Environmental Fate and Transport at the Terrestrial-Atmospheric

10

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Environmental Fate and Transport at the Terrestrial-Atmospheric Interface DAVID C. BOMBERGER, JULIA L. GWINN, WILLIAM R. MABEY, DANIEL TUSE, and TSONG WEN CHOU SRI International, Menlo Park, CA 94025

Simple models are used to i d e n t i f y the dominant f a t e or transport path of a m a t e r i a l near the terrestrial -atmospheric interface. The models are based on p a r t i t i o n i n g and f u g a c i t y concepts as w e l l as first -order transformation kinetics and second-order transport k i n e t i c s . Along with a c o n s i d e r a t i o n of the chemical and b i o l o g i c a l transformations, t h i s approach determines if the m a t e r i a l i s l i k e l y to v o l a t i l i z e r a p i d l y , leach downward, or move up and down i n the soil profile in response to precipitation and evapotranspiration. This determination can be u s e f u l f o r p r e l i m i n a r y r i s k assessments or f o r choosing the appropriate more complete terrestrial and atmospheric models f o r a study of environmental f a t e . The models are i l l u s t r a t e d using a set of p e s t i c i d e s with widely d i f f e r e n t behavior patterns. Organic m a t e r i a l s , such as p e s t i c i d e s or wastes placed on or near the s o i l surface, can undergo various f a t e s . They can o x i d i z e , hydrolyze, photolyze, v o l a t i l i z e , or biodegrade. They can be c a r r i e d o f f the land surface i n t o nearby streams by s o i l e r o s i o n or they can be leached i n t o the s o i l by p r e c i p i t a t i o n or irrigation. In a d d i t i o n , there are i n t e r a c t i o n s among the various f a t e processes; f o r example, leaching i n t o the s o i l retards volatilization. A l s o , every compound behaves d i f f e r e n t l y . In assessing the o v e r a l l impact that a compound might have on the environment, i t i s necessary to i d e n t i f y the important f a t e processes and quantify t h e i r e f f e c t s and i n t e r a c t i o n s . T h i s can be done at s e v e r a l l e v e l s of completeness. One approach i s to use simple screening estimates to determine what i s expected to be the one or two dominant f a t e processes. On the other hand, a l l f a t e processes can be studied i n great d e t a i l , and laboratory or a c t u a l f i e l d experiments may be used to gain a f u l l understanding of what happens to a p a r t i c u l a r compound.

0097-6156/83/0225-0197$06.00/0 © 1983 American Chemical Society

Swann and Eschenroeder; Fate of Chemicals in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


198

FATE OF CHEMICALS IN THE ENVIRONMENT

The most economical route i s probably to use screening studies to determine the dominant f a t e processes and then study only those i n d e t a i l . In t h i s paper we review some simple screening techniques that can be used to quantify v o l a t i l i z a t i o n and leaching rates at the s o i l / a i r i n t e r f a c e . V o l a t i l i z a t i o n and leaching rates are then compared with estimates of transformation rates to determine the compound's o v e r a l l f a t e and i d e n t i f y the process r e q u i r i n g f u r t h e r study i f a more exact f a t e assessment i s required. Method Soil Diffusion. Water-soluble m a t e r i a l i n the s o i l includes m a t e r i a l d i s s o l v e d i n the s o i l water, m a t e r i a l d i s s o l v e d i n the s o i l a i r , and m a t e r i a l adsorbed to the s o i l s o l i d s . The s o i l w a t e r - s o i l a i r e q u i l i b r i u m p a r t i t i o n i n g i s described by Henry's law: C. = HC A w

(1) '

v

o

when C^ i s the gas phase concentration ( m o l e / l i t e r or g/cm ) and C i s the l i q u i d phase concentration i n the same u n i t s . H i s the Henry's law constant and i s u n i t l e s s . w

The e q u i l i b r i u m p a r t i t i o n i n g between s o l i d s i s described by:

soil

water

and

C

= K F C s oc oc w C = K C s p w

v

soil

(2)

'

where C i s the concentration adsorbed .on the s o i l (g/g or mole/g), K i s the p a r t i t i o n c o e f f i c i e n t on s o i l organic carbon, and F i s the f r a c t i o n of the s o i l s o l i d that i s organic. Then, i f we define: g

Q C

V

=

s o i l volume (cm )

0 A

=

volume f r a c t i o n of s o i l that i s a i r

6 w

=

volume f r a c t i o n of s o i l that i s water

d

=

s p e c i f i c g r a v i t y of s o i l s o l i d s (g/cm )

o

s o i l bulk density,


10.

BOMBERGER E T A L .

Terrestrial-Atmospheric Interface

the t o t a l amount of m a t e r i a l i n the s o i l Total material

=

V9 C AA A

A

V 9C „

+

H


K F C )° ^ C

A

H

d

(3) V

/

pK F

we define a as:

|

r\ A

The

can be expressed as:

+ vfl-9 -9 A w

9

For convenience,

199

=

A

9 W ,

~~if

pK F OC O C

—

g

/1 \

—

4

d i s t r i b u t i o n between the three phases can then be described by \^ = F r a c t i o n compound i n a i r phase = \ \

9^/a

= F r a c t i o n compound i n water phase = g

^

w

= F r a c t i o n compound i n s o l i d phase = 1 -

(5)

^

a

\^ -

\^

(7)

Goring ( 1 ) showed that f o r many compounds, d i f f u s i o n i n the s o i l occurs p r i m a r i l y i n only one of the three s o i l phases. F o r v o l a t i l e compounds ( H > 1 0 ), d i f f u s i o n was claimed to occur i n the a i r phase. For n o n v o l a t i l e compounds ( H < 3 x 10~ ) d i f f u s i o n , i f i t i s important, occurs i n the water phase. V o l a t i l e Compounds. For v o l a t i l e chemicals, Goring (_1) showed that diffusion i n the s o i l could be described mathematically as though i t occurred only i n the a i r phase.

at

-

=

s o i l ~~T oz

8

wjjere z i s the coordinate d i r e c t i o n normal to the s o i l s u r f a c e . D was defined as: soil

D

i o i i - —r

9

A where D i s the d i f f u s i o n c o e f f i c i e n t of the chemical i n f r e e a i r and T^ i s a c o r r e c t i o n f o r the p o r o s i t y of the s o i l . The f a c t o r a c o r r e c t s f o r the f a c t that the chemical i s adsorbed on the s o i l s o l i d s and d i s s o l v e d i n the s o i l water. As a chemical


200

FATE OF

CHEMICALS IN THE

ENVIRONMENT

d i f f u s e s i n the s o i l a i r , i t i s assumed to be replaced immediately by m a t e r i a l from the s o l i d and water phases. This has the e f f e c t of slowing the development of a concentration p r o f i l e i n the s o i l a i r , which, i n turn, retards the o v e r a l l rate at which the chemical i s transported by d i f f u s i o n . T^ i s given by Hamaker (2): ,10/3


(10) (9. + 9 ) A w Several derivation:

important

assumptions

2

are

required

for

the

No transport by water movement occurs. D i f f u s i o n c o e f f i c i e n t i s a constant. P o r o s i t y c o r r e c t i o n i s constant i n both time and space. Chemicals move between the three s o i l phases much more r a p i d l y than they d i f f u s e i n the a i r phase. This means that they appear to be i n e q u i l i b r i u m . Adsorption i s r e v e r s i b l e . The l a s t two assumptions are the most c r i t i c a l and are probably v i o l a t e d under f i e l d c o n d i t i o n s . Smith et a l . (3) found that at l e a s t a half-hour was required to achieve adsorption e q u i l i b r i u m between a chemical i n the s o i l water and on the s o i l s o l i d s . S o l u t i o n of the d i f f u s i o n equation has shown that many v o l a t i l e compounds have t h e o r e t i c a l d i f f u s i o n h a l f - l i v e s i n the s o i l of s e v e r a l hours. Under a c t u a l f i e l d c o n d i t i o n s , the time required to achieve adsorption e q u i l i b r i u m w i l l r e t a r d d i f f u s i o n , and d i f f u s i o n h a l f - l i v e s i n the s o i l w i l l be longer than p r e d i c t e d . Numerous studies have reported m a t e r i a l bound i r r e v e r s i b l y to s o i l s , which would cause apparent d i f f u s i o n h a l f - l i v e s i n the f i e l d to be longer than p r e d i c t e d . There i s another underlying assumption that i s also very important—that the s o i l stays wet. I t i s w e l l e s t a b l i s h e d that organics bind much more s t r o n g l y to dry s o i l than they do to wet soil. K increases in^yalue when the s o i l d r i e s out. This means that i n a dry s o i l (0 = 0 ) the value of a given by Equation 4 i s too low and the estYmated d i f f u s i o n c o e f f i c i e n t i s too l a r g e . Spencer and C l i a t h (4) measured very dramatic increases in s o r p t i o n of lindane as s o i l moisture was decreased below the amount required f o r monolayer coverage. E h l e r s et a l . (5) showed decreases i n the e f f e c t i v e s o i l d i f f u s i o n c o e f f i c i e n t as the s o i l dried. Because under f i e l d conditions the s o i l can dry out, predicted s o i l d i f f u s i o n h a l f - l i v e s w i l l almost always be shorter than those that a c t u a l l y occur. Q c



10.

BOMBERGER E T A L .

201


Farmer (6) reviewed the various d i f f u s i o n models f o r s o i l and developed s o l u t i o n s f o r s e v e r a l of these models. An appropriate model f o r f i e l d studies i s a nonsteady state model that assumes that m a t e r i a l i s mixed i n t o the s o i l to a depth L and then allowed to d i f f u s e both to the surface and more deeply i n t o the s o i l . Material d i f f u s i n g to the surface i s immediately removed by d i f f u s i o n and convection i n the a i r above the s o i l . The e f f e c t of t h i s assumption i s to make the concentration of a d i f f u s i n g compound zero at the s o i l surface. With these boundary conditions the s o l u t i o n to Equation 8 can be converted to the u s e f u l form:

4

D

/ soil / — ^ 5 —

f =

T

t

-L 1 - exp (—T

2

1 1 + erfc

I L /— T

2

(ID

where f i s the f r a c t i o n of the m a t e r i a l o r i g i n a l l y i n the s o i l that remains at any time t . I t can be shown that f =0.5 when 2 A L /4D . - t = 1.04, which means that the h a l f - l i f e f o r d i f f u s i o n soil . ' i s L /4.16D . soil N o n v o l a t i l e Compounds. The same formal development can be used to develop d i f f u s i o n equations f o r n o n v o l a t i l e compounds. The r e s u l t i s : 2

4

1

2

oC

dC *o z W

DT

where D i s the compound's d i f f u s i o n c o e f f i c i e n t i n f r e e water and T i s a c o r r e c t i o n f a c t o r f o r the s o i l p o r o s i t y . w

Diffusion c o e f f i c i e n t s i n water are much smaller than diffusion coefficients i n air. As an example, f o r oxygen, D 1.75 x 10" cm /s but D - 2.1 x 10" cm / s . Consequently, the p r e d i c t e d s o i l d i f f u s i o n h a l f - l i v e s f o r v o l a t i l e compounds range from hours to days, whereas the p r e d i c t e d half-lives for n o n v o l a t i l e compounds range from weeks to months. This long diffusion half-life f o r nonvolatile compounds results in a v i o l a t i o n of one of the d e r i v a t i o n assumptions: that no transport by water movement occurs. Over s e v e r a l weeks, a s i g n i f i c a n t f r a c t i o n of s o i l water w i l l evaporate, and the movement of water through the pores becomes the p r i n c i p a l transport mechanism f o r dissolved nonvolatile organics. The water evaporation rate determines the compound's mass transport rate, and o v e r a l l v o l a t i l i z a t i o n rates w i l l be slow. 1

2

w

5



202


Mass Transport and Leaching. Organic substances can be moved through the s o i l by s o i l water flowing downward as a r e s u l t of i n f i l t r a t i o n induced by p r e c i p i t a t i o n and i r r i g a t i o n and upward as a r e s u l t of e v a p o t r a n s p i r a t i o n . The rate of movement w i l l be a f f e c t e d by the adsorption e q u i l i b r i u m of the substance between s o i l water and s o i l s o l i d s . In general, substances that are s t r o n g l y adsorbed move much more slowly than the s o i l water, and those that are weakly adsorbed may move at the same r a t e . Several mathematical models have been proposed for describing this movement. The e a r l i e r models were developed from the study of chromatography (7_, S_, 9). These models assumed a pointwise e q u i l i b r i u m adsorption throughout the s o i l p r o f i l e . Davidson et a l . (10) also assumed t h i s pointwise e q u i l i b r i u m . Oddson et a l . (11) and Lindstrom et a l . (12) used a k i n e t i c adsorption model. All these models could be solved a n a l y t i c a l l y , but required steady-state s o i l water c o n d i t i o n s . These models are of l i m i t e d use because i n a c t u a l f i e l d s i t u a t i o n s s o i l water content and movement change with time. Davidson et a l . (13) developed numerical s o l u t i o n s of the d i f f e r e n t i a l equation f o r solute transport f o r a model that included both the t r a n s i e n t and steady-state s o i l water conditions a f t e r p r e c i p i t a t i o n events. The model also included the e f f e c t s of d i f f u s i o n and d i s p e r s i o n . The model did not, however, include the e f f e c t s of e v a p o t r a n s p i r a t i o n . E v a p o t r a n s p i r a t i o n causes a movement of water to the s o i l surface that c a r r i e s organic m a t e r i a l with i t . This water movement i s responsible f o r the w e l l known "wick e f f e c t , " (14) and not i n c l u d i n g i t severely l i m i t s the u t i l i t y of the model. Because the more complicated model that required numerical s o l u t i o n s t i l l neglected important e f f e c t s , we chose to use a simple a n a l y t i c a l model f o r convenience. We chose Oddson s because i t s major features had been v e r i f i e d by Huggenberger (15, 16) f o r lindane, one of the compounds i n our study. Oddson included the k i n e t i c s of adsorption by assuming that the rate of adsorption i s p r o p o r t i o n a l to the d i f f e r e n c e between the amount that has already adsorbed and the e q u i l i b r i u m value: f

oC ^ dt

= p(K C - C ) p w s

8 (1/h) i s a constant d e s c r i b i n g e q u i l i b r i u m i s achieved.

the

rate

v

at

which

(14) '

adsorption


10.

BOMBERGER ET A L .

The overall described by:

transport

5C


203

Terrestrial-Atmospheric Interface of

chemical

dC

by

water

movement i s

dC

where v i s the s u p e r f i c i a l water v e l o c i t y i n the s o i l . [The s u p e r f i c i a l v e l o c i t y i s equal ^to ^the rate at which water i s a p p l i e d to the s o i l surface cm /cm -s .] T h i s formulation does not include the e f f e c t s of d i f f u s i o n or d i s p e r s i o n , which makes the s o l u t i o n s t r a c t a b l e . Oddson solved an i n i t i a l value problem that described the convective movement of an organic down from the s o i l surface f o r the f o l l o w i n g s p e c i f i c c o n d i t i o n s : No organic

chemical i s present

i n the s o i l s

originally.

The organic chemical i s placed on the s o i l surface and c a r r i e d i n t o the s o i l by water a p p l i e d to the s o i l s u r f a c e . Assuming the r e s u l t i n g concentration at the surface i s constant at C (compound s o l u b i l i t y ) f o r the time T that i t takes to d i s s o l v e a l l the m a t e r i a l and i s zero t h e r e a f t e r , we have t

C(0,t) = C

g a t

for 0 < t < T

and C(0,t) - 0 f o r t > T The a c t u a l s o l u t i o n s that were developed are not shown here because they are not needed f o r a screening a n a l y s i s . The major features of the s o l u t i o n s include the f o l l o w i n g : Kp i n f l u e n c e s the depth of maximum concentration of organic in s o l u t i o n , but does not a f f e c t the value of that concentration. The organic chemical i n s o l u t i o n w i l l move through the s o i l as a gaussian peak. The lower K , the more spread out the peak w i l l be. The depth of movement of maximum concentration i s equal to the depth of water penetration d i v i d e d by pK /6 . For f i e l d s t u d i e s , an appropriate value of 0 to u s e i s the f i e l d capacity, which is the water content that develops i n a s o i l that i s saturated and then allowed to d r a i n f r e e l y . p

The concentration of m a t e r i a l adsorbed on the s o i l a l s o moves down as a peak. The p o s i t i o n of maximum adsorbed m a t e r i a l i s about the same as f o r the maximum concentration i n s o l u t i o n .



204


These two conclusions are the important r e s u l t s from the model because they enable us to say how deep i n t o the s o i l p r o f i l e the majority of an organic chemical penetrates due to water inputs. I f V centimeters of water are a p p l i e d to a s o i l surface, then the water penetrates the s o i l to a depth of V/0 . If V is s u f f i c i e n t to d i s s o l v e a l l the organic chemical present, then depth where the maximum concentration of chemical i n the s o i l w i l l be found i s V/pKp. The rate of adsorption, which is described by the parameter 8 , a f f e c t s the shape of the concentration peak. Large values of 8 cause sharp peaks, whereas small values cause wide peaks. Various values have been found f o r 8 (11). They range from 0.025 to 5.0, with the lower values that imply a slow approach to e q u i l i b r i u m a s s o c i a t e d with s o i l s high i n organic content. I t appears a l s o that 8 may not r e a l l y be a constant, being l a r g e r when adsorption i s f a r from e q u i l i b r i u m and smaller later. For s o i l s with the smaller values of 8 , some organic m a t e r i a l may be found at the depth of water penetration even though the bulk of the organic may be much nearer to the surface. Soil. The f a t e of organics i s h i g h l y dependent on the s o i l p r o p e r t i e s . The models f o r d i f f u s i o n and mass transport show that h i g h l y organic s o i l s r e t a r d d i f f u s i o n and mass transport by strengthening the s o r p t i v e a t t r a c t i o n of the s o i l p a r t i c l e s for o r g a n i c s . Highly porous and dry s o i l s (9 large) f o s t e r d i f f u s i o n because they o f f e r adequate a i r space f o r d i f f u s i o n . For t h i s i n v e s t i g a t i o n we chose a s o i l with " t y p i c a l p r o p e r t i e s " so that the behavior of d i f f e r e n t compounds could be compared. The s o i l has a p o r o s i t y of 50%, the s o i l p a r t i c l e s themselves have a density of 2.5 g/cm (a c l a y ) , and the bulk density of the dry s o i l i s 1.25 g/cm . The s o i l f i e l d capacity i s 30% and i t s organic content i s 1%. S o i l organic contents can vary from almost zero f o r sandy s o i l s to 11% or more f o r organic mucks. Two percent is considered highly organic, and 1% is fairly r e p r e s e n t a t i v e of a g r i c u l t u r a l s o i l . 3

Diffusion Coefficients. Diffusion coefficients i n a i r were estimated using the F u l l e r , S c h e t l l e r , and Giddings c o r r e l a t i o n (17) even though f o r some compound measured values were a v a i l a b l e . Adsorption P a r t i t i o n C o e f f i c i e n t s . Experimental K values were used when a v a i l a b l e ; otherwise, the K „ values were estimted. We oc used a c o r r e l a t i o n between aqueous s o l u b i l i t y ( C « ) and K that contained data f o r p e s t i c i d e s and a group of polar and nonpolar organic chemicals c o l l e c t e d by Kenega and Goring (18) and Smith et a l . (3): log K = -0.27 - 0.782 l o g C (16) Q C

g

t


Q C

10.

BOMBERGER E T A L .

205



This r e l a t i o n s h i p does not include the c o r r e c t i o n recommended by Yalkowsky (19) f o r i n c l u d i n g d i f f e r e n c e s between s o l i d and l i q u i d compounds because Yalkowsky*s work was not a v a i l a b l e at the beginning of the study. Yalkowsky showed that s o l i d s and l i q u i d s do not f i t w e l l i n t o the same c o r r e l a t i o n unless the s o l i d s o l u b i l i t i e s are corrected f o r the entropy of m e l t i n g . The e r r o r introduced by not i n c l u d i n g the c o r r e c t i o n i s not s i g n i f i c a n t f o r t h i s screening a n a l y s i s . Henry's Law Constants. When a v a i l a b l e , experimental values of Henry's law constants were used. When experimental values could not be found, values were estimated using the method o u t l i n e d by Mackay (2

Environmental Fate and Transport at the Terrestrial-Atmospheric

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