Processes and Factors Affecting Transport of Pesticides to Ground

1 Processes and Factors Affecting Transport of Pesticides to Ground Water 1

2

H . H . Cheng and W. C. Koskinen 1

Department of Agronomy and Soils, Washington State University, Pullman, WA 99164 Southern Weed Science Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Stoneville, MS 38776

Downloaded by 109.161.238.159 on November 3, 2015 | http://pubs.acs.org Publication Date: July 17, 1986 | doi: 10.1021/bk-1986-0315.ch001

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The process of transporting pesticides down through the soil horizons and the vadose zone into the ground water is affected by a number of other processes taking place in the soil profile. The sorption process can retard or retain the chemicals from moving with leaching water. The commonly used pesticides, mostly being organic chemicals, can be degraded partially or completely to inorganic end products by chemical, photochemical, or biochemical means. The degradation process reduces or eliminates the presence of pesticides in the environment. Pesticides can also be transported to the atmosphere by volatilization or to surface water by runoff from soil, or be removed from soil by plant uptake. Whether circumstances for transport of pesticides to groundwater exist will depend upon a combination of factors including the nature of the pesticide chemical, the properties and conditions of the soil, and climatic and environmental variables. Realistic assessment of the potential for transport of pesticides to groundwater must include simultaneous evaluation of all the processes and factors that may impact the transport process. The t o p i c o f t h i s symposium i s b o t h t i m e l y and o f s p e c i a l c o n c e r n t o many s e c t o r s o f t h e p u b l i c as i t combines two c o n t r a s t i n g s u b j e c t matters. Groundwater, b e i n g an e s s e n t i a l n a t u r a l r e s o u r c e , has been assumed t o be p r i s t i n e i n q u a l i t y and must n o t be v i o l a t e d by contamination. A f t e r a l l , 86% o f t h e water i n t h e U n i t e d S t a t e s i s s t o r e d i n a q u i f e r s ; over 50% o f t h e U. S. p o p u l a t i o n and 95% o f r u r a l U. S. u s e groundwater as d r i n k i n g w a t e r . On t h e o t h e r hand, any m e n t i o n o f p e s t i c i d e s o f t e n connotes s u b s t a n c e s t h a t a r e t o x i c , hazardous, and even life-threatening. A tremendous amount o f p e s t i c i d e s i s produced and used i n t h e U. S. each y e a r . I n 1984, 1.1 b i l l i o n pounds was s y n t h e s i z e d ; even t h i s amount was down from t h e I. 5 b i l l i o n pounds produced i n 1975 ( 1 ) . 0097-6156/ 86/ 0315-O002$06.00/ 0 © 1986 A m e r i c a n C h e m i c a l Society

In Evaluation of Pesticides in Ground Water; Garner, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


1.

C H E N G A N D KOSKINEN

Affecting Transport of Pesticides to Ground Water

3

Some 68% o f the p e s t i c i d e s was used f o r a g r i c u l t u r a l p u r p o s e s , 17% by i n d u s t r i e s and commercial c o n c e r n s , 8% by homes and gardens, and 7% by governmental u s e s . Over 60% o f the p e s t i c i d e s produced a r e h e r b i c i d e s , the o t h e r s b e i n g m o s t l y i n s e c t i c i d e s and f u n g i c i d e s . There i s i n c r e a s i n g e v i d e n c e f o r the p r e s e n c e of a v a r i e t y of p e s t i c i d e s i n groundwater a t c e r t a i n l o c a t i o n s , such as i n s h a l l o w , unprotected a q u i f e r s or i n k a r s t formations. A r e p o r t by the U. S. Environmental P r o t e c t i o n Agency (EPA) i n 1984 (2) l i s t e d some 12 pesticides t h a t have been d e t e c t e d i n d r i n k i n g water w e l l s i n Florida, New York, California, Hawaii, and other locations. R e c e n t l y , the EPA announced i t s i n t e n t i o n t o i n c l u d e 60 p e s t i c i d e s i n its d r i n k i n g water survey (3·), more than half of which are herbicides. The seriousness of the potential contamination of groundwater by p e s t i c i d e s c e r t a i n l y cannot be m i n i m i z e d . A l t h o u g h the c o n c e r n s by EPA f o r groundwater q u a l i t y a r e t o be l a u d e d , the p l a n n e d program on groundwater m o n i t o r i n g , p a r t i c u l a r l y the d r i n k i n g w a t e r w e l l s s u r v e y , i s o f l i m i t e d v a l u e . The m o n i t o r i n g program c o u l d a t b e s t a l e r t the p e o p l e o f e x i s t i n g h a z a r d . I t would not be u s e f u l i n a s s e s s i n g the magnitude o f the p o t e n t i a l danger n o r i n d e d u c i n g the s o u r c e s o f c o n t a m i n a t i o n . An e f f e c t i v e management program must not o n l y i n c l u d e a groundwater m o n i t o r i n g e f f o r t but a l s o s u p p o r t a p l a n n e d program of r e s e a r c h t h a t can i s o l a t e and i d e n t i f y the p o t e n t i a l causes o f c o n t a m i n a t i o n by p e s t i c i d e s and d e v i s e ways t o m i n i m i z e o r e l i m i n a t e the s o u r c e s o f c o n t a m i n a t i o n . The h e a r t of such a program must be t h e s e a r c h f o r a b a s i c u n d e r s t a n d i n g of the p r o c e s s e s and f a c t o r s t h a t a f f e c t the t r a n s p o r t of p e s t i c i d e s t o groundwater. The

Setting

It i s not always o b v i o u s t h a t groundwater i n most c a s e s is a renewable r e s o u r c e . Few o f the a q u i f e r s t h a t s u p p l y d r i n k i n g water a r e of a n c i e n t o r i g i n . Most o f them a r e dynamic i n n a t u r e , and a r e r e c h a r g e d r e p e a t e d l y by water moving s l o w l y o r r a p i d l y downward from the s u r f a c e o f the e a r t h through the s o i l p r o f i l e t o a l e v e l t h a t accumulates the w a t e r . In the p r o c e s s , d i s s o l v e d c h e m i c a l s i n c l u d i n g p e s t i c i d e s can a l s o be c a r r i e d downward i n t o the s o i l p r o f i l e and e v e n t u a l l y r e a c h the groundwater. Thus, t h e c o n c e r n f o r c o n t a m i n a t i o n of groundwater i s l e g i t i m a t e and j u s t i f i e d . A number o f papers of t h i s symposium a r e d e a l i n g w i t h the s p e c i f i c p r o c e s s o f t r a n s p o r t of p e s t i c i d e s t o the groundwater. The o b j e c t i v e o f t h i s paper i s to s e t the s t a g e f o r t h i s symposium by r e l a t i n g the t r a n s p o r t p r o c e s s t o other processes occurring simultaneously i n s o i l that a f f e c t the transport process. We w i l l attempt t o v i e w the t r a n s p o r t p r o c e s s i n p e r s p e c t i v e o f the f a t e o f p e s t i c i d e s i n the t o t a l environment, and to s h a r e some c a u t i o n s needed i n a s s e s s i n g these p r o c e s s e s . The n a t u r a l environment can be viewed as c o n s i s t i n g of s e v e r a l e n v i r o n m e n t a l zones ( F i g u r e 1 ) , e x t e n d i n g from the atmosphere and the above-ground c r o p zone i n t o the s o i l from the s u r f a c e down, through the r o o t zone, the u n s a t u r a t e d s o i l o r vadose zone below, e v e n t u a l l y down t o the s a t u r a t e d zone where ground water i s s i t u a t e d . For a p e s t i c i d e t o contaminate groundwater, the c h e m i c a l must f i r s t r e a c h the s o i l , e i t h e r d i r e c t l y by a p p l i c a t i o n o r i n d i r e c t l y such as by drift. F o l i a r - a p p l i e d chemicals are s u b j e c t to photodecomposition, p l a n t a b s o r p t i o n , d r i p p i n g t o s o i l , o r f a l l i n g on s o i l when p l a n t s


In Evaluation of Pesticides in Ground Water; Garner, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986. Upward

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Evaporation

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WATER PROCESSES

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Degradation (chemical/biological) Soil Retention Transport.

Degradation (chemical/biological) Soil Retention

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Degradation (chemical/biological) Soil Retention Root uptake Transport .

SurfaceSubsurface application Washoff Transport

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PESTICIDE

F i g u r e 1. P r o c e s s e s a f f e c t i n g water movement and p e s t i c i d e t r a n s p o r t i n v a r i o u s e n v i r o n m e n t a l zones.

Saturated-zone (groundwater)

Unsaturated soil zone below root depth (vadose zone)

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die. Once i n t h e s o i l , t h e c h e m i c a l must be t r a n s p o r t e d , u s u a l l y by w a t e r , t h r o u g h t h e v a r i o u s e n v i r o n m e n t a l zones down t o t h e ground water. F a c t o r s t h a t a f f e c t water i n f i l t r a t i o n and movement w i t h i n a zone o r from one zone t o a n o t h e r w i l l a l s o a f f e c t t h e l o c a t i o n o f t h e chemical. In a d d i t i o n t o moving downward, water c a n a l s o be t r a n s p o r t e d upward t o t h e s o i l s u r f a c e and e v a p o r a t e d i n t o t h e atmosphere, t a k e n up by p l a n t s and t r a n s p i r e d , o r d i s c h a r g e d o v e r s o i l s u r f a c e i n t o s u r f a c e water b o d i e s . P e s t i c i d e s c a n be t r a n s p o r t e d by water i n s i m i l a r manners. B e f o r e t h e s e t r a n s p o r t p r o c e s s e s c a n be p r o p e r l y characterized a t any p a r t i c u l a r setting, a l l t h e p r o c e s s e s and f a c t o r s t h a t impact p e s t i c i d e t r a n s p o r t must be c h a r a c t e r i z e d . Two major t y p e s o f p r o c e s s e s t h a t c a n a f f e c t t h e amount o f p e s t i c i d e s present and a v a i l a b l e f o r t r a n s p o r t t h r o u g h t h e s o i l p r o f i l e a r e r e t e n t i o n and t r a n s f o r m a t i o n . The r e t e n t i o n p r o c e s s e s do not a f f e c t t h e t o t a l amount o f p e s t i c i d e p r e s e n t i n t h e s o i l b u t c a n d e c r e a s e o r e l i m i n a t e t h e amount a v a i l a b l e f o r t r a n s p o r t . On t h e o t h e r hand, t h e t r a n s f o r m a t i o n p r o c e s s e s a c t u a l l y r e d u c e o r t o t a l l y eliminate t h e amount of p e s t i c i d e present and a v a i l a b l e f o r transport. Retention

Processes

The l i t e r a t u r e abounds w i t h r e f e r e n c e s on t h e r e t e n t i o n o f p e s t i c i d e s i n s o i l s ( e . g . , b_,5) . The term ' r e t e n t i o n ' i s used h e r e i n an a l l encompassing s e n s e , b u t i t i s most f r e q u e n t l y equated w i t h a d s o r p t i o n or s i m p l y s o r p t i o n . In a s t r i c t sense, a d s o r p t i o n i s a r e v e r s i b l e process i n v o l v i n g n o n s p e c i f i c a t t r a c t i o n of a chemical to the s o i l p a r t i c l e s u r f a c e and r e t e n t i o n o f t h e c h e m i c a l on t h e s u r f a c e f o r a l o n g e r o r s h o r t e r p e r i o d o f time depending on t h e a f f i n i t y of the chemical t o the surface. However, whether a c h e m i c a l i s a c t u a l l y sorbed to a p a r t i c u l a r surface i s o f t e n not confirmed by t h e technique used t o c h a r a c t e r i z e r e t e n t i o n . Few o f t h e t e c h n i q u e s g e n e r a l l y used f o r r e t e n t i o n c h a r a c t e r i z a t i o n c a n d i f f e r e n t i a t e t h e mechanisms i n v o l v e d i n a t t r a c t i n g t h e c h e m i c a l t o t h e s o i l s u r f a c e . For i n s t a n c e , t h e commonly used b a t c h e q u i l i b r a t i o n method m e r e l y d e t e r m i n e s t h e d e c r e a s e o r d i s a p p e a r a n c e o f a c h e m i c a l from s o l u t i o n when s o i l i s added t o t h e s o l u t i o n , under t h e assumption t h a t what does n o t remain i n t h e s o l u t i o n would be a d s o r b e d . T h i s method does not p r o v i d e any i n f o r m a t i o n on t h e mechanism o f a d s o r p t i o n , o r t h e strength of adsorption, o r whether the reduction i n solution c o n c e n t r a t i o n was r e l a t e d t o a d s o r p t i o n a t a l l . S t u d i e s have shown t h a t t h i s method c o u l d l e a d t o e r r o n e o u s e s t i m a t i o n o f a d s o r p t i o n i f p r e c a u t i o n was n o t t a k e n i n e l i m i n a t i n g o r a c c o u n t i n g f o r d e g r a d a t i o n of t h e c h e m i c a l d u r i n g t h e a d s o r p t i o n - d e s o r p t i o n p r o c e s s e s ( 6 , 7 ) . A number o f q u i c k - t e s t t e c h n i q u e s have been used w i d e l y t o e s t i m a t e t h e e x t e n t o f s o r p t i o n o f p e s t i c i d e s t o s o i l s , and t h e s e e s t i m a t e s a r e o f t e n used i n p e s t i c i d e t r a n s p o r t models. The most commonly used t e c h n i q u e i s t o d e t e r m i n e t h e r a t i o o f d i s t r i b u t i o n o f a c h e m i c a l , o f t e n a t one c o n c e n t r a t i o n , between t h e s o l u t i o n and s o i l s o l i d phases (K,) o r s i m p l y t h e d i s t r i b u t i o n between w a t e r and o c t a n o l phases ?K ) . The u s e o f Κ , as an i n d e x o f a d s o r p t i o n assumes t h a t t h e R e t r i b u t i o n r a t i o i s c o n s t a n t o v e r a range o f c o n c e n t r a t i o n s of t h e c h e m i c a l i n t h e s o i l . I n o t h e r words, t h e amount o f c h e m i c a l adsorbed i n c r e a s e s l i n e a r l y w i t h t h a t r e m a i n i n g i n the s o l u t i o n . The l i n e a r r e l a t i o n s h i p may be v a l i d o v e r a narrow



6

E V A L U A T I O N O F PESTICIDES IN G R O U N D WATER

range of c o n c e n t r a t i o n s . However, a p l o t t i n g o f v a l u e s t a k e n from the l i t e r a t u r e v s . the e q u i l i b r i u m c o n c e n t r a t i o n of p e s t i c i d e i n s o l u t i o n r e a d i l y d e m o n s t r a t e s the d i s p a r i t y o f t h e values (8). Most literature values indicate that p e s t i c i d e adsorption data o b t a i n e d by the b a t c h e q u i l i b r a t i o n method can be b e t t e r f i t t e d t o a c u r v i l i n e a r e q u a t i o n , such as the F r e u n d l i c h e q u a t i o n . The wide d i s p a r i t i e s i n the F r e u n d l i c h a d s o r p t i o n c o n s t a n t s (K^) r e p o r t e d i n the literature should be critically evaluated, as the true s i g n i f i c a n c e o f the d a t a o b t a i n e d by d i f f e r e n t s t u d i e s can be a consequence of the parameters used i n the testing procedure. K o s k i n e n and Cheng (9) have shown t h a t v a r i a t i o n s i n e x p e r i m e n t a l p a r a m e t e r s c o u l d r e s u l t i n v a r i a t i o n s o f up t o s e v e r a l f o l d . As a r e s u l t , m o b i l i t y o f the p e s t i c i d e 2,4,5-T ( 2 , 4 , 5 - t r i c h l o r o p h e n o x y a c e t i c a c i d ) i n t h e P a l o u s e s i l t loam s o i l , f o r i n s t a n c e , c o u l d be c l a s s i f i e d as low o r h i g h depending on the e x p e r i m e n t a l p a r a m e t e r s used. Many s t u d i e s have shown t h a t p e s t i c i d e a d s o r p t i o n can be c o r r e l a t e d t o t h e s o i l o r g a n i c m a t t e r c o n t e n t s , but not w i t h s o i l m i n e r a l o r c l a y c o n t e n t s . Thus, a d s o r p t i o n c o n s t a n t s have o f t e n been e x p r e s s e d i n terms o f the s o i l o r g a n i c m a t t e r o r o r g a n i c carbon c o n t e n t s (K ) (4). Such c o r r e l a t i o n s have o f t e n been shown t o be statisticalfy significant (8). Significant correlations between sorption (K ) and estimates of sorption using water octanol p a r t i t i o n (K° ) (10), reverse-phase high-performance l i q u i d chroma C

tographic

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(R ) t

( 1 1 ) , and

water

solubility

(S) o f

the

c h e m i c a l (12) have l e d t o wide use o f t h e s e p a r a m e t e r s t o e s t i m a t e Κ . These e s t i m a t e s may be adequate as a f i r s t approximation, e s p e c i a l l y f o r hydrophobic, nonionic chemicals. However, a rough c o r r e l a t i o n between s o i l s o r p t i o n o f a c h e m i c a l and i t s Κ , R , or S ow t does not a u t o m a t i c a l l y impart any t h e o r e t i c a l meaning t o the e m p i r i cal measurement. Such f a c t o r s as s o i l s o l u t i o n composition and temperature c a n a f f e c t t h e amount o f p e s t i c i d e s adsorbed but cannot be a c c o u n t e d f o r by t h e s e i n d i r e c t methods. Thus, any measurement w h i c h does n o t t a k e i n t o c o n s i d e r a t i o n the r o l e o f s o i l o r the e n v i r o n m e n t a l v a r i a b l e s i n t h e a d s o r p t i o n p r o c e s s can a t b e s t be s i m p l y an e s t i m a t i o n . C e r t a i n c a u t i o n s s h o u l d a l s o be mentioned so t h a t the meaning of t h e s e c o r r e l a t i o n s i s not extended beyond what the d a t a w a r r a n t . A significant c o r r e l a t i o n between s o r p t i o n and soil organic carbon c o n t e n t s does not i m p l y t h a t o n l y one mechanism o f s o r p t i o n i s i n v o l v e d o r t h a t a l l p e s t i c i d e s i n t e r a c t w i t h a l l components o f s o i l organic matter by the same mechanism. Furthermore, the l a c k o f c o r r e l a t i o n between p e s t i c i d e s o r p t i o n and s o i l m i n e r a l or clay c o n t e n t s s h o u l d n o t be t a k e n t o mean t h a t a d s o r p t i o n on s o i l m i n e r a l s i s not i m p o r t a n t . A d s o r p t i o n of o r g a n i c c h e m i c a l s on s o i l m i n e r a l s u r f a c e s i s a w e l l - e s t a b l i s h e d f a c t (13,14)· The l a c k o f c o r r e l a t i o n may o n l y i m p l y t h a t s o i l m i n e r a l c o n t e n t by w e i g h t i s not a good index f o r the e x t e n t of m i n e r a l s u r f a c e a v a i l a b l e f o r a d s o r p t i o n . The s i g n i f i c a n c e o f a d s o r p t i o n on m i n e r a l s u r f a c e s s h o u l d not be i g n o r e d , e s p e c i a l l y i n m a t e r i a l s low i n o r g a n i c m a t t e r , such as the g e o l o g i c a l m a t e r i a l s i n the vadose zone u n d e r n e a t h the s u r f a c e s o i l . The c o m p l e x i t y o f the a d s o r p t i o n p r o c e s s s h o u l d be u n d e r s t o o d by t h o s e i n t e r e s t e d i n a s s e s s i n g the impact o f s o r p t i o n on the t r a n s p o r t of c h e m i c a l s t o the groundwater. Mechanisms o r f o r c e s i n v o l v e d i n a d s o r p t i o n can range from v a n d e r Waals-London f o r c e s , hydrogen t


1.



1


bonding, l i g a n d and i o n exchange, charge t r a n s f e r , i o n - d i p o l e and d i p o l e - d i p o l e f o r c e s , hydrophobic bonding, to chemisorption ( 5 ) . Any s i m p l i s t i c r e p r e s e n t a t i o n o f a d s o r p t i o n such as d e p i c t i n g t h e p r o c e s s as p a r t i t i o n i n g i n t o an o r g a n i c phase i s t o i g n o r e the t r u e n a t u r e of the p r o c e s s e s i n v o l v e d as p o i n t e d out by M i n g l e g r i n and G e r s t l ( 1 5 ) . Even c h a r a c t e r i z a t i o n of s o r p t i o n based s o l e l y on s o l u t e and s o l v e n t f a c t o r s may not be adequate. The c o m p o s i t i o n o f t h e aqueous phase can affect the pesticide-soil-solution equilibrium of certain pesticides. F o r example, s o l u b l e o r g a n i c m a t t e r and m e t a l i o n s i n soil solution can complex o r b i n d small organic molecules and s t a b i l i z e them i n the aqueous phase ( 1 6 ) . K o s k i n e n and Cheng (9) found t h a t l e a c h i n g s o i l d e c r e a s e d the amount of s o l u b l e o r g a n i c s i n s o i l s o l u t i o n and i n c r e a s e d a d s o r p t i o n o f 2,4,5-T i n the s o i l s o l i d phase. C u r r e n t l y a number o f l a b o r a t o r i e s a r e d e v o t i n g c o n s i d e r a b l e e f f o r t i n d e v e l o p i n g methodology to c h a r a c t e r i z e the v a r i o u s bonds i n v o l v e d i n a d s o r p t i o n , many of which a r e not r e a d i l y r e v e r s i b l e by simple e q u i l i b r a t i o n with water (e.g., 17). Thus r e t e n t i o n cannot be viewed as p r o c e s s e s t h a t m e r e l y r e t a r d t r a n s p o r t o f p e s t i c i d e s i n w a t e r , but can b i n d the c h e m i c a l s i r r e v e r s i b l y t o s o i l s u r f a c e s ( t h e so-called 'bound r e s i d u e ' ) and t o t a l l y remove t h e c h e m i c a l s from transport. Such p r o c e s s as c h e m i s o r p t i o n may not be e a s i l y d i s t i n g u i s h e d w i t h c h e m i c a l t r a n s f o r m a t i o n o f the compound i n the soil media ( 1 8 ) . Transformation

Processes

The term ' t r a n s f o r m a t i o n ' i s used h e r e t o encompass a l l changes i n the c h e m i c a l s t r u c t u r e o r c o m p o s i t i o n of the p e s t i c i d e compound. The c h e m i c a l s t r u c t u r e may be m o d i f i e d by such r e a c t i o n s as o x i d a t i o n , r e d u c t i o n , h y d r o l y s i s , s u b s t i t u t i o n and removal o f f u n c t i o n a l groups, complexation with metal i o n s , p o l y m e r i z a t i o n , and others. The s t r u c t u r e may a l s o be b r o k e n down i n t o fragments o f the o r i g i n a l m o l e c u l e and e v e n t u a l l y i n t o i n o r g a n i c e n d p r o d u c t s , such as H^O, CO^, h a l i d e , ammonium, p h o s p h a t e , and o t h e r s a l t s . The term d e g r a d a t i o n s h o u l d o n l y be a s s o c i a t e d w i t h the breakdown p r o c e s s , a l t h o u g h i t i s commonly used t o d e s c r i b e o t h e r t r a n s f o r m a t i o n p r o c e s s e s . One s h o u l d be aware that while most transformation processes modify the s t r u c t u r e t o d e t o x i f y the c h e m i c a l , o t h e r r e a c t i o n s may l e a d t o more toxic products. A n o t a b l e example i s t h e f o r m a t i o n o f 3,3'4,4't e t r a c h l o r o a z o b e n z e n e by c o n d e n s a t i o n o f 3 , 4 - d i c h l o r o a n i l i n e w h i c h i s a d e g r a d a t i o n p r o d u c t o f many a n i l i d e h e r b i c i d e s ( 1 9 ) . Only by degradation can a pesticide be totally eliminated from the environment. P e s t i c i d e s can be t r a n s f o r m e d by c h e m i c a l , p h o t o c h e m i c a l , and b i o c h e m i c a l means. S o i l can p r o v i d e the c o n d i t i o n s o r s e r v e as the c a t a l y s t o r component f o r c h e m i c a l r e a c t i o n s . C h e m i c a l r e a c t i o n s a r e m e d i a t e d by such s o i l p r o p e r t i e s as pH o r c a t a l y z e d by s o i l m i n e r a l s (20). P h o t o l y s i s of a c h e m i c a l can r e s u l t d i r e c t l y from a b s o r b i n g r a d i a t i o n o r i n d i r e c t l y by r e a c t i o n w i t h a n o t h e r c h e m i c a l w h i c h i s a c t i v a t e d by absorbed r a d i a t i o n . However, the predominant means of transformation i s m i c r o b i a l or enzymatic. Mechanisms o f these r e a c t i o n s have been e x t e n s i v e l y reviewed and summarized (21-23). A t t e n t i o n s h o u l d be g i v e n t o e x p e r i m e n t a l methods b o t h f o r characterizing the process and for assessing the kinetics of



8


degradation. C h a r a c t e r i z a t i o n o f d e g r a d a t i o n under f i e l d c o n d i t i o n s is o f t e n complicated by other processes such as r e t e n t i o n and t r a n s p o r t a c t i n g s i m u l t a n e o u s l y on t h e c h e m i c a l ( 2 4 ) . S t u d i e s on p e r s i s t e n c e , d i s s i p a t i o n , d i s a p p e a r a n c e , o r l o s s o f e f f e c t i v e n e s s of p e s t i c i d e s a r e u s u a l l y not a d i r e c t measure o f p e s t i c i d e d e g r a d a t i o n . The i n f l u e n c e o f a l l p r o c e s s e s must be s o r t e d out b e f o r e t h a t a t t r i b u t a b l e t o d e g r a d a t i o n can be e v a l u a t e d . The k i n e t i c s o f p e s t i c i d e d e g r a d a t i o n i s a f f e c t e d by (a) the quantity and availability of the pesticides, (b) presence of m i c r o o r g a n i s m s o r enzyme systems c a p a b l e o f d e g r a d i n g the p e s t i c i d e , and (c) a c t i v i t y l e v e l o f t h e m i c r o o r g a n i s m s as a f f e c t e d by the nutrients available to sustain the microbial population; by environmental c o n d i t i o n s such as temperature, moisture, oxygen s u p p l y , a e r a t i o n ; and by v a r i o u s s o i l p a r a m e t e r s . Many models f o r e s t i m a t i n g t h e f a t e o f p e s t i c i d e s i n t h e environment assume t h a t d e g r a d a t i o n can be s i m p l y e x p r e s s e d as a f i r s t o r d e r r e a c t i o n w i t h r e s p e c t t o p e s t i c i d e c o n c e n t r a t i o n , a l t h o u g h the inadequacy o f t h i s approach has been p o i n t e d out r e p e a t e d l y ( e . g . , 25-27). Expressions for biodégradation rates should i n c l u d e c o n s i d e r a t i o n s of the s u b s t r a t e c o n c e n t r a t i o n as w e l l as t h e a c t i v i t i e s o f microorganisms or enzyme systems p r e s e n t , such as the Monod e q u a t i o n or the analogous M i c h a e l i s - M e n t e n e q u a t i o n f o r enzyme k i n e t i c s . Lewis et al. (27) have c a l l e d a t t e n t i o n t o t h e need t o have a m u l t i p h a s i c approach in expressing the kinetics of pesticide degradation, depending on the c o n c e n t r a t i o n o f the c h e m i c a l i n t h e environment. A number o f s t u d i e s from A l e x a n d e r ' s l a b o r a t o r y (28) have shown t h a t the k i n e t i c s o f d e g r a d a t i o n c a l c u l a t e d from the r a t e s n o r m a l l y used f o r such s t u d i e s may not be a p p l i c a b l e f o r p e s t i c i d e s at e x t r e m e l y low c o n c e n t r a t i o n s . The same c o u l d a l s o be s a i d f o r v e r y h i g h c o n c e n t r a t i o n s of p e s t i c i d e s (e.g. 29). In a d d i t i o n t o the amount of p e s t i c i d e p r e s e n t , the d e g r a d a t i o n r a t e c o u l d be a f f e c t e d by t h e a v a i l a b i l i t y o f t h e c h e m i c a l f o r degradation. Ogram e t a l . (30) have r e c e n t l y p r e s e n t e d evidence s u g g e s t i n g t h a t o n l y the 2,4-D (2,4-dichloro phenoxyacetic acid) i n s o i l s o l u t i o n , but not t h a t adsorbed on s o i l c o l l o i d s , c o u l d be degraded by s o i l m i c r o b e s b o t h i n s o i l s o l u t i o n and sorbed on s o i l colloids. Other c o n s i d e r a t i o n s s h o u l d a l s o be g i v e n t o the n a t u r e and q u a n t i t y of soil m i c r o b i a l biomass p r e s e n t in relation to n u t r i e n t a v a i l a b i l i t y (26,28,31) and the a d a p t a b i l i t y o f m i c r o b e s , e i t h e r by n a t u r a l s e l e c t i o n o r by g e n e t i c m a n i p u l a t i o n , t o a t t a c k and u t i l i z e the p e s t i c i d e c h e m i c a l ( 3 2 ) . Most of our knowledge on pesticide degradation has been accumulated from s t u d i e s w i t h s u r f a c e s o i l s and under l a b o r a t o r y incubation conditions. However, i t i s d i f f i c u l t to p r e d i c t the b e h a v i o r o f p e s t i c i d e s under f i e l d c o n d i t i o n s from d a t a o b t a i n e d under a c o n t r o l l e d l a b o r a t o r y c o n d i t i o n . The d e g r a d a t i o n i n s o i l c o n t a i n i n g p l a n t s may be e n t i r e l y d i f f e r e n t . Root exudates and decaying root fragments can provide energy and nutrients for m i c r o b i a l growth (33) and l e a d t o an a c c e l e r a t e d m i n e r a l i z a t i o n o f p e s t i c i d e s i n the r h i z o s p h e r e (34,35). The p r e s e n c e o f p l a n t s w i l l also affect soil water potential, which i n turn affects soil m i c r o b i a l a c t i v i t i e s and the d e g r a d a t i o n p r o c e s s e s . Furthermore, i n d i c a t i o n s a r e t h a t o r g a n i c c h e m i c a l s do degrade i n the vadose zone and i n groundwater ( e . g . , 36-38), but the mechanisms and k i n e t i c s of d e g r a d a t i o n are m o s t l y unknown. More a t t e n t i o n i s needed t o b e t t e r


1.


c h a r a c t e r i z e the mechanisms and the vadose zone.


Transport

9

Affecting Transport of Pesticides to Ground Water k i n e t i c s of p e s t i c i d e d e g r a d a t i o n

in

Processes

A l t h o u g h the downward t r a n s p o r t of p e s t i c i d e s by water i s of u l t i m a t e c o n c e r n i n e v a l u a t i n g the p o t e n t i a l of groundwater c o n t a m i n a t i o n , other modes o f pesticide transport should also be taken into consideration. These p r o c e s s e s i n c l u d e the upward t r a n s p o r t by w a t e r to s o i l s u r f a c e , evaporation o r v o l a t i l i z a t i o n from s o i l surface, t r a n s p o r t by water i n s u r f a c e r u n o f f o r by s o i l p a r t i c l e s i n e r o s i o n , and uptake by plants. The volatilization process involves two s t a g e s : the upward movement t o s o i l s u r f a c e and the escape from the s o i l surface. V o l a t i l i z a t i o n i s a f u n c t i o n of the v a p o r p r e s s u r e of the p e s t i c i d e and i s a f f e c t e d by p e s t i c i d e c o n c e n t r a t i o n , soil-water content, adsorptivity of the soil, diffusion rate in soil, t e m p e r a t u r e , and a i r movement (39-41). A model a c c o u n t i n g f o r most of the p r o c e s s e s a f f e c t i n g v o l a t i l i z a t i o n has been t e s t e d by J u r y e t al. (42). A l t h o u g h v o l a t i l i z a t i o n l o s s e s a r e u s u a l l y most r a p i d i m m e d i a t e l y f o l l o w i n g p e s t i c i d e a p p l i c a t i o n , the c o n t i n u e d slow l o s s over an extended p e r i o d i n a w a t e r - d e f i c i e n t environment such as i n the a r i d west can a l s o be s i g n i f i c a n t . P e s t i c i d e t r a n s p o r t by s u r f a c e r u n o f f and s o i l erosion i s a f u n c t i o n of time l a g between r a i n f a l l and a p p l i c a t i o n ; the c h e m i c a l n a t u r e and p e r s i s t e n c e of the p e s t i c i d e ; the h y d r o l o g i c a l , s o i l , and v e g e t a t i v e c h a r a c t e r i s t i c s o f the f i e l d ; and the method and t a r g e t of a p p l i c a t i o n (43). Wauchope (44) found t h a t u n l e s s s e v e r e r a i n f a l l o c c u r r e d s h o r t l y a f t e r p e s t i c i d e a p p l i c a t i o n , t o t a l l o s s e s f o r the m a j o r i t y o f p e s t i c i d e s due t o r u n o f f were l e s s t h a n 0.5% of the amount a p p l i e d i n most c a s e s , a l t h o u g h s i n g l e - e v e n t l o s s e s from s m a l l p l o t s o r watersheds can be much g r e a t e r . In a s s e s s i n g the f a t e of p e s t i c i d e s i n the e n v i r o n m e n t , the p r o c e s s o f p l a n t u p t a k e and i t s consequences have o f t e n been i g n o r e d , even though i t s importance i s r e a d i l y r e c o g n i z e d i n any s t u d y on the e f f i c a c y of the c h e m i c a l (45,46). P l a n t s not o n l y degrade p e s t i c i d e s and enhance t h e i r d e g r a d a t i o n , but t h e y can also participate i n pesticide transport. I f the p e s t i c i d e i s not degraded a f t e r b e i n g t a k e n up by p l a n t s , the p e s t i c i d e c o u l d be p a s s e d t h r o u g h the f o o d c h a i n when the plants are harvested and consumed, o r c o u l d be r e c y c l e d back t o s o i l i f the p l a n t p a r t s f a l l back on the ground and are not removed. Systems Approach A number of m o d e l i n g approaches a t t e m p t i n g t o d e p i c t the p r o c e s s of downward transport of pesticides to the groundwater have been published (47) or presented at this symposium. A conceptual framework f o r any such c o n s i d e r a t i o n s w i l l i n c l u d e not only the r e t e n t i o n , t r a n s f o r m a t i o n , and t r a n s p o r t p r o c e s s e s i n v o l v e d , but a l s o the f a c t o r s a f f e c t i n g a l l the p r o c e s s e s as i n p u t s t o the model, before the outcome of a l l p r o c e s s e s a c t i n g s i m u l t a n e o u s l y on the p e s t i c i d e s can be p r e d i c t e d ( F i g u r e 2 ) . Some o f the major f a c t o r s can be b r o a d l y d i v i d e d as f o l l o w s :


10


I. P e s t i c i d e f a c t o r s : Chemical p r o p e r t i e s : s t r u c t u r e , s o l u b i l i t y , v o l a t i l i t y A p p l i c a t i o n methods: f o r m u l a t i o n , r a t e , mode D e g r a d a t i o n p a t t e r n s : pathways, m e t a b o l i t e formation


II.

Soil factors: S o i l p r o p e r t i e s : type and amount of o r g a n i c m a t t e r , c l a y , and amorphous m a t e r i a l s , pH, s t r u c t u r e , permeability S o i l conditions: moisture, aeration, nutrient status, microbial a c t i v i t y , h e t e r o g e n e i t y , d e p t h t o water t a b l e Land forms: t o p o g r a p h y , s l o p e l e n g t h and s t e e p n e s s , drainage

I I I . Plant

factors:

Species c h a r a c t e r i s t i c s Stage of growth Root system and rhizosphere IV.

Environmental f a c t o r s : Temperature Precipitation A i r movement Radiation

This l i s t i s by no means an e x h a u s t i v e one, but i t r e v e a l s the m u l t i f a r i o u s i n t e r a c t i o n s o f the f a c t o r s i n v o l v e d t h a t a f f e c t a l l the processes. What we have attempted i n t h i s p r e s e n t a t i o n was t o p o i n t out some of the d i f f i c u l t i e s and p i t f a l l s one s h o u l d be aware of i n any attempt t o model p e s t i c i d e t r a n s p o r t as w e l l as o t h e r f a c t o r s a f f e c t i n g the f a t e of p e s t i c i d e i n the environment. While m o d e l i n g can be an i m p o r t a n t t o o l f o r e s t i m a t i o n of p e s t i c i d e movement and fate i n the environment, the current l a c k o f knowledge of the mechanisms and i n t e r a c t i o n s of factors and processes a f f e c t i n g p e s t i c i d e b e h a v i o r i n the environment has l e d t o assumptions and simplifications i n the systems t o be modeled. Errors either i n estimation s i m p l i f i c a t i o n s or inherent in the assumptions are d i f f i c u l t to q u a n t i f y . Moreover, e r r o r s associated with inputs f o r each f a c t o r or p r o c e s s i n the model can be compounded by e r r o r s i n subsequent i n t e r a c t i o n s . Thus p r e d i c t i v e v a l u e s o b t a i n e d from many c u r r e n t models must a l l be a c c e p t e d w i t h c a u t i o n i f they a r e to be used f o r assessment p u r p o s e s . As a f i n a l i t e m of f o o d f o r t h o u g h t , we would l i k e t o m e n t i o n t h a t one sometimes has the i m p r e s s i o n t h a t p e s t i c i d e movement i n t o groundwater i s an i n e v i t a b i l i t y , e s p e c i a l l y i f the c h e m i c a l moves beyond the r o o t zone i n t o the s u b s o i l where m i c r o b i a l a c t i v i t i e s a r e much lower and d e g r a d a t i o n would be l e s s e n e d . We a r e reminded of the s o i l genesis process i n that a great d e a l of s o l u b l e organic matter has been l e a c h e d from the s u r f a c e s o i l down i n t o the s u b s o i l o v e r c e n t u r i e s and even m i l l e n n i a o f s o i l development. The q u e s t i o n i s why have we not seen a m a s s i v e amount of n a t u r a l o r g a n i c m a t t e r i n




Inputs


Processes

Output

Organisms (plant, microbes)

F i g u r e 2 . I n t e r a c t i o n s o f t h e f a c t o r s and p r o c e s s e s the f a t e o f p e s t i c i d e s i n t h e s o i l environment.

affecting


12

EVALUATION OF PESTICIDES IN GROUND WATER

the groundwater? Perhaps we have n o t y e t a t t a i n e d n e a r l y t h e l e v e l of u n d e r s t a n d i n g o f a l l t h e p r o c e s s e s i n v o l v e d i n the r e t e n t i o n , transformation, and t r a n s p o r t o f c h e m i c a l s i n t h e s o i l as we s h o u l d have. The c h a l l e n g e i s s t i l l i n f r o n t o f u s . Acknowledgments J o i n t c o n t r i b u t i o n from Department o f Agronomy and S o i l s , Washington State U n i v e r s i t y , P u l l m a n , and S o u t h e r n Weed S c i e n c e L a b o r a t o r y , USDA-ARS, S t o n e v i l l e , MS. S c i e n t i f i c Paper No. 7215. C o l l e g e o f Agriculture and Home Economics R e s e a r c h C e n t e r , Washington State U n i v e r s i t y , P u l l m a n , P r o j e c t 1858.


Literature Cited

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