Spatial Variability of Pesticide Sorption and Degradation Parameters

6 Spatial Variability of Pesticide Sorption and Degradation Parameters 1

P. S. C. Rao , K. S. V. Edvardsson, L . T. Ou, R. E . Jessup, P. Nkedi-Kizza, and A. G. Hornsby

Downloaded by CORNELL UNIV on October 13, 2016 | http://pubs.acs.org Publication Date: July 17, 1986 | doi: 10.1021/bk-1986-0315.ch006

Soil Science Department, University of Florida, Gainesville, F L 32611

Data were collected at two field sites, one in Florida and the other in Georgia, to evaluate the variability in pesticide concentrations and in pesticide sorption and degradation parameters. The observed variability can be attributed to intrinsic factors leading to inherent variability, and to extrinsic factors resulting in imposed variability. Data are presented to demonstrate the predominant effects of the method of pesticide application and tillage operations in introducing significant extrinsic variability in pesticide concentrations. Variability introduced at the soil surface may persist as the pesticide leaches to deeper depths. At both field sites, a close relationship was found between the spatial variations in soil organic carbon content and pesticide sorption coefficients. At the Georgia field site, small variations (coefficient of variation < 30%) were noted in measured pesticide degradation half-lives in soil samples collected from several locations and four morphologic horizons. Spatial patterns in pesticide sorption and degradation parameters did not correspond to spatial boundaries of the soil series. Recent r e p o r t s o f i n c r e a s i n g i n c i d e n c e o f p e s t i c i d e s and t o x i c o r g a n i c p o l l u t a n t s i n groundwater ( i , 2 ) have prompted a number o f l a b o r a t o r y and f i e l d i n v e s t i g a t i o n s o f t h e p r o c e s s e s and f a c t o r s i n f l u e n c i n g p e s t i c i d e b e h a v i o r i n s o i l s and groundwater. Several s i m u l a t i o n models have been d e v e l o p e d f o r f o r e c a s t i n g p e s t i c i d e

7

C u r r e n t address: A g r o n o m y a n d S o i l Science Department, University o f H a w a i i , H o n o l u l u , H I 96822 0097-6156/ 86/ 0315-0100S06.00/ 0 © 1986 A m e r i c a n C h e m i c a l Society

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


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f a t e i n s o i l s ( 3, A ) . To v a l i d a t e t h e s e models and t o u s e them f o r e i t h e r management o r r e g u l a t o r y p u r p o s e s , s e v e r a l s i t e - s p e c i f i c v a l u e s f o r s o i l and p e s t i c i d e parameters a r e needed. A t y p i c a l f i e l d s i t e , v a r y i n g i n a r e a from about 1 t o 10 h a , may i n c l u d e s e v e r a l s o i l s e r i e s . The model parameter v a l u e s may be d i f f e r e n t n o t o n l y f o r each o f t h e s e s o i l s e r i e s , b u t may a l s o v a r y considerably within a single series. Such v a r i a b i l i t y i n a number of s o i l h y d r a u l i c p r o p e r t i e s ( e . g . , s o i l h y d r a u l i c c o n d u c t i v i t y , s o i l water f l u x , e t c . ) has been w i d e l y r e p o r t e d i n t h e l i t e r a t u r e ( 5 - 1_ ). The model parameter v a l u e s f o r a g i v e n l o c a t i o n i n t h e f i e l d may a l s o v a r y w i t h p r o f i l e depth depending upon s o i l h o r i z o n a t i o n as w e l l as a f u n c t i o n o f t h e s o i l and e n v i r o n m e n t a l f a c t o r s ( e . g . , s o i l a e r a t i o n , temperature, e t c . ) . S i n c e s o i l and e n v i r o n m e n t a l f a c t o r s undergo dynamic changes w i t h t i m e , model parameters a r e a l s o e x p e c t e d t o e x h i b i t t e m p o r a l v a r i a b i l i t y . A t p r e s e n t , o n l y l i m i t e d d a t a a r e a v a i l a b l e t o c h a r a c t e r i z e such s p a t i a l and t e m p o r a l v a r i a b i l i t y i n p e s t i c i d e s o r p t i o n and degradat i o n parameters r e q u i r e d i n s e v e r a l s i m u l a t i o n models. In t h i s paper, we w i l l d i s c u s s d a t a c o l l e c t e d as a p a r t o f two r e c e n t f i e l d s t u d i e s , one i n G e o r g i a and t h e o t h e r i n F l o r i d a . The o b j e c t i v e o f b o t h s t u d i e s was t o m o n i t o r t h e e n v i r o n m e n t a l dynamics of p e s t i c i d e s i n t h e c r o p r o o t zone and t o u s e t h e s e d a t a t o e v a l u a t e t h e p r e d i c t i v e c a p a b i l i t y o f s e v e r a l s i m u l a t i o n models. S p e c i f i c a t t e n t i o n w i l l be f o c u s e d on t h e d a t a c h a r a c t e r i z i n g t h e s p a t i a l v a r i a b i l i t y o f p e s t i c i d e s o r p t i o n and d e g r a d a t i o n paramet e r s measured a t t h e s e two f i e l d s i t e s . Description of the F i e l d

Sites

The G e o r g i a f i e l d s t u d y was a c o o p e r a t i v e e f f o r t between t h e U.S. E n v i r o n m e n t a l P r o t e c t i o n Agency, t h e U.S. G e o l o g i c a l Survey, and the U n i v e r s i t y o f F l o r i d a . The 4.5-ha f i e l d s i t e , p l a n t e d t o peanuts, was l o c a t e d near A l b a n y , GA. A t t h i s s i t e d a t a a r e b e i n g c o l l e c t e d to characterize p e s t i c i d e migration within the unsaturated and t h e s a t u r a t e d zones. The c r i t e r i a used i n s e l e c t i n g t h i s f i e l d s i t e and i n d e s i g n i n g t h e s t u d y a r e d i s c u s s e d by C a r s e l e t a l . ( 8 ). Cooper ( 9 ) p r e s e n t e d a d e t a i l e d g e o h y d r o l o g i c description of this s i t e . B u l k samples o f s u r f a c e s o i l (0-20 cm) were c o l l e c t e d u s i n g a bucket auger from 20 l o c a t i o n s , d e s i g n a t e d as t h e p r i m a r y s i t e s ( 8 ), and an a d d i t i o n a l 16 random l o c a t i o n s at t h i s f i e l d s i t e . Each sample was i d e n t i f i e d by i t s l o c a t i o n w i t h r e f e r e n c e t o an a r b i t r a r y g r i d ( 8 ). S o i l o r g a n i c carbon c o n t e n t (OC) o f t h e s e samples was determined by d r y combustion method u s i n g a LECO Carbon A n a l y z e r . Pesticide sorption coeffic i e n t s (K,) were determined u s i n g t h e b a t c h e q u i l i b r i u m t e c h n i q u e s ( 10, 11_ The p e s t i c i d e s used were: a l d i c a r b [ ( m e t h y l t h i o ) p r o p i o n a l d e h y d e 0 - ( m e t h y l c a r b a m o y l ) oxime]; m e t o l a c h l o r [ 2 - c h l o r o - N - ( 2 - e t h y l - 6 - m e t h y l p h e n y l ) - N - (2-methoxy-1-methyl e t h y l ) acetamide; and d i u r o n [ 3 - ( 3 , 4 - d i c h l o r o p h e n y l ) - 1,1-dimethyl u r e a ] . A t t h e 20 p r i m a r y l o c a t i o n s , s o i l c o r e s were a l s o c o l l e c t e d from each o f t h e f o u r m o r p h o l o g i c h o r i z o n s (0-20, 25-45, 48-63, and 94-107 cm) u s i n g a s e p t i c sampling t e c h n i q u e s ( 12 ) . A l d i c a r b and m e t o l a c h l o r d e g r a d a t i o n i n t h e s e s o i l samples under a e r o b i c c o n d i t i o n s was measured u s i n g b a t c h i n c u b a t i o n t e c h n i q u e s ( 13^ ). These


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o r

d a t a were used t o c a l c u l a t e t h e h a l f - l i v e s ( t j ^ ) ^ disappearance of t h e p a r e n t compound. The second f i e l d s t u d y was c o n d u c t e d on a 0.8-ha c i t r u s g r o v e , l o c a t e d n e a r Davenport, F l o r i d a . T h i s s t u d y was a c o o p e r a t i v e e f f o r t between t h e U n i v e r s i t y o f F l o r i d a and t h e Union C a r b i d e A g r i c u l t u r a l P r o d u c t s Company, I n c . The s o i l a t t h i s s i t e i s c l a s s i f i e d as C a n d l e r sand ( h y p e r t h e r m i c , T y p i c Q u r t z i p s a m m e n t s ) , w h i c h i s t y p i c a l o f t h e deep, sandy, and w e l l - d r a i n e d s o i l s p l a n t e d t o c i t r u s on t h e c e n t r a l r i d g e o f F l o r i d a . B u l k samples o f s u r f a c e s o i l (0-15 and 15-30 cm) were c o l l e c t e d u s i n g bucket auger a t 29 l o c a t i o n s , 12.5 m a p a r t , on a n o r t h - s o u t h (N-S) t r a n s e c t . On a w e s t - e a s t (W-E) t r a n s e c t , s o i l samples were c o l l e c t e d i n a s i m i l a r manner a t a n o t h e r 17 l o c a t i o n s . The N-S and W-E t r a n s e c t s s h a r e d one s o i l sample a t t h e p o i n t o f t h e i r i n t e r s e c t i o n . Additional s o i l samples were t a k e n from 26 s i t e s s e l e c t e d randomly w i t h i n t h e field. The e x a c t l o c a t i o n o f each s a m p l i n g s i t e was n o t e d and was used t o i d e n t i f y t h e samples. OC v a l u e s f o r t h e s e s o i l s were d e t e r m i n e d by t h e d r y combustion method. A l d i c a r b v a l u e s were measured u s i n g t h e b a t c h e q u i l i b r i u m t e c h n i q u e . Types o f S p a t i a l

Variability

Rao and Wagenet ( 14 ) have proposed t h a t t h e t o t a l v a r i a b i l i t y observed i n a given s o i l property i s t h e sum o f i n t r i n s i c and extrinsic variability. The former a r i s e s from i n h e r e n t v a r i a b i l i t y i n s o i l p r o p e r t i e s due t o p e d o g e n i c p r o c e s s e s , w h i l e t h e l a t t e r i s t h e r e s u l t o f v a r i o u s s o i l and c r o p management p r a c t i c e s a t a specific field site. They s u g g e s t e d t h a t t h e s p a t i a l v a r i a b i l i t y i n s o i l p r o p e r t i e s such as s o i l h y d r a u l i c c o n d u c t i v i t y o r p e s t i c i d e s o r p t i o n c o e f f i c i e n t s might be i n f l u e n c e d p r i m a r i l y by i n t r i n s i c f a c t o r s , whereas v a r i a b i l i t y i n p e s t i c i d e c o n c e n t r a t i o n s and f l u x e s might e x h i b i t t h e combined i n f l u e n c e o f b o t h i n t r i n s i c and e x t r i n sic factors. G i v e n d a t a on t o t a l v a r i a b i l i t y , i t i s n o t always p o s s i b l e t o q u a n t i f y t h e c o n t r i b u t i o n s o f each t y p e o f v a r i a b i l i t y . In t h i s paper, we w i l l p r e s e n t d a t a r e p r e s e n t i n g b o t h t y p e s o f variability. Extrinsic

Variability

P e s t i c i d e s and f e r t i l i z e r s a r e u s u a l l y a p p l i e d a t t h e s o i l s u r f a c e e i t h e r i n g r a n u l a r o r l i q u i d f o r m u l a t i o n s and a r e s u b s e q u e n t l y i n c o r p o r a t e d by some s o r t o f t i l l a g e o p e r a t i o n ( e . g . , d i s c i n g ) . T h i s o p e r a t i o n can i n d u c e e x t r i n s i c v a r i a b i l i t y i n a g r o c h e m i c a l c o n c e n t r a t i o n s and f l u x e s i n s o i l s due t o v a r i a b i l i t y i n a p p l i c a t i o n i t s e l f (random i f b r o a d c a s t ; nonrandom i f banded) and t h e t i l l a g e o p e r a t i o n . Weed s c i e n t i s t s have examined such v a r i a b i l i t y , i n p a r t i c u l a r as r e l a t e d t o h e r b i c i d e e f f i c a c y ( 15 - 21_ ). Such e x t r i n s i c v a r i a b i l i t y i n p e s t i c i d e c o n c e n t r a t i o n s can a l s o pose s e r i o u s problems i n o b t a i n i n g r e p r e s e n t a t i v e s o i l samples f o r p e s t i c i d e concentration determinations. A t t h e F l o r i d a f i e l d s i t e , we e v a l u a t e d t h e s i g n i f i c a n c e o f t i l l a g e - i n d u c e d e x t r i n s i c v a r i a b i l i t y i n bromide ( B r ) and a l d i c a r b * s t o t a l t o x i c r e s i d u e s (TTR) v a r i a b i l i t y . Bromide was a p p l i e d i n a 3-m s t r i p c e n t e r e d between t h e c i t r u s t r e e s t h a t were p l a n t e d about 8 m a p a r t . A c o n c e n t r a t e d KBr


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s o l u t i o n was s p r a y e d on t h e s o i l s u r f a c e u s i n g a t r a c t o r - m o u n t e d boom s p r a y r i g . The s p r a y n o z z l e s were s e t such t h a t s o l u t i o n s p r a y from a d j a c e n t n o z z l e s o v e r l a p p e d and p r o v i d e d e s s e n t i a l l y a uniform Br a p p l i c a t i o n . On t h e same 3-m s t r i p t o w h i c h Br was a p p l i e d , a l d i c a r b (Temik 15-G; g r a n u l a r f o r m u l a t i o n ) was a p p l i e d i n 16 p a r a l l e l bands t h a t were spaced 20-cm a p a r t . Br and a l d i c a r b were t h e n i n c o r p o r a t e d i n t o t h e s o i l t o a depth o f about 10 cm u s i n g a t r a c t o r - m o u n t e d m u l t i p l e d i s k implement w i t h 24 d i s k s t h a t were s e t about 12.5 cm a p a r t . Immediately f o l l o w i n g d i s c i n g , s o i l samples were c o l l e c t e d t o a depth o f 30 cm as shown s c h e m a t i c a l l y i n F i g u r e 1. Note t h a t a t o t a l o f 16 samples were t a k e n , 8 each on two p a r a l l e l t r a n s e c t s s e t 0.67 m a p a r t and p e r p e n d i c u l a r t o t h e a p p l i c a t i o n band. S o i l samples were c o l l e c t e d i n t h i s manner i n each o f t h e f o u r q u a d r a n t s o f t h e f i e l d . The v a r i a t i o n s i n Br and a l d i c a r b TTR c o n c e n t r a t i o n s a c r o s s t h e a p p l i c a t i o n band a r e shown i n F i g u r e s 2 and 3. Note t h a t b o t h Br and a l d i c a r b TTR c o n c e n t r a t i o n s a r e h i g h l y v a r i a b l e w i t h i n t h e a p p l i c a t i o n band, w i t h s e v e r a l o r d e r s o f magnitude d i f f e r e n c e i n c o n c e n t r a t i o n s even i n a d j a c e n t s o i l samples. Note t h a t even though Br was a p p l i e d e s s e n t i a l l y u n i f o r m l y t o t h e s o i l s u r f a c e , t h e v a r i a t i o n s i n Br c o n c e n t r a t i o n s a r e q u i t e s i m i l a r t o t h a t o f a l d i c a r b TTR c o n c e n t r a t i o n s . T h i s s u g g e s t s t h a t p o s t - a p p l i c a t i o n d i s c i n g was t h e p r i m a r y s o u r c e o f t h e o b s e r v e d e x t r i n s i c v a r i a b i l i ty. These r e s u l t s a r e c o n s i s t e n t w i t h t h e f i n d i n g s o f o t h e r workers ( 8, 18, 21^ ). We have a l s o o b s e r v e d s i m i l a r v a r i a b i l i t y p a t t e r n s i n a l d i c a r b TTR c o n c e n t r a t i o n s i n f i e l d s t u d i e s conducted d u r i n g 1983 a t two o t h e r l o c a t i o n s i n F l o r i d a ( 22 ). S o i l samples c o l l e c t e d up t o 4-m depth d u r i n g 1983, and up t o a d e p t h o f 10 m i n 1984 showed t h a t such v a r i a b i l i t y w i l l p e r s i s t t h r o u g h o u t t h e y e a r and a t a l l d e p t h s . We a r e p r e s e n t l y a n a l y z i n g o u r 1983 and 1984 a l d i c a r b and B r f i e l d d a t a i n o r d e r t o c a l c u l a t e a s t a t i s t i c a l l y v a l i d "average" c o n c e n t r a t i o n s which can be, i n t u r n , compared w i t h t h e v a l u e s p r e d i c t e d by s i m u l a t i o n models. A d i s c u s s i o n o f t h e s e model v a l i d a t i o n e f f o r t s i s beyond t h e scope o f t h i s paper. Intrinsic Variability

i n PC and

V a r i a t i o n s i n OC v a l u e s i n s o i l samples c o l l e c t e d a l o n g t h e N-S and W-E t r a n s e c t s a t t h e F l o r i d a s i t e a r e shown i n F i g u r e 4. Note t h a t f o r both t h e t r a n s e c t s , OC i n s o i l s from t h e 0-15 cm depth were more v a r i a b l e t h a n i n samples c o l l e c t e d from t h e 15-30 cm d e p t h . V a r i o g r a m s ( 14 ) c a l c u l a t e d u s i n g t h e s e d a t a i n d i c a t e d t h a t f o r t h e 0-15 cm depth, OC v a l u e s i n s o i l samples c o l l e c t e d w i t h i n a s e p a r a t i o n d i s t a n c e ( i . e . , l a g ) o f 15 m would be s p a t i a l l y c o r r e lated. On t h e o t h e r hand, OC v a l u e s f o r t h e 15-30 cm d e p t h a r e s p a t i a l l y independent. OC d a t a f o r b o t h depth i n c r e m e n t s c o u l d be f i t t e d t o a normal f r e q u e n c y d i s t r i b u t i o n ; t h e n o r m a l i t y was c o n f i r m e d by t h e Kolmogorov- Smirnov D - s t a t i s t i c ( 23 ). The c o e f f i c i e n t o f v a r i a t i o n (CV) i n OC d a t a f o r both depths was l e s s t h a n 20%. The measured d a t a f o r OC and a l d i c a r b were used t o g e n e r a t e 3-dimensional p l o t s d e p i c t i n g t h e i r s p a t i a l v a r i a t i o n s a t t h e Florida field site. These p l o t s , shown i n F i g u r e 5, i n d i c a t e a c l o s e , b u t n o t e x a c t , c o r r e s p o n d e n c e i n t h e s p a t i a l p a t t e r n s o f OC and K,. The OC d a t a and K, v a l u e s f o r t h r e e p e s t i c i d e s measured




FIGURE 1. Schematic diagram showing t h e s a m p l i n g d e s i g n used t o e v a l u a t e t h e e x t r i n s i c v a r i a b i l i t y i n bromide and a l d i c a r b TTR c o n c e n t r a t i o n s a t t h e Florida site.


6.

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DISTANCE, meters FIGURE 2. V a r i a t i o n s i n bromide ( B r ) c o n c e n t r a t i o n s i n s o i l samples t a k e n a c r o s s t h e a p p l i c a t i o n band. V e r t i c a l arrows i n d i c a t e t h e w i d t h o f t h e a p p l i c a t i o n band.



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DISTANCE, meters FIGURE 3. V a r i a t i o n s i n a l d i c a r b TTR c o n c e n t r a t i o n s i n s o i l samples t a k e n a c r o s s t h e a p p l i c a t i o n band. V e r t i c a l arrows i n d i c a t e t h e w i d t h o f t h e a p p l i c a t i o n band.


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D I S T A N C E ALONG T R A N S E C T , meters FIGURE 4. V a r i a t i o n s i n s o i l o r g a n i c c a r b o n c o n t e n t (OC) a t two depths (0-15 and 15-30 cm) i n samples t a k e n a l o n g two t r a n s e c t s a t t h e F l o r i d a s i t e .


108


u s i n g t h e s u r f a c e s o i l (0-20 cm) samples c o l l e c t e d a t t h e G e o r g i a s i t e a r e p r e s e n t e d i n F i g u r e 6; t h e s e p l o t s were g e n e r a t e d i n manner s i m i l a r t o t h o s e shown i n F i g u r e 5. As w i t h t h e d a t a f o r t h e F l o r i d a s i t e , s p a t i a l p a t t e r n s i n p e s t i c i d e K, v a l u e s and OC values are very s i m i l a r . These r e s u l t s may be a n t i c i p a t e d because OC i s t h e s i n g l e most i m p o r t a n t d e t e r m i n a n t o f for nonionic p e s t i c i d e s ( Γ1, 24 - 2 6 ) . Thus, i t would appear t h a t g i v e n a v a l u e r e f e r e n c e d t o OC, denoted as Κ , the s p a t i a l v a r i a t i o n s i n Κj f o r a p e s t i c i d e a t a f i e l d s i t e may be e s t i m a t e d by m e a s u r i n g t h e v a r i a t i o n s i n OC. Green e t a l . ( 27 ) p r e s e n t d a t a f o r nematicide s o r p t i o n to support t h i s c o n c l u s i o n .


I n t r i n s i c V a r i a b i l i t y of Degradation H a l f - L i v e s On t h e b a s i s o f an a n a l y s i s o f p u b l i s h e d d a t a , Rao and D a v i d s o n ( 1_1 ) n o t e d t h a t f o r s e v e r a l p e s t i c i d e s t h e v a r i a t i o n i n p e s t i c i d e degradation h a l f - l i v e s (two) among s e v e r a l s o i l s was s u r p r i s i n g l y s m a l l (CV < 100%), e s p e c i a l l y g i v e n t h e range i n s o i l t y p e s and t h e e n v i r o n m e n t a l c o n d i t i o n s a t w h i c h d e g r a d a t i o n was measured. Data c o l l e c t e d by Ou e t a l . ( 28 ) f o r d e g r a d a t i o n o f s e v e r a l p e s t i c i d e s i n s e l e c t e d U.S. s o i l s c o n f i r m e d t h i s o b s e r v a t i o n . More r e c e n t l y , Walker and Brown ( 29 ) measured t h e d e g r a d a t i o n o f two t r i a z i n e h e r b i c i d e s i n s o i l samples c o l l e c t e d from s e v e r a l l o c a t i o n s w i t h i n a 0.64-ha f i e l d . They r e p o r t e d a s m a l l v a r i a t i o n i n t . ,^ v a l u e s (CV < 25 %) f o r b o t h h e r b i c i d e s . D a t a p r e s e n t e d by Walker and Z i m d h a l ( 30 ) i n d i c a t e t h a t t h e h a l f - l i v e s f o r p e s t i c i d e degrada t i o n i n s o i l s c o l l e c t e d from t h r e e s t a t e s i n t h e U.S. d i d n o t v a r y by more t h a n a f a c t o r o f 2. I t s h o u l d be n o t e d t h a t i n a l l o f t h e above c i t e d s t u d i e s p e s t i c i d e d e g r a d a t i o n was measured i n s o i l s t h a t had been a i r - d r i e d and r e - w e t t e d t o some d e s i r e d s o i l - w a t e r c o n t e n t . I n h e r e n t d i f f e r e n c e s i n t h e d i v e r s i t y and t h e s i z e o f m i c r o b i a l p o p u l a t i o n s may have been r e d u c e d by a i r - d r y i n g t h e s o i l s , w h i c h c o u l d e x p l a i n t h e absence o f s i g n i f i c a n t v a r i a b i l i t y i n laboratory-measured p e s t i c i d e degradation r a t e s . I n c o n t r a s t t o measuring OC o r v a l u e s , t h e measurement o f p e s t i c i d e degradation h a l f - l i v e s ( ) ^ s o i l s i s a much more d i f f i c u l t and time-consuming t a s k . Hence, t h e s p a t i a l v a r i a t i o n s i n t - i 2 c o u l d n o t be a s s e s s e d i n as much d e t a i l as we d i d w i t h OC and ί,. M e t o l a c h l o r and a l d i c a r b TTR h a l f - l i v e s were measured i n a s e l e c t e d number o f s o i l samples c o l l e c t e d from t h e G e o r g i a f i e l d site. S o i l samples were s e l e c t e d t o r e p r e s e n t t h e t h r e e major s o i l s e r i e s p r e s e n t a t t h e s i t e ( C l a r e n d o n , A r d i l l a , and T i f t o n ) and t h e f o u r major m o r p h o l o g i c s o i l h o r i z o n s (0-20, 25-46, 48-63, and 94-107 cm) w i t h i n t h e c r o p r o o t zone. S o i l samples c o l l e c t e d from 4 depths a t 10 s i t e s were u s e d t o c h a r a c t e r i z e a l d i c a r b TTR d e g r a dation rates. M e t o l a c h l o r h a l f - l i v e s were measured i n s o i l s t a k e n from f o u r d e p t h s a t one s i t e f o r each s o i l s e r i e s and a l s o i n samples c o l l e c t e d a t two depths (0-20 cm and 94-107 cm) a t 6 s i t e s . The d a t a f o r o b s e r v e d v a r i a t i o n s i n m e t o l a c h l o r t . ,^ a r e p r e s e n t e d i n T a b l e I . Of t h e t h r e e sampling s i t e s w i t n m t h e C l a r e n d o n s e r i e s , t h e s h o r t e s t d i s t a n c e between two s i t e s (G-13 t o K-15) was about 60 m, w h i l e t h e s i t e s K-15 and H-9 were t h e f a r t h e s t , l o c a t e d about 100 m from each o t h e r . Of t h e f i v e s a m p l i n g l o c a t i o n s , t h e g r e a t e s t s e p a r a t i o n d i s t a n c e was about 200 m between J-4 ( T i f t o n ) and J-18 ( A r d i l l a ) . M e t o l a c h l o r t , v a l u e s measured n

1

n


RAO ET A L .

A : ORGANIC CARBON

B : ALDICARB K


109


CONTENT

D

FIGURE 5. S p a t i a l v a r i a t i o n s i n s o i l o r g a n i c c a r b o n c o n t e n t (OC) and a l d i c a r b s o r p t i o n c o e f f i c i e n t (K. ) a t t h e F l o r i d a s i t e .

A : ORGANIC CARBON CONTENT

C : METOLACHLOR K

d

Β : ALDICARB K

d

D: DIURON K

d

FIGURE 6. S p a t i a l v a r i a t i o n s i n OC and K, v a l u e s f o r t h r e e p e s t i c i d e s ( a l d i c a r b , m e t o l a c h l o r , and d i u r o n ) at the Georgia s i t e .


110


i n a l l s u r f a c e s o i l s (0-20 cm) were e s s e n t i a l l y i d e n t i c a l . The t / v a l u e s a r e l a r g e r by about a f a c t o r o f 2 f o r t h e s u b s o i l s (25-46, 48-63, 94-107 cm) and t e n d t o be more v a r i a b l e (CV < 3 8 % ) . The m e t o l a c h l o r h a l f - l i v e s shown i n T a b l e I a r e s m a l l e r t h a n t h o s e r e p o r t e d by e a r l i e r workers ( 30, 31^ ). 1

?

Table I. V a r i a t i o n s i n Metolachlor Degradation H a l f - L i v e s ( d a y s ) Measured i n S o i l s from t h e G e o r g i a S i t e

S o i l Depth Increment Soil

0-20


(cm)

Series 25-46

48-63

94-107

C l a r e n d o n (G-13) C l a r e n d o n (H- 9) C l a r e n d o n (K-15) Ardilla (J-18) Ardilla (B-13) Tifton (J-4)

19 19 17 18 14 19

17 ^ N.D. N.D. 34 N.D. 31

44 N.D. N.D. 44 N.D. 39

41 35 20 53 68 41

Average CV (%)

18 11

27 33

42 7

43 38

* **

Code f o r s a m p l i n g Not d e t e r m i n e d

Table I I .

l o c a t i o n on an a r b i t r a r y

V a r i a t i o n s i n A l d i c a r b TTR H a l f - L i v e s ( d a y s ) Measured i n S o i l s from t h e G e o r g i a S i t e

S o i l Depth (cm)

S o i l Series Clarendon

0-20 25-46 48-63 94-107

42 51 51 67

Average *

grid

(26) (26) (26) (21)

53 (28)

Ardilla

45 44 47 54

(24) (46) (36) (30)

48 (30)

Average Tifton

37 28 39 63

(11) (25) (13) ( 5)

42 (34)

42 42 47 62

(21) (38) (26) (21)

48 (31)

numbers i n p a r e n t h e s i s a r e % CV

The measured v a r i a b i l i t y i n a l d i c a r b TTR d e g r a d a t i o n h a l f - l i v e s a r e summarized i n T a b l e I I . Data shown f o r C l a r e n d o n s o i l were a v e r a g e d o v e r 4 s i t e s , and t h e v a l u e s f o r A r d i l l a and T i f t o n s o i l s a r e averages f o r 3 s i t e s . Among t h e s a m p l i n g s i t e s , J-4 and J-18 were t h e f a r t h e s t a p a r t (about 233 m), whereas t h e s i t e s 1-15 and K-15 were t h e c l o s e s t (about 30 m). The g r e a t e s t



6.

RAO ET A L .

111


d i s t a n c e between two sampling s i t e s w i t h i n a s o i l s e r i e s was: 126 m f o r C l a r e n d o n ; 157 m f o r A r d i l l a ; and 195 m f o r T i f t o n . The v a r i a t i o n s i n a l d i c a r b TTR t - , ^ v a l u e s w i t h d e p t h and l o c a t i o n a r e somewhat l a r g e r (CV < 50%) t h a n t h o s e f o r m e t o l a c h l o r t - / ^ . However, t h e v a r i a t i o n i n TTR t - , ~ v a l u e s w i t h i n a s o i l s e r i e s a r e e q u a l t o o r l a r g e r t h a n t h e v a r i a t i o n s among t h e s e r i e s . An o v e r a l l average t- ,~ f o r a l d i c a r b TTR d e g r a d a t i o n i n t h e s e s o i l s ( T a b l e I I ) was 48 days (CV = 31 % ) . I t s h o u l d be n o t e d t h a t i n o u r s t u d y , s o i l samples c o l l e c t e d from t h e f i e l d were s t o r e d a t t h e same water c o n t e n t a t w h i c h t h e y were sampled. P e s t i c i d e d e g r a d a t i o n was measured i n t h e s e s o i l s under i d e n t i c a l e n v i r o n m e n t a l c o n d i t i o n s ( i . e . , s u b s t r a t e concen t r a t i o n , a e r a t i o n s t a t u s , t e m p e r a t u r e , and s o i l - w a t e r content). Thus, t h e l a c k o f s p a t i a l v a r i a b i l i t y i n p e s t i c i d e d e g r a d a t i o n rates suggests that the inherent c a p a c i t y o f the s o i l microorgan isms t o degrade t h e s e p e s t i c i d e s might be s i m i l a r a t t h e G e o r g i a site. T h i s does n o t n e c e s s a r i l y imply t h a t t h e a c t u a l i n s i t u d e g r a d a t i o n r a t e s would n o t v a r y s p a t i a l l y o r t e m p o r a l l y w i t h i n a field. L o c a l s o i l e n v i r o n m e n t a l f a c t o r s may be e x p e c t e d t o v a r y as a r e s u l t o f s p a t i a l and t e m p o r a l v a r i a t i o n s i n o t h e r s o i l p r o p e r t i e s as w e l l as v a r i a t i o n s i n c r o p / s o i l management p r a c t i c e s ( i r r i g a t i o n , r a i n f a l l , f e r t i l i z a t i o n , e t c . ) . Thus, a knowledge o f t h e r e l a t i o n s h i p between p e s t i c i d e d e g r a d a t i o n r a t e s and s o i l e n v i r o n m e n t a l f a c t o r s and t h e v a r i a b i l i t y o f t h e s e f a c t o r s i n a f i e l d i s necessary f o r p r e d i c t i n g the s p a t i a l v a r i a b i l i t y of p e s t i c i d e residue concentrations. We a r e n o t aware o f any pub l i s h e d data f o r assessing i n s i t u s p a t i a l v a r i a t i o n s i n p e s t i c i d e degradation rates. Such d a t a have been c o l l e c t e d f o r s p a t i a l v a r i a t i o n s i n d e n i t r i f i c a t i o n r a t e s ( 32, 33> ) . S o i l - w a t e r c o n t e n t (Θ) and t e m p e r a t u r e ( T ) a r e t h e two major s o i l environmental f a c t o r s that c o n t r o l p e s t i c i d e degradation rates. Walker ( 34 ) and Walker and Barnes ( 35 ) have p r o p o s e d a model f o r p e s t i c i d e p e r s i s t e n c e i n s o i l s . I n t h e i r model t h e Θ-depgndence o f t ^ ,^ i s d e s c r i b e d by a power f u n c t i o n ί^^,^ Α Θ , where A and Β a r e c o n s t a n t s ] , w h i l e t h e r e l a t i o n s h i p between t ^ y ^ and Τ i s d e s c r i b e d by t h e A r r h e n i u s e q u a t i o n . Walker and co-workers ( 36 - 40 ) d e t e r m i n e d t h e v a l u e s o f t h e n e c e s s a r y model p a r a m e t e r s from l a b o r a t o r y measurements o f p e s t i c i d e degrada t i o n under c o n t r o l l e d c o n d i t i o n s . These parameter v a l u e s and weather d a t a were used as model i n p u t s t o p r e d i c t t h e p e r s i s t e n c e of s e v e r a l p e s t i c i d e s i n f i e l d p l o t s a t s e v e r a l l o c a t i o n s . Walker e t a l . ( 41 ) summarized t h e r e s u l t s o f an i n t e r n a t i o n a l c o o p e r a t i v e s t u d y , i n which s i m a z i n e h e r b i c i d e d e g r a d a t i o n was measured i n s o i l s c o l l e c t e d from 21 l o c a t i o n s i n 11 c o u n t r i e s . As i n e a r l i e r s t u d i e s ( 36 - 40 ) , model parameters were e s t i m a t e d on t h e b a s i s l a b o r a t o r y s t u d i e s , and were used t o p r e d i c t s i m a z i n e p e r s i s t e n c e i n f i e l d p l o t s a t 16 l o c a t i o n s . They a l s o s i m u l a t e d s i m a z i n e d e g r a d a t i o n i n f i e l d p l o t s a t 5 o t h e r l o c a t i o n s f o r w h i c h companion l a b o r a t o r y d a t a were n o t a v a i l a b l e . I n a l l t h e s e s t u d i e s ( 36 =

41 ), t h e model g e n e r a l l y u n d e r e s t i m a t e d t h e amount o f p e s t i c i d e residues remaining i n t h e s o i l . Walker and co-workers c o n s i d e r e d t h e model t o be s u f f i c i e n t l y a c c u r a t e f o r p r a c t i c a l a p p l i c a t i o n s g i v e n t h e u n c e r t a i n t i e s i n measured p e s t i c i d e c o n c e n t r a t i o n s and the p o s s i b i l i t y t h a t processes other than m i c r o b i a l degradation ( e . g . , l e a c h i n g p a s t t h e s a m p l i n g depth; v o l a t i l i z a t i o n ; and


112


c h e m i c a l o r p h o t o l y t i c d e g r a d a t i o n ) may have been r e s p o n s i b l e f o r pesticide dissipation.


Summary We have attempted t o show t h a t b o t h i n t r i n s i c and e x t r i n s i c f a c t o r s c o n t r i b u t e t o t h e o b s e r v e d s p a t i a l v a r i a b i l i t y i n p e s t i c i d e concentrations. The method o f p e s t i c i d e a p p l i c a t i o n and subsequent t i l l a g e o p e r a t i o n s can have a major impact on t h e e x t r i n s i c v a r i a b i l i t y i n t h e measured p e s t i c i d e c o n c e n t r a t i o n s and f l u x e s . Such v a r i a b i l i t y needs t o be t a k e n i n t o a c c o u n t i n d e s i g n i n g s o i l s a m p l i n g s t r a t e g i e s and i n i n t e r p r e t i n g t h e f i e l d d a t a . Intrinsic v a r i a b i l i t y i n p e s t i c i d e s o r p t i o n and d e g r a d a t i o n p a r a m e t e r s appears t o be s m a l l (CV < 3 0 % ) . Because p e s t i c i d e s o r p t i o n c o e f f i c i e n t s a r e s t r o n g l y c o r r e l a t e d t o s o i l o r g a n i c carbon c o n t e n t s , t h e r e i s a c l o s e c o r r e s p o n d e n c e between t h e measured s p a t i a l d i s t r i b u t i o n s o f OC and K^. Thus, a f i e l d - a v e r a g e v a l u e may be e s t i m a t e d g i v e n t h e average OC v a l u e . P e s t i c i d e degradation rates i n s o i l samples c o l l e c t e d from d i f f e r e n t l o c a t i o n s and s u b j e c t e d t o i d e n t i c a l e n v i r o n m e n t a l c o n d i t i o n s were n o t v a r i a b l e . However, i n s i t u d e g r a d a t i o n r a t e s may v a r y as f u n c t i o n o f v a r i a t i o n s i n s o i l environmental c o n d i t i o n s . Acknowledgments F i n a n c i a l s u p p o r t f o r t h i s s t u d y was p r o v i d e d , i n p a r t , by t h e C o o p e r a t i v e Agreement No. CR-810464 between t h e U.S. E n v i r o n m e n t a l P r o t e c t i o n Agency and t h e U n i v e r s i t y o f F l o r i d a ; and a g r a n t from t h e Union C a r b i d e A g r i c u l t u r a l P r o d u c t s Co., I n c . A s s i s t a n c e o f R.F. C a r s e l and C.N. Smith, AERL, USEPA, Athens, GA; Ms. S.C. Cooper, USGS, A l b a n y , GA; and J . F . McNabb, RSKERL, USEPA, Ada, OK i n c o l l e c t i n g t h e s o i l samples a t t h e G e o r g i a s i t e i s acknowledged. R.L. Jones and R.R. Romine, U n i o n C a r b i d e , a s s i s t e d i n c o l l e c t i n g t h e s o i l samples a t t h e F l o r i d a s i t e and a n a l y z e d t h e samples f o r bromide ancjl^aldicarb TTR. We a r e g r a t e f u l t o Union C a r b i d e f o r Ojroviding C - a l d i c a r b and t o C i b a - G e i g y f o r s u p p l y i n g C - m e t o l a c h l o r used i n t h e s o r p t i o n and d e g r a d a t i o n s t u d i e s . F i n a l l y , we a p p r e c i a t e Ms. L i n d a Lee's a s s i s t a n c e i n d a t a a n a l y s i s and i n d r a f t i n g t h e f i g u r e s .

Literature Cited

1.

Cohen, S.Z.; Creeger, S.M.; Carsel, R.F.; Enfield, C.G. In "Treatment and Disposal of Pesticide Wastes"; Kreuger, R.F.; Seiber, J.N., Eds.; Symposium Series No. 259; American Chemical Society: Washington, 1984; pp. 297-325.

2.

Pye, V.; Patrick, R.; Quarels, J. Groundwater Contamination in the United States; University of Pennsylvania Press: Philadelphia, 1983; 315 p.


RAO

3. 4. 5. 6.

et al.


Carsel, R.F.; Smith, C.N.; Mulkey, L.A.; Dean, J.D.; Jowsie, P. EPA-600/3-84-109; U.S. Environmental Protection Agency: Athens, 1984; 216 p. Wagenet, R.J.; Rao, P.S.C. Weed Sci. 1986, 34, In Press. Nielsen, D.R.; Biggar, J.W.; Erh, K.T. Hilgardia 1973, 42, 215-260. Biggar, J.W.; Nielsen, D.R. Water Resour. Res. 1976, 12, 78-84.


7.

Nielsen, D. R.; Bouma, J. (Eds.). Soil Spatial Variability; Proc. Workshop of ISSS and SSSA; Pudoc: Wageningen, 1985; 243 p. 8. Carsel, R.F.; Smith, C.N.; Parrish, R.S.; Mulkey, L.A.; Payne, Jr., W.R. In "Evaluation of Pesticides in Groundwater"; Honeycutt, R.; Ed.; Symposium Series; American Chemical Society: Washington, 1986. 9. Cooper, S.C. In "Evaluation of Pesticides in Groundwater"; Honeycutt, R.; Ed.; Symposium Series; American Chemical Society: Washington, 1986. 10. Green, R.E.; Davidson, J.M.; Biggar, J.W. In "Agrochemicals in Soil"; Banin, Α.; Kafkafi, U.; Eds.; Pergamon Press: New York, 1980, pp. 73-80. 11. Rao, P.S.C.; Davidson, J.M. In "Environmental Impact of Nonpoint Source Pollution"; Overcash, M.R.; Davidson, J.M.; Eds.; Ann Arbor Science Publishers: Ann Arbor, 1980; pp. 23-67. 12. Wilson, J.T.; McNabb, J.F.; Balkwill, D.L.; Ghiorse, W.C. Groundwater 1983, 21, 134-142. 13. Ou, L.T.; Edvardsson, K.S.V.; Rao, P.S.C. J. Agric. Food Chem. 1985, 33, 72-78. 14. Rao, P.S.C.; Wagenet, R.J. Weed Sci. 1986, 34, In Press. 15. Clay, D.V.; Scott, K.G.; Weed Res. 1973; 13, 42-50. 16. Horrman, W.D.; Kardhuber, B.; Ramskiner, K.A.; Eberle, D.O. Proc. European Weed Res. Symp. 1973, pp. 129-140. 17. Polzin, W.J.; Brown, Jr., I.F.; Manthey, J.Α.; Probst, G.W. Pestic. Monit. J. 1971, 4, 209-215. 18. Robinson, E.L. Weed Sci. 1976, 24, 420-422.



114

EVALUATION

OF

PESTICIDES

IN

GROUND

19. Wauchope, R.D.; Chandler, J.M.; Savage, K.E. Weed Sci. 1977, 25, 193-196. 20. Taylor, A.W.; Freeman, H.P.; Edwards, W.M. J. Agric. Food Chem. 1971, 19, 832-836. 21. Thompson, Jr., L.T.; Skroch, W.A.; Beasley, E.O. Pesticide Incorporation: Distribution of Dye by Tillage Implements; North Carolina Agricultural Extension Service: Raleigh, 1981, 32 p. 22. Hornsby, A.G.; Rao, P.S.C.; Nkedi-Kizza, P.; Wheeler, W.B.; Jones, R.L. In "Characterization and Monitoring of the Vadose (Unsaturated) Zone"; Nielsen, D.M.; Curl, M.; Eds.; National Water Well Association: Worthington, 1983; pp. 936-958. 23. Rao, P.V.; Rao, P.S.C.; Davidson, J.M.; Hammond, L.C. Soil Sci. Soc. Amer. J. 1979, 43, 274-278. 24. Karickhoff, S.W. Chemosphere 1981, 10, 833-846. 25. Rao, P.S.C.; Nkedi-Kizza, P.; Davidson, J.M.; Ou, L.T. In "Agricultural Management and Water Quality"; Schaller, F.; Bailey, G.; Eds.; Iowa State Univ. Press: Ames, 1983; pp. 126-140. 26. Kenaga, E.E.;Goring, C.A.I. In "Proc. Third Aquatic Toxicology Symposium"; Eaton, E.G.; Parrish, P.R.; Hendricks, A.G.; Eds.; American Society for Testing and Materials: Philadelphia, 1980; pp. 78-115. 27. Green, R.E.; Cheng-Tsu, M.Y. ; Lee, CC Agron. Abstr. 1985, p. 25. 28. Ou, L.T.; Rao, P.S.C.; Wheeler, W.B. In "Estimation of Parameters for Modeling the Behavior of Selected Pesticides and Orthophosphate"; Rao, P.S.C.; Berkheiser, V.E.; and Ou, L.T.; Eds., EPA-600/3-84-019; U.S. Environmental Protection Agency: Athens, 1984; pp. 48-88. 29. Walker, Α.; Brown, P.A. Crop. Prot. 1983, 2, 17-25. 30. Walker, Α.; Zimdhal, R.L. Weed Res. 1981, 21, 255-265. 31. Bouchard, D.C.; Lavy, T.L.; Marx, D.B. Weed Sci. 1982, 30, 629-632. 32. Ryden, J.C.; Lund, L.J. J. Environ. Qual., 1980, 9, 387-393. 33. Ryden, J.C.; Lund, L.J. Soil Sci. Soc. Amer. J., 1980, 44, 505-511.


WATER


6.RAOETAL.


34. 35. 36. 37. 38. 39. 40. 41.

Walker, A. J. Environ. Qual. 1974, 3, 396-401. Walker, Α.; Barnes, A. Pest. Sci. 1981, 123-132. Walker, A. Pest. Sci. 1976, 7, 41-49. Walker, A. Pest. Sci. 1976, 7, 50-58. Walker, A. Pest. Sci. 1976, 7, 59-64. Smith, A.E.; Walker, A. Pest. Sci. 1977, 8, 449-456. Walker, A. Weed Res. 1978, 18, 305-313. Walker, Α.; Hance, R.J.; Allen, J.G.; Briggs, G.G.; Chen, Y-L.; Gaynor, J.D.; Hogue, E.J.; Malquor, Α.; Moody, K.; Moyer, J.R.; Pestemer, W.; Rahman, Α.; Smith, A.E.; Streibig, J.C.; Torstensson, N.T.L.; Widyanato, L.S.; Zandvoort, Z. Weed Res. 1983, 23, 373-383. RECEIVED April 7, 1986


Spatial Variability of Pesticide Sorption and Degradation Parameters

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