f the ground waters a n a l y s e d . Consequently, they most d i r e c t l y r e f l e c t the annual p r e c i p i t a t i o n . However, the h i g h e s t value i n these s h a l l o w a q u i f e r s , 0.4 dpm/fc, i s s t i l l much lower than the value expected i n the Northeast from p r e c i p i t a t i o n (20 dpm/fc). Thus, l e s s than 2% o f the Pb-210 s u p p l i e d to a t e r r a i n from the r a i n i s t r a n s m i t t e d t o ground water. I f we c o n s i d e r the deep w e l l s analyzed by Holtzman (8) as tapping a q u i f e r s i n which the water has a r e s i d e n c e time o f about 100 y e a r s , we note t h a t the a c t i v i t y r a t i o s o f Pb-210/Ra-226 a r e so low as t o p r e d i c t a mean r e s i d e n c e time f o r Pb-210 o f about 2
Church; Marine Chemistry in the Coastal Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
12. BENNiNGER E T A L .
Pb-210 as a Heavy Metal Tracer
205
one month (with d e v i a t i o n s by a f a c t o r o f ten around t h i s v a l u e ) . Such a s h o r t residence time i n dominantly limestone and sandstone a q u i f e r s i m p l i e s t h a t any heavy metals i n j e c t e d i n t o the ground water system w i l l be adsorbed on the rocks c l o s e t o the p o i n t o f i n j e c t i o n , and very l i t t l e can be s u p p l i e d t o the streams d r a i n ing the adjacent ground water r e s e r v o i r .
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The Fate of Pb-210 i n Streams The p r i n c i p a l sources o f d i s s o l v e d Pb-210 t o streams are d i r e c t p r e c i p i t a t i o n onto the water s u r f a c e and s u r f a c e r u n o f f , although some t e r r i g e n o u s Pb-210 i s s u p p l i e d by ground water d i s c h a r g e . In mining areas a c i d mine waters supply Pb-210 as w e l l . What has been learned about the behavior o f Pb-210 i n normal s o i l p r o f i l e s and i n ground water i n d i c a t e s t h a t very l i t t l e Pb-210 can reach the streams i n the d i s s o l v e d s t a t e under normal weathering c o n d i t i o n s . However, the i n j e c t i o n o f Pb-210 i n t o streams by a c i d mine waters does provide a t e s t o f the behavior o f s o l u b l e Pb-210 i n a normal stream system. The f i r s t study o f Pb-210 i n r i v e r waters impacted by mine drainage was c a r r i e d out on the Colorado R i v e r system ( 9 ) . The source waters were as high as 14 dpm Pb-210/fc because o 7 l e a c h i n g of t a i l i n g s from uranium mining operations i n t h i s s e m i - a r i d r e g i o n . As the r i v e r continued downstream t h i s high value was found to be reduced t o 0.29 dpm/fc. T h i s i n d i c a t e s t h a t processes w i t h i n the Colorado R i v e r remove Pb-210 onto p a r t i c l e s . A d e t a i l e d Pb-210 study of the Susquehanna R i v e r system i n the northeastern U.S. has been done. The West Branch of the Susquehanna R i v e r i s massively a f f e c t e d by mine drainage from coal mining and i s high i n d i s s o l v e d (
