Evaluation of Pesticides in Ground Water - American Chemical Society

2 Soil Characteristics Affecting Pesticide Movement into Ground Water Charles S. Helling and Timothy J. Gish

Downloaded by PURDUE UNIVERSITY on May 23, 2013 | http://pubs.acs.org Publication Date: July 17, 1986 | doi: 10.1021/bk-1986-0315.ch002

Agricultural Research Service, U.S. Department of Agriculture, Beltsville, M D 20705

Processes that modify convective transport of pesticides through soil and into groundwater include adsorption/desorption, degradation, volatilization, runoff, and plant uptake. These processes, in turn, are affected by soil characteristics, climate, pesticide properties, and agricultural practices. A screening model based on the convection-dispersion equation (assuming 1st-order degradation) was used to rank several soil properties that may affect atrazine leaching. Transport was most retarded by low hydraulic conductivity and high soil organic matter content; increased bulk density attenuated leaching to a lesser extent. A literature survey, with emphasis on atrazine, aldicarb, and DBCP (pesticides that have leached to groundwater), tended to confirm that sandy soils (with high hydraulic conductivity and low organic matter) were usually associated with leaching. Restricted drainage has led to lateral subsurface movement or occurrence of residues in perched groundwater. At the other extreme, karst topography allowed rapid recharge and high probability of pesticide leaching. Groundwater i s e s t i m a t e d t o s u p p l y 40-50% o f U.S. d r i n k i n g water needs, and c o n s t i t u t e s at l e a s t p a r t o f t h e water s o u r c e f o r 75% o f American c i t i e s ( 1 ) . About 95% o f t h e r u r a l p o p u l a t i o n depends on groundwater f o r t h e i r d r i n k i n g water. R e l i a n c e on groundwater f o r domestic consumption and a g r i c u l t u r a l uses becomes i n c r e a s i n g l y important i n t h e more a r i d Western s t a t e s . Groundwater c o n t a m i n a t i o n i n t h e U.S. was r e v i e w e d by Pye e t a l . i n 1983 (1)· The most common s o u r c e s o f such c o n t a m i n a t i o n i n c l u d e d human and a n i m a l wastes, i n d u s t r i a l w a s t e s , p e t r o l e u m p r o d u c t s , l a n d f i l l l e a c h a t e , and ( a l o n g c o a s t a l r e g i o n s ) s a l t w a t e r i n t r u s i o n . Cont a m i n a t i o n from t h e a g r i c u l t u r a l use o f p e s t i c i d e s was a p p a r e n t l y f a r l e s s common. N e v e r t h e l e s s , by 1984, Cohen e t a l . (2) r e p o r t e d t h a t This chapter not subject to U.S. copyright. Published 1986, American Chemical Society

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

2.

H E L L I N G A N D GISH

Soil Characteristics Affecting Pesticide Movement

15


12 p e s t i c i d e s were found i n groundwaters o f 18 s t a t e s as a c o n s e quence o f such u s e . T y p i c a l p o s i t i v e r e s i d u e l e v e l s v a r i e d w i d e l y , a l t h o u g h most commonly t h e y ranged from c a . 1-100 ppb. The e x t e n t o f p e s t i c i d e l e a c h i n g depends on a c o m b i n a t i o n o f f a c t o r s that include the physicochemical c h a r a c t e r i s t i c s o f the pesticide (or i t s degradation products), s o i l properties, climate, and agronomic f a c t o r s such as t h e t i m i n g , r a t e , and method o f p e s t i c i d e a p p l i c a t i o n , t h e use o f i r r i g a t i o n , and t h e i n f l u e n c e o f c r o p c o v e r . Whether a c t u a l c o n t a m i n a t i o n o f groundwater o c c u r s i s a l s o i n f l u e n c e d by t h e depth t o groundwater and t h e p e r m e a b i l i t y o f overlying s o i l . In t h i s paper, we w i l l f i r s t f o c u s on s o i l f a c t o r s t h a t a f f e c t p e s t i c i d e l e a c h i n g through t h e r o o t zone and i n t o t h e s u b s o i l . The general c o n v e c t i o n - d i s p e r s i o n equation with f i r s t - o r d e r degradation w i l l be used t o c h a r a c t e r i z e t h e e f f e c t s o f v a r i o u s s o i l p r o p e r t i e s on a t r a z i n e movement. I n t h e remainder o f t h e paper, we w i l l d i s c u s s l a b o r a t o r y and f i e l d experiments t h a t focus on a d s o r p t i o n and l e a c h ing. G e n e r a l i z a t i o n s from t h e s e s t u d i e s w i l l be compared w i t h s o i l p r o p e r t i e s at s i t e s o f known groundwater c o n t a m i n a t i o n . F i n a l l y , the r e s u l t s from t h e t r a n s p o r t model a n a l y s i s w i l l be used i n c o n j u n c t i o n w i t h t h e l i t e r a t u r e r e v i e w t o propose a h i e r a r c h i c a l r a n k i n g o f properties a f f e c t i n g leaching. Background:

Soil

Properties

S o i l s c a n be c h a r a c t e r i z e d i n many ways, depending, f o r example, on whether p r i m a r y c o n c e r n r e l a t e s t o agronomic a p p l i c a t i o n s , e n g i n e e r ing u t i l i t y , o r s o i l g e n e s i s . From t h e s t a n d p o i n t o f p r e d i c t i n g p e s t i c i d e t r a n s p o r t , a s e r i e s o f p h y s i c a l , c h e m i c a l , and b i o l o g i c a l properties some t r a n s i e n t c o u l d be c o n s i d e r e d . For convenience, we have l i s t e d many such parameters i n o u t l i n e form w i t h i n T a b l e I . Numerous r e f e r e n c e s d e s c r i b e them and t h e i r a n a l y s i s (3-7). In a d d i t i o n t o t h e c l a s s i f i c a t i o n o f p r o p e r t i e s i n T a b l e I , t h e s i t e c a n be c h a r a c t e r i z e d f u r t h e r as i n T a b l e I I . S u r f a c e o r s u b s u r f a c e d r a i n a g e systems, i f p r e s e n t , would be important additional descriptions. Background:

Other R e l a t e d

Properties

M e t e r e o l o g i c a l c o n d i t i o n s a f f e c t t r a n s p o r t o f water and s o l u t e s s i n c e , i n t h e absence o f i r r i g a t i o n , t h e y determine how much water r e a c h e s t h e s o i l s u r f a c e , what t h e i n t e n s i t y and f r e q u e n c y o f t h a t p r e c i p i t a t i o n i s , and how much water i s r e c y c l e d from t h e s o i l v i a évapotranspiration l o s s e s . Temperature i n f l u e n c e s t h e r a t e o f p e s t i c i d e d e g r a d a t i o n and t h e r a t e s o f water and p e s t i c i d e v o l a t i l i z a t i o n . P r o p e r t i e s o f t h e p e s t i c i d e s t r o n g l y a f f e c t the tendency t o l e a c h and degrade, but a r e a d d r e s s e d elsewhere i n t h i s symposium. A g r i c u l t u r a l f a c t o r s such as t h e manner and t i m i n g o f p e s t i c i d e a p p l i c a t i o n and the c r o p p i n g t i l l a g e p r a c t i c e have p o t e n t i a l impact on t h e u l t i m a t e fate of a chemical. These v a r i o u s n o n s o i l f a c t o r s a r e l i s t e d i n Table I I I .


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

16

Table


A.

I.

Classification

o f Some S o i l

P h y s i c a l composition 1. S o i l t e x t u r e (% Sand, s i l t , 2. S o i l organic matter content

Properties

clay; gravel) (% OM)

B.

Chemical c o m p o s i t i o n 1. Clay mineralogy 2. O r g a n i c m a t t e r type

C.

Physical properties 1. Bulk d e n s i t y 2. F i e l d moisture capacity 3. Hydraulic conductivity 4. Pore s i z e d i s t r i b u t i o n , macropores; tendency t o c r a c k on d r y i n g

D.

Chemical p r o p e r t i e s 1. pH 2. Cation-exchange c a p a c i t y 3. % Base s a t u r a t i o n 4. Redox p o t e n t i a l , Eh

E.

F.

Transient s o i l properties 1. S o i l moisture content 2. S o i l temperature

(CEC); anion-exchange

capacity

(volumetric)

Biological/biochemical properties 1. Number and type o f m i c r o o r g a n i s m s 2. A c t i v i t y o f s p e c i f i c enzymes

Table

II.

Classification

o f Some Macro S o i l

Properties

A.

Surface 1. Relief 2. Slope

B.

Subsurface 1. P r o f i l e changes ( t y p e , d e p t h , and a r e a l homogeneity) 2. Restricting layers 3. Depth t o groundwater ( p e r c h e d and u n c o n f i n e d aquifer)


2.

HELLING AND GISH

Table I I I .


A.

Β.


C l a s s i f i c a t i o n of Nonsoil Factors P o t e n t i a l l y T r a n s p o r t t o Groundwater

Climate 1. R a i n f a l l (temporal d i s t r i b u t i o n , 2. Temperature 3. E v a p o t r a n s p i r a t i o n

17

Affecting

intensity)

Pesticide properties 1. S o i l a d s o r p t i o n c o e f f i c i e n t (K) 2. Water s o l u b i l i t y 3. O c t a n o l : water p a r t i t i o n c o e f f i c i e n t ( K ) 4. I o n i z a t i o n c o n s t a n t ( p K , pK^) 5. C h e m i c a l and b i o l o g i c a l s t a b i l i t y ( p e r s i s t e n c e i n s o i l s ) 6. V o l a t i l i t y o w

a

C.

Pesticide application 1. Formulation 2. Method o f a p p l i c a t i o n ( f o l i a r , s o i l s u r f a c e , s o i l incorporât ion) 3. Rate 4. Timing 5. H i s t o r y o f p e s t i c i d e use ( a c c e l e r a t e d d e g r a d a t i o n ; buildup)

D.

Agricultural practices 1. Cropland a) Conventional t i l l a g e b) Conservation t i l l a g e c) Irrigation 2. Noncropland a) Fallow b) Rangeland, f o r e s t s , e t c . 3. S o i l amendments


18


Processes A f f e c t i n g Leaching The d i s t r i b u t i o n o f p e s t i c i d e s throughout the s o i l p r o f i l e , as a f u n c t i o n o f t i m e , r e p r e s e n t s the i n t e g r a t i o n o f p r o c e s s e s such as mass flow, d i f f u s i o n , a d s o r p t i o n / d e s o r p t i o n , d e g r a d a t i o n , v o l a t i l i z a t i o n , r u n o f f , and p l a n t uptake ( t h e l a t t e r , m a i n l y as i t a f f e c t s water movement i n the r o o t z o n e ) . These have been the s u b j e c t o f many r e v i e w s ( 8 - 1 2 ) , and t h e r e f o r e o n l y l i m i t e d a t t e n t i o n w i l l be g i v e n t o the s u b j e c t i n the f o l l o w i n g s e c t i o n s .


P e s t i c i d e Transport

Model

M o d e l s f o r d e s c r i b i n g p e s t i c i d e t r a n s p o r t on a f i e l d s c a l e g e n e r a l l y f a l l i n t o one o f two c a t e g o r i e s , d e t e r m i n i s t i c or s t o c h a s t i c . Determ i n i s t i c models seek t o account f o r p e s t i c i d e l e a c h i n g by d e s c r i b i n g the mechanisms g o v e r n i n g v o l a t i l i z a t i o n , a d s o r p t i o n , degradation, c o n v e c t i o n and d i f f u s i o n , w h i l e at the same time a c c o u n t i n g f o r the p h y s i c a l and c h e m i c a l c h a r a c t e r i s t i c s o f the s o i l medium. Due t o the complex n a t u r e o f such an i n t e r a c t i o n , i t i s o f t e n n e c e s s a r y t o make assumptions which are at b e s t a f i r s t a p p r o x i m a t i o n o f what o c c u r s under f i e l d c o n d i t i o n s . S t o c h a s t i c models assume t h a t a l t h o u g h p e s t i c i d e movement on a s m a l l homogeneous s c a l e obeys c e r t a i n p h y s i c a l laws, the random component a s s o c i a t e d w i t h those laws i n a heterogeneous system w i l l override t h e i r deterministic behavior. Consequently, transport i s a s c e r t a i n e d by e v a l u a t i n g t r a n s f e r f u n c t i o n models or by e v a l u a t i n g the p r o b a b i l i t y d i s t r i b u t i o n o f some t r a n s p o r t p r o c e s s ( 1 3 ) . The p r o b a b i l i t y d e n s i t y f u n c t i o n (PDF) f o r a s p e c i f i c t r a n s p o r t process, on a f i e l d s i t e , can be combined w i t h d e t e r m i n i s t i c t h e o r y t o account for s p a t i a l heterogeneity (14-18). However, a s o l e l y s t o c h a s t i c model f o r s c r e e n i n g p e s t i c i d e s may be o f l i t t l e v a l u e f o r s e v e r a l reasons. F i r s t , the PDF f o r a s p e c i f i c s o i l p r o p e r t y would r e q u i r e the a n a l y s i s o f numerous s o i l samples. Second, a p u r e l y s t o c h a s t i c model l a c k s the a b i l i t y o f p r e d i c t i n g the l o c a t i o n o f p o t e n t i a l l y h a z a r d o u s areas w i t h i n a f i e l d . Third, i f large variations in s o i l or s o i l water p r o p e r t i e s are p r e s e n t , t h e y may a f f e c t the r e l a t i v e b e h a v i o r o f most p e s t i c i d e s i n a s i m i l a r manner. Our t h e o r e t i c a l development o f a s c r e e n i n g model w i l l focus on the d e t e r m i n i s t i c approach s i n c e i t may s t i l l be a p p l i e d i n a semis t o c h a s t i c manner. T h i s model i s c o n d u c i v e to a n a l y z i n g the e f f e c t of a s p e c i f i c s o i l property while holding others constant. Our t h e o r e t i c a l c o n s i d e r a t i o n o f p e s t i c i d e t r a n s p o r t b e g i n s ( E q u a t i o n 1) w i t h an e x p r e s s i o n o f the r a t e at which water moves through s o i l , where J i s the water f l u x (volume o f water f l o w i n g through a c r o s s s e c t i o n o f a r e a per t i m e ) , 1// i s the m a t r i c p o t e n t i a l , VH i s the h y d r a u l i c g r a d i e n t , and K(y) i s the h y d r a u l i c c o n d u c t i v i t y . w

J

w

= -KOf)VH

F r e q u e n t l y termed the Buckingham-Darcy e q u a t i o n , E q u a t i o n 1 be used t o d e s c r i b e p e s t i c i d e movement through a s o i l volume by employing mass b a l a n c e e q u a t i o n s f o r b o t h water and p e s t i c i d e


(1)

may

2.


19


VJ

W

= 0

mass b a l a n c e f o r water

(2a)

9t

+

V J

= S

φ

mass b a l a n c e f o r p e s t i c i d e

(2b)

3t where C i s t h e volume-averaged p e s t i c i d e c o n c e n t r a t i o n , J i s t h e s o l u t e f l u x , θ i s t h e v o l u m e t r i c water c o n t e n t , and φ i s a r e a c t i o n term d e s c r i b i n g t h e s t a b i l i t y o r t h e r a t e o f p l a n t uptake f o r a particular pesticide. The s o l u t e f l u x o f t h e p e s t i c i d e c o n s i s t s o f two terms. The f i r s t term c o r r e s p o n d s t o t h e c o n v e c t i v e o r b u l k t r a n s p o r t o f t h e p e s t i c i d e w i t h t h e moving s o i l s o l u t i o n ; t h e second, a d i f f u s i o n d i s p e r s i o n term, a c c o u n t s f o r t h e random t h e r m a l m o t i o n o f t h e p e s t i c i d e m o l e c u l e s (19) as w e l l as any hydrodynamic d i s p e r s i o n t h a t may o c c u r due t o v a r i a t i o n s i n t h e pore water v e l o c i t y ( 2 0 ) . The mathe matical representation of J i s


r

s

g

D7C

(3)

r

where D i s t h e d i f f u s i o n - d i s p e r s i o n c o e f f i c i e n t . Combining E q u a t i o n s 2a, 2b, and 3, a s o l u t e t r a n s p o r t e q u a t i o n i n one d i m e n s i o n c a n be w r i t t e n where

3 r c

R

9t

2

^ C 3C = D ~ - V — r

3χ

3χ2

r

- ruC r

(4a)

r

where

R = l

+

! ^

(4b)

and S = KC

(4c)

r

Here, i s the s o i l bulk d e n s i t y , μ i s the f i r s t - o r d e r d e g r a d a t i o n coefficient, i s the d i s t r i b u t i o n c o e f f i c i e n t f o r the s o i l / w a t e r phases, V i s t h e average pore water v e l o c i t y (V = J / 8 ) , χ i s t h e s o i l d e p t h , S i s t h e adsorbed c o n c e n t r a t i o n p e r u n i t o f mass, and R i s a dimensionless v a r i a b l e . The assumption o f a l i n e a r a d s o r p t i o n i s o t h e r m , = Κ in E q u a t i o n 4b, may be v a l i d under low s o l u t i o n c o n c e n t r a t i o n s ( 2 1 ) . However, Rao and D a v i d s o n (22) showed t h a t t h i s assumption c o u l d produce e r r o r s w i t h i n a f a c t o r o f 2 o r 3. A d d i t i o n a l l y , Κ c a n be e s t i m a t e d from s o i l OM c o n t e n t ( 2 2 ) . C o n s e q u e n t l y , t h e s o l u t i o n w


EVALUATION OF PESTICIDES IN GROUND WATER

20

o f E q u a t i o n 4a, s u b j e c t e d t o a p p r o p r i a t e i n i t i a l and boundary c o n d i t i o n s , can be used t o p e r f o r m a s e n s i t i v i t y a n a l y s i s on the e f f e c t o f f^, OM and J on p e s t i c i d e movement. To s o l v e E q u a t i o n 4a, boundary c o n d i t i o n s are imposed t h a t d e s c r i b e the i n i t i a l s o i l c o n d i t i o n s w i t h r e s p e c t t o the p e s t i c i d e and the method o f c h e m i c a l a p p l i c a t i o n . I n i t i a l l y , t h e r e may be some f i n i t e c o n c e n t r a t i o n i n the s o i l due t o the p r e v i o u s y e a r ( s ) o f pes t i c i d e a p p l i c a t i o n . T h i s r e s i d u a l c o n c e n t r a t i o n w i l l be denoted i n the s o l u t i o n as (see Appendix I ) . I f the p e s t i c i d e i s a p p l i e d as a one-time a p p l i c a t i o n (per growing s e a s o n ) , a p u l s e boundary c o n d i t i o n at the s u r f a c e and a f l u x bottom boundary c o n d i t i o n are well s u i t e d (23). The s o l u t i o n o f E q u a t i o n 4a w i t h t h e s e r e s t r i c t i o n s i s g i v e n by van Genuchten and A l v e s ( 2 4 ) . A l t h o u g h the d e t e r m i n i s t i c model p r e s e n t e d assumes s t e a d y s t a t e c o n d i t i o n s , l a b o r a t o r y s t u d i e s have shown t h a t s o l u t e t r a n s p o r t under t r a n s i e n t f l o w c o n d i t i o n s may be approximated by assuming an e q u i v a l e n t u n i f o r m water f l u x and water c o n t e n t ( 2 5 - 6 ) .


w

Model Assumptions The model p r e s e n t e d can be used f o r s c r e e n i n g n o n v o l a t i l e p e s t i c i d e s under v a r i o u s f i e l d c o n d i t i o n s . The assumptions b e h i n d i t s d e r i v a t i o n should be c l e a r l y s t a t e d , as we attempt t o do f o r the f o l l o w i n g a t t e n u a t i o n and t r a n s p o r t mechanisms. A d s o r p t i o n . The s o l u t i o n used t o e v a l u a t e the p e s t i c i d e t r a n s p o r t e q u a t i o n , E q u a t i o n 4a, assumes a l i n e a r a d s o r p t i o n i s o t h e r m t h a t i s c o n s t a n t w i t h d e p t h . However, l i n e a r i t y may not be the case f o r some p e s t i c i d e s and the a d s o r p t i o n c o e f f i c i e n t w i l l almost never be constant with depth. The r a t i o n a l e f o r u s i n g a l i n e a r model i s i n i t i a l l y based on the F r e u n d l i c h i s o t h e r m 1

S = KC /

11

(5)

where Κ i s the F r e u n d l i c h c o n s t a n t , and 1/n i s an exponent t h a t g e n e r a l l y ranges between c a . 0.5 and 1.2. I f 1/n i s assumed t o be 1, the r e s u l t i n g e q u a t i o n i s l i n e a r , i . e . , Κ = i n E q u a t i o n 4c. Al though the v a l i d i t y i s s t i l l under d i s c u s s i o n , K a r i c k h o f f (21) c o n s i dered t h a t f o r the low s o l u t i o n c o n c e n t r a t i o n s t y p i c a l l y a s s o c i a t e d w i t h p e s t i c i d e s , the l i n e a r model i s a p p r o p r i a t e . Rao and D a v i d s o n (22) showed t h a t the assumption o f l i n e a r i t y c o u l d produce e r r o r s w i t h i n a f a c t o r o f 2 or 3. The v a l u e o f Κ i s c r i t i c a l s i n c e i t i n d i c a t e s the p r o p o r t i o n o f p e s t i c i d e i n the m o b i l e water phase. Κ has o f t e n been used t o p r e d i c t the e x t e n t o f l e a c h i n g by assuming o n l y c o n v e c t i v e movement and a d s o r p t i o n i n a r e t a r d a t i o n f a c t o r , R, as i n E q u a t i o n 4b (14, 27-8). S i n c e the a d s o r p t i o n c o e f f i c i e n t i s c r i t i c a l t o the t h e o r e t i c a l development, c a u t i o n s h o u l d be e x e r c i s e d i n u s i n g a p a r t i c u l a r Κ v a l u e f o r a p a r t i c u l a r s o i l and p e s t i c i d e . The most common method used t o measure a d s o r p t i o n i s by the 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 , i n which s o i l samples are e q u i l i b r a t e d w i t h a s e r i e s o f p e s t i c i d e c o n centrations. However, the e q u i l i b r i u m time i s c r i t i c a l , and may not r e p r e s e n t a d s o r p t i o n under f i e l d c o n d i t i o n s where the p e s t i c i d e i s moving i n the s o l u t i o n phase. C o n s e q u e n t l y , f l o w e q u i l i b r i u m methods have a l s o been d e v e l o p e d ( 2 9 ) .


2.



21

The a d s o r p t i o n Κ c a n a l s o be e s t i m a t e d from t h e s o i l OM c o n t e n t . O r g a n i c m a t t e r has been shown t o be a p r i m a r y s i t e f o r a d s o r p t i o n ( u n l e s s t h e p e s t i c i d e i s permanently c h a r g e d ) . As a r e s u l t , t h e a d s o r p t i o n c o e f f i c i e n t may be approximated by t h e e q u a t i o n Κ = £

o

K

c

(6)

o c

where K i s t h e c o e f f i c i e n t o f l i n e a r a d s o r p t i o n n o r m a l i z e d on o r g a n i c carbon and f i s the f r a c t i o n a l content o f organic carbon ( 2 2 ) . K c a n o f t e n be o b t a i n e d from p r e v i o u s l y p u b l i s h e d v a l u e s o r e s t i m a t e d from t h e o c t a n o l : w a t e r p a r t i t i o n c o e f f i c i e n t (30). One advantage o f employing E q u a t i o n 6 i s t h a t i t a l l o w s one t o model t h e e f f e c t o f o r g a n i c m a t t e r on t r a n s p o r t . A d d i t i o n a l l y , the use o f E q u a t i o n 6 has o f t e n r e s u l t e d i n r e d u c i n g t h e c o e f f i c i e n t o f v a r i a t i o n a s s o c i a t e d w i t h Κ (22, 31-2). Thus, E q u a t i o n 6 w i l l be employed i n s o l v i n g E q u a t i o n 4 a . o c

Q

C


Q C

D i f fus ion-D i s p e r s i o n . A d i f f u s i o n - d i s p e r s i o n c o e f f i c i e n t was used i n t r a n s p o r t E q u a t i o n 4. Depending upon t h e water v e l o c i t y , e i t h e r d i f f u s i o n o r d i s p e r s i o n would be t h e d o m i n a t i n g mechanism. F o r r e l a t i v e l y m o b i l e c h e m i c a l s , t h e v a r i a b i l i t y i n D may be l i n k e d d i r e c t l y to t h e water v e l o c i t y ( 3 3 - 4 ) . C o n s e q u e n t l y , D would be dominated by hydrodynamic d i s p e r s i o n w i t h D = SV, where d i s p e r s i v i t y ε ranges from 0.1-4 cm ( 3 5 ) . On t h e o t h e r hand, d i f f u s i o n may be t h e dominant mech anism c o n t r o l l i n g t h e magnitude o f D, e s p e c i a l l y i f water movement i s slow. I n a f i e l d s e t t i n g , t h e time between p r e c i p i t a t i o n e v e n t s w i l l be much g r e a t e r than t h e d u r a t i o n o f p r e c i p i t a t i o n e v e n t s , a l l o w i n g more time f o r d i f f u s i o n t h a n d i s p e r s i o n . T h i s being the case, the d i f f u s i o n c o e f f i c i e n t c a n be e s t i m a t e d by u s i n g the M i l l i n g t o n and Q u i r k t o r t u o s i t y model (36)

D = (θ

1 Ο / 3

2

/0 ) D

(7)

w a t e r

where 0 i s t h e p o r o s i t y o f t h e b u l k s o i l and D c o e f f i c i e n t i n water; f o r most p e s t i c i d e s , D mated as 4.3 X 1 0 ~ m day" . 5

2

i s the d i f f u s i o n c a n be a p p r o x i

w a t e r

w a t e r

1

Convection. Convection i s the bulk transport o f p e s t i c i d e with the moving s o i l s o l u t i o n . C o n s e q u e n t l y , t h e water v e l o c i t y i s t h e major mechanism g o v e r n i n g c o n v e c t i v e t r a n s p o r t . Numerous s t u d i e s have shown t h a t a s t o c h a s t i c r e p r e s e n t a t i o n o f t h e water v e l o c i t y does a b e t t e r job o f d e s c r i b i n g chemical t r a n s p o r t (14-8). However, f o r a s c r e e n i n g mode, an a c c u r a t e d e p i c t i o n o f t h e water v e l o c i t y i s n o t e s s e n t i a l s i n c e v e l o c i t y v a r i a t i o n s would a f f e c t t h e r e l a t i v e b e h a v i o r o f most c h e m i c a l s i n a s i m i l a r manner once t h e p a r t i t i o n c o e f f i c i e n t between t h e l i q u i d and adsorbed phases has been e s t a b lished. P r e d i c t i o n s o f t h e average water v e l o c i t y ( e x p e c t e d o r mean v a l u e ) have been made f o r f i e l d experiments by m o n i t o r i n g meteoro l o g i c a l events and s u b t r a c t i n g e s t i m a t e s o f t h e évapotranspiration from t h e water i n p u t s ( 3 3 ) .



22


Dégradât i o n . P e s t i c i d e d e g r a d a t i o n i s a complex phenomenon s i n c e the p r o c e s s may be p u r e l y c h e m i c a l or dependent upon the p r e s e n c e o f microorganisms. The assumption o f f i r s t - o r d e r k i n e t i c s may be m i s l e a d i n g s i n c e the d e g r a d a t i o n r a t e w i l l depend upon temperature as w e l l as the p a r t i c u l a r phases i n which the p e s t i c i d e r e s i d e s . Thus, a s p e c i f i c p e s t i c i d e may degrade by d i f f e r e n t mechanisms at d i f f e r e n t rates. Assuming i s o t h e r m a l c o n d i t i o n s , the v o l u m e t r i c water c o n t e n t w i l l d i c t a t e the o v e r a l l r a t e o f d e g r a d a t i o n ( 3 7 ) . The water c o n t e n t a f f e c t s both a e r a t i o n as w e l l as the f r a c t i o n o f p e s t i c i d e u n d e r g o i n g d e g r a d a t i o n i n the s o l u t i o n phase. In a d d i t i o n , the s o i l pH w i l l a l s o a f f e c t the r a t e o f d e g r a d a t i o n f o r some p e s t i c i d e s ( 3 8 ) . Assuming a f i r s t - o r d e r d e g r a d a t i o n p r o c e s s , the d e g r a d a t i o n r a t e may be measured a c c o r d i n g t o μ = In ( C / C ) t "

1

(8a)

0

or e s t i m a t e d from p u b l i s h e d v a l u e s o f t\/2> p a r t i c u l a r p e s t i c i d e , as i n E q u a t i o n 8b μ » In (1/2)

P e s t i c i d e Transport

t

1

/

~

9

1

t

n

e

half-life

= 0.693 t ^ "

of a

1

(8b)

Simulations

The r e l a t i v e importance o f f^, 0M, and J on p e s t i c i d e movement was a c c o m p l i s h e d by c o n d u c t i n g a s e r i e s o f computer s i m u l a t i o n s and sub j e c t i n g the r e s u l t s t o a s e n s i t i v i t y a n a l y s i s . The s e n s i t i v i t y a n a l y s i s compares the peak c o n c e n t r a t i o n s i n the l i q u i d phase, s i n c e t h i s phase w i l l be the major v e h i c l e f o r p e s t i c i d e t r a n s p o r t t o groundwater. The a t r a z i n e a p p l i c a t i o n r a t e used i n the s i m u l a t i o n s was e q u i v a l e n t t o 2.8 kg/ha o f a c t i v e i n g r e d i e n t . The d i f f u s i o n d i s p e r s i o n c o e f f i c i e n t was assumed t o be c o n s t a n t , 1 cm day" . The ranges chosen f o r 9^, 0M, J , and θ i n the s i m u l a t i o n s were 0.81.45 g m~3, 1-5%, 1-4 cm d a y " , and 0.15-0.35 m^ water/m^ s o i l , r e spectively. These ranges c o r r e s p o n d t o t y p i c a l f i e l d v a l u e s . Since a f a m i l y o f c u r v e s was g e n e r a t e d , o n l y a few r e p r e s e n t a t i v e c u r v e s w i l l be shown, F i g u r e l a - c . So t h a t v i s u a l comparisons can be made between d i f f e r e n t s i m u l a t i o n s , the a t r a z i n e p r o f i l e c o r r e s p o n d i n g t o ?b = 0.8, θ = 0.15, and J = 1 cm d a y " was used i n F i g u r e l a - c . A l l s i m u l a t i o n s assume t h a t 20 days have t r a n s p i r e d s i n c e a t r a z i n e a p p l i c a t i o n and t h a t μ = 0.0098 d a y " (27, 39). In F i g u r e l a the e f f e c t o f an i n c r e a s e d b u l k d e n s i t y on the a t r a z i n e c o n c e n t r a t i o n p r o f i l e s i s shown, w h i l e h o l d i n g $ J , and 0M c o n s t a n t . The s o i l b u l k d e n s i t y o f 0.8 g cm~3 c o r r e s p o n d s t o a v e r y l i g h t s o i l a n d ( o r ) a s o i l t h a t has been r e c e n t l y plowed, w h i l e ?b 1.45 g cm~3 r e p r e s e n t s a s o i l t h a t has a n a t u r a l h i g h d e n s i t y or a s o i l t h a t has been compacted by farm implements. As the s o i l d e n s i t y i n c r e a s e s , the maximum p e s t i c i d e c o n c e n t r a t i o n i n the l i q u i d phase d e c r e a s e s . A d d i t i o n a l l y , the maximum p e s t i c i d e c o n c e n t r a t i o n w i l l o c c u r c l o s e r t o the s o i l s u r f a c e as 9^ i n c r e a s e s . S i n c e Pfc i n c r e a s e s w i t h depth f o r most a g r i c u l t u r a l s o i l s , F i g u r e l a i n d i c a t e s t h a t p e s t i c i d e movement would be more r e t a r d e d as i t moves t h r o u g h the s o i l p r o f i l e , a l l o t h e r f a c t o r s h e l d c o n s t a n t . S i n c e the c o e f f i c i e n t s o f v a r i a t i o n f o r ^ are g e n e r a l l y between 5-10%, assuming a c o n s t a n t d e n s i t y c o u l d i n t r o d u c e e r r o r s w i t h i n a f a c t o r o f 2. w

2

w

1

ν

1

1

ν

w

1

v t

=


w

2.


23


0.8

g cm-3

1.2 g

1.45

cm"

g

3

cm"

3


10h

Evaluation of Pesticides in Ground Water - American Chemical Society

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