Principles Governing Environmental Mobility and ... - ACS Publications

Chapter 8

Principles Governing Environmental Mobility and Fate James N. Seiber

Downloaded by UNIV OF SYDNEY on September 26, 2015 | http://pubs.acs.org Publication Date: March 12, 1987 | doi: 10.1021/bk-1987-0336.ch008

Department of Environmental Toxicology, University of California, Davis, CA 95616

During the past several years, much attention has been devoted to understanding the physical and chemical properties, processes, and principles governing the environmental behaviour and fate of chemicals. The goal is to be able to predict how chemicals behave before release occurs and to use this capability in the design and regulation of chemicals proposed for use in pest control and other environmental applications. This effort has included improving the data base of key physical and chemical properties, understanding the processes which underly movement to air, biota, and groundwater, and developing models for predicting mobility and persistence. The modelling approach will be illustrated with examples of pesticide volatilization from water and the fate of pesticides in aquatic field use situations. The role of field experiments in validating predictive models w i l l also be discussed.

Predicting how c h e m i c a l s behave i n the environment i s a major t a s k f a c i n g s c i e n c e today. S o c i e t y i s no l o n g e r s a t i s f i e d t o know t h a t we can p r o v i d e answers on where c h e m i c a l s go and how l o n g they p e r s i s t by c o n d u c t i n g a n a l y s e s o f e n v i r o n m e n t a l samples a f t e r use occurs. f a t h e r , i t demands premarket o r preuse t e s t s which c a n l e a d t o p r e d i c t i o n , w i t h a h i g h degree o f c e r t a i n t y , t h a t t h e c h e m i c a l i n q u e s t i o n w i l l n o t pose adverse e n v i r o n m e n t a l risks. Such p r o c e s s e s as f o o d c h a i n a c c u m u l a t i o n , c o n t a m i n a t i o n o f s u r f a c e or groundwaters, undue p e r s i s t e n c e i n s o i l o r water, and movement to sensitive environments through the a i r a r e of p a r t i c u l a r concern. F u l f i l l i n g these e x p e c t a t i o n s f o r premarket e n v i r o n m e n t a l s a f e t y t e s t i n g i s a l a r g e o r d e r ; i t r e q u i r e s t h a t much i n f o r m a t i o n be available on p h y s i c o c h e m i c a l p r o p e r t i e s , the e n v i r o n m e n t a l compartments available t o the chemical i n i t s zone o f u s e , p r o c e s s e s which can t r a n s f e r the c h e m i c a l between compartments and t r a n s f o r m t h e c h e m i c a l w i t h i n each compartment, and those e x t r i n s i c properties o f the environment which i n f l u e n c e both the c o u r s e and

0097-6156/87/0336-0088$06.00/0 © 1987 American Chemical Society

In Pesticides; Ragsdale, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


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Environmental Mobility and Fate

r a t e of such p r o c e s s e s . Given the c o m p l e x i t y and h e t e r o g e n e i t y o f the e n v i r o n m e n t — f u n c t i o n s of both l o c a t i o n from one environment t o a n o t h e r and time w i t h i n any g i v e n e n v i r o n m e n t — i t i s p r e s e n t l y n o t p o s s i b l e to provide q u a n t i t a t i v e l y accurate p r e d i c t i o n . Yet the demands o f s o c i e t y , e v e r more f r e q u e n t l y c o n t a i n e d i n r e g u l a t i o n s , r e q u i r e t h a t s c i e n c e do the b e s t j o b p o s s i b l e i n t h i s a r e a . The s u b j e c t of t h i s c h a p t e r i s the measurement of the key p h y s i c o c h e m i c a l p r o p e r t i e s which govern f a t e , and the use of these p r o p e r t i e s f o r p r e d i c t i n g the e n v i r o n m e n t a l b e h a v i o u r of p e s t i c i d e s . Our a b i l i t y t o i d e n t i f y and measure the key p h y s i c o c h e m i c a l properties which i n f l u e n c e behaviour and fate has improved considerably (J_). There e x i s t g u i d e l i n e s and, i n some c a s e s , d e t a i l e d d i r e c t i o n s f o r d e t e r m i n i n g such p h y s i c a l p r o p e r t i e s as water s o l u b i l i t y (S), octanol-water p a r t i t i o n c o e f f i c i e n t (K. ), b i o c o n c e n t r a t i o n f a c t o r (BCF), vapor p r e s s u r e ( P p ) , Henry's law c o n s t a n t (H), and s o i l s o r p t i o n c o e f f i c i e n t ( K o r K ) , and the rates of such chemical processes as hydrolysis, photolysis, o x i d a t i o n , metabolism by p l a n t s and a n i m a l s , and b i o d e g r a d a t i o n (24) • As i l l u s t r a t e d i n F i g u r e 1, t h i s i n f o r m a t i o n a l o n g w i t h s e v e r a l parameters which d e s c r i b e the "environment" i n t o which the c h e m i c a l i s t o be p l a c e d (a pond i n the example) p r o v i d e the starting point for making p r e d i c t i o n s on intercompartraental d i s t r i b u t i o n and p e r s i s t e n c e — the f i r s t s t e p i n d e f i n i n g the e n v i r o n m e n t a l f a t e f o r a g i v e n c h e m i c a l or group of c h e m i c a l s . The second step o f t e n i n v o l v e s the use of p h y s i c a l or mathematical/computer models. Combining the "benchmark" p r o p e r t i e s ( f i r s t s t e p ) w i t h d a t a from models (second s t e p ) a l l o w s one t o draw a p r o f i l e of e x p e c t e d b e h a v i o u r . T h i s i n f o r m a t i o n can be v e r y u s e f u l t o those d e v e l o p i n g c h e m i c a l s f o r e v e n t u a l r e l e a s e t o the environment o r p r o p o s i n g new uses f o r e x i s t i n g c h e m i c a l s (j^) • C e r t a i n l y , i f one i s t o r e l y on models, a t h i r d s t e p must o c c u r which i n v o l v e s v a l i d a t i n g the model p r e d i c t i o n s by comparing them w i t h r e s u l t s from f i e l d s t u d i e s . I t i s model v a l i d a t i o n t h a t perhaps i s i n most need of immediate a t t e n t i o n , p a r t i c u l a r l y i f one i s a i m i n g t o r e g u l a t e based upon model i n f o r m a t i o n as appears t o be the case for EPA and several state regulatory agencies. U n f o r t u n a t e l y , the a b i l i t y of models t o p r o v i d e numbers may have engendered the n o t i o n t h a t f i e l d t e s t s a r e no l o n g e r needed, a n o t i o n t h a t must be d i s p e l l e d i f we a r e t o improve p r e d i c t i v e c a p a b i l i t y t o the p o i n t of r e g u l a t o r y r e l i a b i l i t y . QW

V

d

o c

Physicochemical Properties Polarity. A b a s i c concept underlying v i r t u a l l y a l l physical p r o p e r t i e s , and the a s s o c i a t e d d i s t r i b u t i o n s i n v o l v i n g them, i s t h a t of m o l e c u l a r p o l a r i t y . S t r i c t l y speaking, p o l a r i t y r e f e r s to the unevenness of charge i n a m o l e c u l e . Water i s c o n s i d e r e d t o be p o l a r because i t i s r e l a t i v e l y n e g a t i v e i n the r e g i o n o f the oxygen atom and p o s i t i v e i n the r e g i o n o f the two hydrogen atoms i n the non-linear structure. The r e l a t i v e l y h i g h d i p o l e moment (1 .85 deByes), measured by o b s e r v i n g the e x t e n t t o which water m o l e c u l e s align themselves when p l a c e d between the p l a t e s of a charged condensor, and the h i g h d i e l e c t r i c c o n s t a n t (80), measured as the a b i l i t y of water t o a c t as an i n s u l a t o r when p l a c e d i n an e l e c t r i c



Sediment

Water

Air

Biomass Dissolved O2

Windspeed Sunlight Intensity Humidity

Figure 1• I n t r i n s i c and e x t r i n s i c properties governing the d i s t r i b u t i o n and fate of a chemical i n a pond environment.

Temp PH Susp. Sed.

h

PROPERTIES PHYSICAL CHEMICAL MW k yd H k photo BCF koxidn Sol. k metab V.P. k micro Kd



8. SEIBER


91

field, confirm this inherent polar character. Nitrobenzene i s s i m i l a r l y c o n s i d e r e d to be a p o l a r a r o m a t i c compound, w i t h a d i p o l e moment of 4.21 deByes and a d i e l e c t r i c c o n s t a n t of 35.7. These v a l u e s are r e a s o n a b l e based upon the s t r o n g p o l a r i z i n g e f f e c t o f the n i t r o s u b s t i t u e n t . We have no problem r a n k i n g n i t r o b e n z e n e , c h l o r o b e n z e n e (u = 1.7 deByes, D = 5.7) and t o l u e n e (u = 0.37 deByes, D = 2.4) i n a p o l a r i t y s e r i e s based upon t h i s p a r t o f the p o l a r i t y c o n c e p t , and t h e i r water s o l u b i l i t e s f a l l r o u g h l y i n the o r d e r e x p e c t e d based upon i t . I t i s even p o s i b l e t o do some r a n k i n g i n s i m p l e s t r u c t u r a l s e r i e s u s i n g the d i p o l e moment c o n t r i butions for various s u b s t i t u e n t groups ( n i t r o , amino, nitrile, etc.). U s e f u l g e n e r a l i z a t i o n s among o t h e r w i s e s i m i l a r compounds a r e t h a t s y m m e t r i c a l ones (eg, c a r b o n t e t r a c h l o r i d e ) a r e l e s s p o l a r than unsymmetrical ones (eg, chloroform), and that compounds c o n t a i n i n g oxygen, n i t r o g e n , and s u l f u r (eg organophosphate and carbamate e s t e r s ) are more p o l a r than hydrocarbons and c h l o r i n a t e d hydrocarbons. The t o t a l i n t e r a c t i o n of s o l u t e w i t h s o l v e n t (or w i t h s o l i d s u r f a c e i n a heterogenous environment) i n v o l v e s s e v e r a l k i n d s of f o r c e s : Polar i n t e r a c t i o n s ( d i p o l e - d i p o l e , dipole-induced d i p o l e ) , hydrogen bonding, and the d i s p e r s i o n i n t e r a c t i o n s which e x i s t between e v e r y p a i r of a d j a c e n t molecules. The latter, r e f e r r e d t o as London or van der Waal's f o r c e s , e x p l a i n the a b i l i t y o f even a p o l a r s u b s t a n c e s t o a s s o c i a t e i n condensed p h a s e s . A p o l a r i t y r a n k i n g i s n o t p o s s i b l e based o n l y on d i e l e c t r i c c o n s t a n t and d i p o l e moment because they do n o t take i n t o a c c o u n t Hbonding; thus, p o l a r i t y s e r i e s are o f t e n c o n s t r u c t e d e m p i r i c a l l y , u s i n g such f a c t o r s as the s o l v e n t s t r e n g t h parameter o b t a i n e d from the o b s e r v e d a b i l i t y of v a r i o u s s o l v e n t s t o e l u t e s o l u t e s from aluminum o x i d e a b s o r b e n t . However, f o r e n v i r o n m e n t a l c h e m i c a l s , a n u m e r i c a l i n d e x f o r p o l a r i t y does not e x i s t ; o n l y the consequences o f p o l a r i t y , as r e f l e c t e d i n measureable p r o p e r t i e s such as water s o l u b i l i t y and 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 , a r e a v a i l a b l e for fate predictions. Water S o l u b i l i t y (S). The measurement of water s o l u b i l i t y i s r e l a t i v e l y straightforward f o r most o r g a n i c compounds, i n v o l v i n g o b s e r v a t i o n of the amount d i s s o l v e d i n water when an e x c e s s of the c h e m i c a l i s a l l o w e d t o r e a c h e q u i l i b r i u m w i t h water a t constant temperature. Centrifugation or filtration removes suspended m a t e r i a l from the s o l u t i o n p r i o r t o measurement. Experimental variations on this basic method can produce rather large discrepancies (_6) • While p r e c i s i o n appears t o be lowest with hydrophobic compounds of very low solubility, a recent remeasurement r e v e a l e d d i s c r e p a n c i e s w i t h l i t e r a t u r e v a l u e s of up t o a f a c t o r of two f o r s e v e r a l p e s t i c i d e s of moderate water s o l u b i l i t y and a f a c t o r of 100 f o r two of them ( r o n n e l and bromophos) iJJ • A recently introduced column method offers the potential of generating s o l u b i l i t y d a t a of good p r e c i s i o n and a c c u r a c y much f a s t e r than p o s s i b l e w i t h the c o n v e n t i o n a l method (8_) • This may s t i m u l a t e an e f f o r t a t r e - m e a s u r i n g a l l p e s t i c i d e s under i d e n t i c a l conditions. Water s o l u b i l i t y i s i n f l u e n c e d by temperature ( T ) , and the d i r e c t i o n g e n e r a l l y i s toward an i n c r e a s e i n s o l u b i l i t y w i t h an increase i n temperature. A r u l e of thumb i s t h a t s o l u b i l i t y of


92

PESTICIDES: MINIMIZING T H E RISKS

s o l i d s and l i q u i d s i n c r e a s e s by a f a c t o r o f 2 f o r a 14° r i s e i n temperature from 10-24°C. However, t h e r e a r e e x c e p t i o n s t o t h i s . The solubility of thiolcarbamates decreases with increasing temperature (9), an e f f e c t a s c r i b e d t o an i n c r e a s e i n resonance contribution of the uncharged -S-C(0)-N= form a t higher temperatures o v e r t h e -S-C(0~)=N = form which predominates a t lower temperatures ( 1 0 ) . Unfortunately, s o l u b i l i t i e s i n the l i t e r a t u r e a r e u s u a l l y g i v e n a t j u s t a s i n g l e temperature s o t h a t t h e r e i s no b a s i s f o r j u d g i n g whether a r e g u l a r o r i n v e r s e r e l a t i o n s h i p e x i s t s between S and T f o r a g i v e n c h e m i c a l . Furthermore, the temperature may n o t always be s p e c i f i e d i n t h e l i t e r a t u r e c i t a t i o n , l e a v i n g a large potential f o r error when u s i n g such values i n fate calculations. Among organophosphates paraoxon has a water s o l u b i l i t y o f 3640 ppm compared w i t h o n l y 12.4 ppm f o r p a r a t h i o n (7_), r e f l e c t i n g t h e much g r e a t e r p o l a r i z i n g e f f e c t o f t h e B=0 m o i e t y when c o n t r a s t e d w i t h P=S. S i m i l a r l y , p h o r a t e s u l f o x i d e (>8000 ppm) i s much more s o l u b l e than p h o r a t e (17.9 ppm) because o f t h e p r e s e n c e o f t h e polarizing S 0 group i n t h e f o r m e r . I t i s thus n o t p o s s i b l e t o e s t i m a t e t h e water s o l u b i l i t y o f a compound based upon t h e v a l u e f o r a c l o s e a n a l o g . The e f f e c t o f v e r y s m a l l changes i n s t r u c t u r e may a l s o h e l p t o e x p l a i n some o f t h e d i s c r e p a n c i e s i n r e p o r t e d s o l u b i l i t i e s i n the l i t e r a t u r e , where a s m a l l c o n t a m i n a t i o n w i t h an analogue o f much h i g h e r s o l u b i l i t y than t h e compound b e i n g subj e c t e d t o measurement c a n produce a l a r g e e r r o r i n the measured value. The water s o l u b i l i t y o f t h e s u p e r c o o l e d l i q u i d exceeds t h a t o f the s o l i d t o r a g i v e n c h e m i c a l above i t s m e l t i n g temperature ( t ) . An approximate f o r m u l a f o r c o n v e r t i n g from one t o t h e o t h e r i s :


+

m

L

°9

S

L

S

solid " ° 9

liquid " ° -

0 0 9 5

(t

m

"

2

5

)

This may be an i m p o r t a n t correction i n environmental fate c a l c u l a t i o n s because t h e l i q u i d form, r a t h e r t h a n t h e s o l i d (upon which s o l u b i l i t y d e t e r m i n a t i o n s a r e u s u a l l y b a s e d ) , may be t h e state of i n t e r e s t i n environmental processes. Obviously, the c o r r e c t i o n becomes l a r g e r f o r compounds o f h i g h e r m e l t i n g p o i n t . C o n s i d e r i n g t h e above f a c t o r s as w e l l as water pH and water p u r i t y , i t i s c l e a r t h a t r e p o r t e d water s o l u b i l i t i e s , p a r t i c u l a r l y t h o s e done a t a s i n g l e temperature w i t h no i n d i c a t i o n o f r e p l i c a t i o n o r o f s o l u t e and s o l v e n t p u r i t y , must be a s s i g n e d a f a i r l y large uncertainty ( a t l e a s t ± 100%) when used f o r c a l c u l a t i n g d i s t r i b u t i o n c o e f f i c i e n t s or other environmental f a t e parameters. When water s o l u b i l i t y i s n o t r e p o r t e d i n t h e l i t e r a t u r e , i t may be e s t i m a t e d from the 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 (K ) o r from s t r u c t u r a l parameters ( 1 1 ) ; i n e i t h e r c a s e an even l a r g e r u n c e r t a i n t y e x i s t s i n the v a l u e . Octanol-Water Partition Coefficient (K ). This partition c o e f f i c i e n t i s perhaps t h e most used d i s t r i b u t i o n c o n s t a n t i n environmental chemistry, underlying calculations o f bi©concent r a t i o n and b i o a c c u m u l a t i o n , s e v e r a l s t r u c t u r e - a c t i v i t y r e l a t i o n ships, and t h e c h o i c e of solvent conditions f o r extractions. Q W


93


8. SEIBER

Partition coefficient has been studied i n great detail and c o m p i l a t i o n s a r e a v a i l a b l e i n the l i t e r a t u r e ( 1 2 ) • The l a b o r a t o r y measurement of K i s f a i r l y s t r a i g h t f o r w a r d , although again e r r o r can c r e e p i n due t o such t h i n g s as f a i l u r e t o use e q u i l i b r a t e d s o l v e n t s , n o n - c o n s t a n t temperature, and i n a c c u r a c y of the measuring technique, c o n t r i b u t i n g to a f a i r l y large u n c e r t a i n t y i n l i t e r a t u r e values. A not a t y p i c a l case i s t h a t f o r methyl p a r a t h i o n , where the l i t e r a t u r e p r o v i d e s a t l e a s t 4 v a l u e s of l o g K (11); Q W

Q W


l Q

9

K

JSow

ow

2.04 2.99 1 .91 3.22

109.6 977.2 81 .3 1659.6

In l i g h t of t h i s example, l i t e r a t u r e v a l u e s of K must be g i v e n a f a i r l y broad l a t i t u d e (± 1 o r d e r of magnitude) u n l e s s they have been c o n f i r m e d by more than one l a b o r a t o r y or by c a l c u l a t i o n from structure. The relatively new area of p r o p e r t y e s t i m a t i o n has been perhaps b e s t d e v e l o p e d f o r K . The methods of e s t i m a t i n g l o g K include: Q W

Q W

a. b. c.

Q W

E s t i m a t i o n from Reverse Phase - HPLC r e t e n t i o n s E s t i m a t i o n from water s o l u b i l i t y E s t i m a t i o n from s t r u c t u r e v i a fragment c o n s t a n t method

C o r r e l a t i o n w i t h r e v e r s e d phase HPLC r e t e n t i o n d a t a i s a t t r a c t i v e as a r a p i d e s t i m a t i o n method because the sample r e q u i r e m e n t s of HPLC i n terms of p u r i t y and q u a n t i t y are n o t s t r i n g e n t . A p o p u l a r e s t i m a t i o n method i s from water s o l u b i l i t y (S) d a t a , g i v e n a l o g log r e g r e s s i o n between S and K f o r a s e r i e s of compounds. An example of a r e g r e s s i o n e q u a t i o n a p p l i c a b l e t o mixed c l a s s e s of c h e m i c a l s (11) i s : Q W

Log

S = 1.37

log K

Q W

+

7.26

where S i s e x p r e s s e d as umol/L. F o r t y - o n e compounds, r a n g i n g from K = 8 to 10 , were used i n the r e g r e s s i o n w i t h a c o r r e l a t i o n c o e f f i c i e n t ( r ) o f 0.903. Other e q u a t i o n s might be more a p t f o r s p e c i f i c types of o r g a n i c c h e m i c a l s (11 ) • The advantage o f u s i n g the s o l u b i l i t y c o r r e l a t i o n t o o b t a i n K i s t h a t no c h e m i c a l i s r e q u i r e d , and one o n l y needs a l i t e r a t u r e v a l u e o f S. The d i s advantage i s t h a t i t i s j u s t an e s t i m a t i o n , and t h e r e i s no way of a s s e s s i n g the a c c u r a c y of i t g i v e n the u n c e r t a i n t y i n l i t e r a t u r e v a l u e s of S d e s c r i b e d above. Related t o the above i s the intriguing possibility that physical properties can be c a l c u l a t e d knowing only molecular structure, completely o b v i a t i n g the need f o r a sample of the s u b s t a n c e or any p r i o r l a b o r a t o r y work w i t h i t . For K , the c a l c u l a t i o n from s t r u c t u r e uses the fragment c o n s t a n t approach (13) or e a r l y v e r s i o n s of i t ( 1 2 ) . B r i e f l y , the method employs e m p i r i c 6

Q W

2

Q W

QW


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PESTICIDES: MINIMIZING THE RISKS

a l l y derived atomic or group fragment constants factors ( f ) : log K

QW

(F) and

structural

= sum of fragments (F) and factors ( f ) .


Values for F and f are compiled i n tables (11, 13). The c a l c u l a t i o n becomes more tedious (and uncertainty i n the result increases) for more complex structures. A computer program has been developed to aid i n the calculation (C. Hansch and A. Leo, personal communication). Bioconcentration Factor. The bioconcentration factor (BCF) i s defined as the r a t i o of the concentration of a chemical i n an organism to the concentration i n the surrounding medium. While BCF i s used most commonly as a measure of d i r e c t p a r t i t i o n i n g of chemical from water to f i s h , i t also has some a p p l i c a b i l i t y to terrestrial species (plants and animals) in contact with contaminated s o i l or water (14)• Some confusion exists i n the l i t e r a t u r e regarding the term "bioconcentration" which, as defined above, implies uptake across membranes from the medium (usually water), and "biomagnification", "bi©accumulation", and "ecological magnification". In the l a t t e r three, dietary transfer of chemical can occur along with d i r e c t partitioning. The major experimental d i s t i n c t i o n i s that bioconcentration experiments are run such that no dietary intake i s involved, while bioaccumulation experiments include contributions from both d i r e c t p a r t i t i o n i n g and dietary intake. In biomagn i f i c a t i o n , the use of an i n t a c t food chain involving two or more trophic levels i s implied. The measurement of bioconcentration i s d i f f i c u l t because the water concentration must remain constant during the run and contact must be maintained u n t i l equilibrium i s reached i n the organism. E q u i l i b r a t i o n , signalled by a plateau i n the concentration vs time plot, may take several days. This e n t a i l s , p a r t i c u l a r l y i n the case of r e l a t i v e l y hydrophobic compounds of low water s o l u b i l i t y , dosing i n a flow-through chamber at levels well below the toxic threshold. A complete experiment involves analysis of samples during the exposure, or "uptake phase", and also following transfer to a clean environment where release (depuration) occurs. Both the parent chemical, from which the bioconcentration factor i s calculated, and known metabolites are analyzed (15)• Experimental variables include, i n addition to those implied above, temperature and species of test organism. The species-tospecies variation alone contributes a v a r i a b i l i t y of ± 50% for the same chemical. Another variable i s the type of tissue sampled. When account i s taken of a l l error sources, values d i f f e r i n g by much less than 1 order of magnitude may not have b i o l o g i c a l significance (15)• This s t i l l leaves room for a reasonable scale as BCF for most organic chemicals f a l l over a wide range, from about 1 (hydrophilic compounds) to over 1,000,000 (hydrophobic chemicals)•


8. SEIBER

95


I f BCF i s not a v a i l a b l e from e x p e r i m e n t a l measurements, i t can be e s t i m a t e d v i a c o r r e l a t i o n e q u a t i o n s from water s o l u b i l i t y ( S ) , octanol-water partition coefficient ( ) soil adsorption coefficient (K ). Of the three, c o r r e l a t i o n s from K are c o n s i d e r e d the most r e l i a b l e because they a r e c u r r e n t l y based on the l a r g e s t body of b i o a s s a y d a t a and because K measurements i n v o l v e a w a t e r - l i p o p h i l i c phase p a r t i t i o n i n g which bears o b v i o u s similarity to water-to-fish partitioning. One recommended c o r r e l a t i o n e q u a t i o n i s (11): K

o

r

o w

Q C

Q W

Q W


l o g BCF = 0.76

log K

Q W

-0.23

I t i s based on d a t a from s e v e r a l i n v e s t i g a t o r s u s i n g a v a r i e t y of f i s h s p e c i e s and 84 o r g a n i c c h e m i c a l s . The l o g - l o g p l o t of t h i s c o r r e l a t i o n shows s u b s t a n t i a l s c a t t e r , u n d e r s c o r i n g the o r d e r - o f magnitude accuracy expected i n results from the use of the correlation. Volatility. F o r vapor p r e s s u r e , the fundamental p r o p e r t y g o v e r n i n g condensed phase-vapor phase distributions, the experimental measurement can be q u i t e t e d i o u s and prone t o s e v e r a l s o u r c e s of error particularly f o r compounds of low volatility. Of the a v a i l a b l e methods, gas s a t u r a t i o n appears t o be the most c o n v e n i e n t and a c c u r a t e ( 1 6 ) , w h i l e e s t i m a t i o n based upon QC r e t e n t i o n d a t a promises a more r a p i d (though perhaps l e s s a c c u r a t e ) method worth f u r t h e r development ( 1 7 ) • Henry's law c o n s t a n t , the a i r - w a t e r d i s t r i b u t i o n r a t i o , i s needed when computing e i t h e r the d i r e c t i o n of e q u i l i b r i u m o r the r a t e of v o l a t i l i z a t i o n from water. I t may be measured e x p e r i m e n t a l l y or c a l c u l a t e d as the r a t i o of vapor p r e s s u r e t o water s o l u b i l i t y (18). Rate C o n s t a n t s . D i s t r i b u t i o n c o e f f i c i e n t s of the type mentioned above t e l l the d i r e c t i o n of t r a n s f e r but not the r a t e of t r a n s f e r or o v e r a l l d i s s i p a t i o n . As noted, the d a t a f o r d i s t r i b u t i o n c o e f f i c i e n t s a r e i m p r e c i s e and f r e q u e n t l y d i f f i c u l t o r i m p o s s i b l e t o f i n d i n the l i t e r a t u r e , and t h e i r e s t i m a t i o n t e c h n i q u e s i n need of f u r t h e r improvement. The s i t u a t i o n i s even l e s s s a t i s f a c t o r y for rate constants, with the possible exception of rate of volatilization from water (Table I). Linear Free Energy R e l a t i o n s h i p s (LFER) o f f e r p o t e n t i a l as e s t i m a t i o n t e c h n i q u e s f o r r e a c t i o n r a t e c o n s t a n t s , w i t h examples b e i n g p r o v i d e d by e s t i m a t i o n o f the second o r d e r r a t e c o n s t a n t f o r h y d r o l y s i s of organophosphate e s t e r s from the pKa of the l e a v i n g group's c o n j u g a t e a c i d or o f b e n z o i c e s t e r s from Hammett sigma-rho v a l u e s (11) • There a r e c u r r e n t l y j u s t a few LFER's a v a i l a b l e , f o r j u s t a few c l a s s e s o f c h e m i c a l s and r e a c t i o n t y p e s , and the d a t a base upon which they have been b u i l t i s f a i r l y s m a l l p a r t i c u l a r l y f o r p e s t i c i d e s . This i s d e f i n i t e l y an a r e a i n need o f a g r e a t l y expanded e f f o r t .


96

PESTICIDES: MINIMIZING THE RISKS Table I. A v a i l a b i l i t y L i t e r a t u r e Sources and

Literature Data Base o f Experimental Values

Rate Constant


of Rate C o n s t a n t Data from from E s t i m a t i o n Techniques

Estimation Techniques

Volatilization

Good

Good

(from

Hydrolysis

Fair

Fair

(LFER, k vs

Photolysis

Fair

Fair

H)

pH)

(UV-Vis e vsX, solar irradiation Uptake fish

data)

by

Poor

Fair

(LRE from

K )

Excretion by f i s h

Poor

Poor

(LRE from

K )

Uptake by soil

Fair

Not

available

Desorption from s o i l

Poor

Not

available

Bi©degradation

Poor

Q u a l i t a t i v e only

Q W

Q W

LFER = L i n e a r Free Energy R e l a t i o n s h i p LRE = L i n e a r R e g r e s s i o n E s t i m a t i o n Environmental

Relevance

The availability of r e l i a b l e measurements o r e s t i m a t e s of water solubility, octanol-water partition coefficient, bioconc e n t r a t i o n f a c t o r , r a t e c o n s t a n t s and the l i k e a l l o w s one t o make qualitative judgements or, through the use of mathematical s i m u l a t i o n models such as EPA's EXAMS ( 1 9 ) , q u a n t i t a t i v e c a l c u l a tions of e n v i r o n m e n t a l distribution and persistence. In the qualitative use, Swann and coworkers (20) c l a s s i f i e d chemical m o b i l i t y i n s o i l based upon r e v e r s e d - p h a s e HPLC r e t e n t i o n d a t a which i n t u r n i s r e l a t e d t o S. The approximate water s o l u b i l i t y equivalents i n t h i s f i r s t - e s t i m a t e c l a s s i f i c a t i o n , with chemical examples, a r e i n T a b l e I I . T h i s c l a s s i f i c a t i o n h o l d s f o r c h e m i c a l s whose p r i m a r y a d s o r p t i o n i n s o i l i s t o o r g a n i c matter, and e x c l u d e s t h o s e c h e m i c a l s (such as p a r a q u a t ) which b i n d i o n i c a l l y t o the s o i l mineral f r a c t i o n . A r e c e n t t a b u l a t i o n of p e s t i c i d e s found i n groundwater had 11 e n t r i e s , 8 o f which r e p r e s e n t e d compounds w i t h water s o l u b i l i t i e s i n e x c e s s of 200 ppm w i t h the r e m a i n i n g t h r e e f a l l i n g i n the range of 3.5 t o 52 ppm ( 2 1 ) .


8. SEIBER



Table I I .

97

R e l a t i o n s h i p Between S o i l M o b i l i t y ( L e a c h i n g ) and Water S o l u b i l i t y (20)

Mobility Class

Water Solubility

V e r y High High Medium Low Slight Immobile

>10 -3000 3000-300 300-30 30-2 2-0.5 t h i o b e n c a r b > MCPA which i s i n agreement w i t h the p r e d i c t i o n based upon Henry's law c o n s t a n t . The a b i l i t y of the model t o generate d a t a s u p p o r t i n g the f i e l d measurement bodes w e l l f o r the f u r t h e r use o f such models i n the f u t u r e . In the case of v o l a t i l i z a t i o n , which i s so d i f f i c u l t t o measure e x p e r i m e n t a l l y , the a v a i l a b i l i t y of a p r e d i c t i v e model would be a welcome development. W h i l e t h i s s t u d y showed the p o t e n t i a l o f EXAMS f o r f o r e c a s t i n g v o l a t i l i z a t i o n o v e r f a i r l y broad time i n t e r v a l s , a more r e f i n e d s t u d y was needed t o s u p p l y e x p e r i m e n t a l f l u x d a t a t o t e s t the c a p a b i l i t y of EXAMS t o model v a r i a t i o n s i n f l u x w i t h time of day, windspeed, and temperature. T h i s was c o n d u c t e d f o r m o l i n a t e u s i n g the same b a s i c f i e l d d e s i g n as b e f o r e b u t w i t h more s a m p l i n g i n t e r v a l s and h e i g h t s , and b e t t e r m i c r o m e t e o r o l o g i c a l equipment. EXAMS was p r o v i d e d w i t h i n p u t s o f temperature, windspeed, and w i t h m o l i n a t e water s o l u b i l i t y and vapor p r e s s u r e c o r r e s p o n d i n g t o the temperatures a t each s a m p l i n g i n t e r v a l . The r e s u l t s (30) showed t h a t EXAMS c o r r e c t l y f o r e c a s t the f l u x maxima and minima measured by the aerodynamic method. The e x p e r i m e n t a l measurements from the f i e l d were somewhat lower than p r e d i c t e d by EXAMS and by the l o s s i n f i e l d water c o n c e n t r a t i o n s of the h e r b i c i d e .



8. SEIBER

101


O v e r a l l , EXAMS appeared t o be q u i t e p r o m i s i n g as a p r e d i c t i v e t o o l f o r e s t i m a t i n g v o l a t i l i z a t i o n l o s s from f l o o d e d r i c e f i e l d s . I t may be u s e f u l f o r e s t i m a t i n g l o s s from o t h e r d i s s i p a t i o n r o u t e s as w e l l , a c a p a b i l i t y of the model n o t t e s t e d i n these e x p e r i m e n t s , and f o r e s t i m a t i n g o v e r a l l d i s s i p a t i o n i n water from a l l r o u t e s o f loss. T h i s c a p a b i l i t y c o u l d be q u i t e u s e f u l f o r c a l c u l a t i n g the effect of water holding intervals on the concentration of herbicides i n rice field e f f l u e n t reaching p u b l i c waterways—a s u b j e c t of much i n t e r e s t i n the e x t e n s i v e r i c e growing r e g i o n s of California's Sacramento Valley (31)—and, more g e n e r a l l y , the c o n c e n t r a t i o n o f v i r t u a l l y any o r g a n i c p o l l u t a n t i n b o d i e s o f water a t v a r i o u s times f o l l o w i n g c o n t a m i n a t i o n . P e s t i c i d e s i n Fogwater. A second s e t of e x p e r i m e n t s d e a l t w i t h the f a t e o f p e s t i c i d e i n the atmosphere, and more s p e c i f i c a l l y w i t h the d i s t r i b u t i o n between vapor and a t m o s p h e r i c m o i s t u r e i n the form o f fog. The ways by which r e s i d u e s can be removed from the a i r i n c l u d e d r y d e p o s i t i o n , t h a t i s , by i m p a c t i o n of p a r t i c l e s or d i r e c t a i r - s u r f a c e exchange of vapors, and wet d e p o s i t i o n f o l l o w i n g e n t r a i n m e n t of p a r t i c l e s or d i s s o l u t i o n o f vapors i n f o g , snow, and rainwater (32)• Hundreds of o r g a n i c c h e m i c a l s have been i d e n t i f i e d i n r a i n w a t e r (33, 34) and a s m a l l e r number i n fogwater (35), b u t a s i d e from the more p e r s i s t e n t h a l o g e n a t e d m a t e r i a l s and h e r b i c i d e s i n r a i n w a t e r (36, 37), p r a c t i c a l l y no measurements have been made of o t h e r p e s t i c i d e s . I n 1983-84, a c o l l e c t i o n system d e s i g n e d by USDA-ARS p e r s o n n e l G l o t f e l t y and L i l j e d a h l a t B e I t s v i l i e , MD, was used t o c o l l e c t fogwater from Maryland. In this collaborative project, an a n a l y t i c a l method (adapted from 38) was d e v e l o p e d f o r p p t concent r a t i o n s of r e p r e s e n t a t i v e p e s t i c i d e s , and p o s i t i v e f i n d i n g s were made of f i v e organophosphates ( i n c l u d i n g d i a z i n o n , m a l a t h i o n and methyl parathion), two triazines ( a t r a z i n e and simazine), an organochlorine (DDT), and s e v e r a l p h t h a l a t e e s t e r s and p o l y c y c l i c aromatic hydrocarbons. In 1984-85, the f o g sampler was b r o u g h t t o C a l i f o r n i a f o r sampling f o g from the C e n t r a l V a l l e y . These " t u l e f o g s " may l i n g e r f o r s e v e r a l days i n the December-March season and, in the p r o c e s s , entrain airborne dusts and p a r t i t i o n chemical vapors. A total of 16 phosphorusand nitrogen-containing compounds were measureably p r e s e n t i n fogwater c o l l e c t e d a t the Kearney A g r i c u l t u r a l C e n t e r near F r e s n o (Table IV) and 16 in a n o t h e r sample c o l l e c t e d near C o r c o r a n i n the c o t t o n - g r o w i n g r e g i o n o f Kings County; s i m i l a r f i n d i n g s were o b t a i n e d f o r samples from o t h e r p a r t s of the C e n t r a l V a l l e y ( 3 9 ) • The c o n c e n t r a t i o n s of several of these chemicals—notably, p-nitrophenol, diazinon, ethylbenzimidazole, p a r a t h i o n , paraoxon, c h l o r p y r i f o s , and DEF— were s u p r i s i n g l y h i g h , e x t e n d i n g t o 30 ppb i n the extreme case of p-nitrophenol. For d i a z i n o n , C e n t r a l V a l l e y f o g had approximately 20 times the c o n c e n t r a t i o n measured i n M a r y l a n d f o g d u r i n g the preceding winter. We were p a r t i c u l a r l y i n t r i g u e d by two f i n d i n g s : 1•

Breakdown p r o d u c t s of p a r a t h i o n (paraoxon and pnitrophenol), trifluralin (ethylbenzimidazole), and chlorpyrifos ( c h l o r p y r i f o s oxon) were surprisingly s i g n i f i c a n t residues.


PESTICIDES: MINIMIZING THE RISKS

102 TABLE IV.

D i s t r i b u t i o n o f Chemicals Between Fog Water and I n t e r s t i t i a l A i r - Kearney A g r i c u l t u r a l C e n t e r , J a n u a r y 13, 1985 (39)

1

C o n c e n t r a t i o n , ng L ~ Fog Water Air (X10°)


Chemical Diazinon Parathion Chlorpyrif OS Methidathion DEF Malathion p-Nitrophenol Simazine Atrazine Paraoxon Methidat h i o n Oxon Diazoxon Chlorpyrif o s Oxon Trifluralin Ethylbenzimidazole PCNB

2.

16,000 12,400 1,020

D i s t r i b u t i o n R a t i o (X10°) Air/Water Experimental L i t e r a t u r e

2.2 3.2 3.3

0.12 0.25 3.2

840

Principles Governing Environmental Mobility and ... - ACS Publications

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