30 Summary of Soil Bound Residues Discussion Session P. C. KEARNEY
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Pesticide Degradation Laboratory, Agricultural Research Service, U. S. Department of Agriculture, Beltsville, Md. 20705
The Discussion Section on Soil Bound Residues considered three major questions: 1. When should soil bound residue studies be initiated? 2. How should soil bound residues be measured? 3. What is the significance of a soil bound residue? Each of these questions was discussed in length by the participants. Summarized below are the salient points of this discussion: When should soil bound studies be initiated? In the early planning of this conference, one of the ideas suggested was the development of c r i t e r i a for determining when pesticide scientists need to i n i t i a t e in-depth studies on soil bound pesticide residues. For example, if after completion of a residue or metabolism study, (a) < 10% of the applied pesticide was bound to soil particles after exhaustive extraction with both nonpolar organic and polar solvents, (b) the compound under study was not a persistent pesticide, and (c) the toxicology of the parent material or suspected metabolites was not a major problem, then in-depth studies on bound pesticide residues were not essential or needed. In contrast, if it were ascertained that binding was a major mechanism for a toxic compound (>10%), then detailed study on the nature, s t a b i l i t y , and chemistry of the bound residue would be mandatory. Conference discussion of this proposal did not yield a favorable reaction. Some of the reasons given were as follows: 1. The methods and interpretation of results gleaned from current methodology are not now s u f f i c i e n t l y developed to merit in-depth investigations. The general consensus was that detailed studies on soil bound residues are still in their infancy and considerable exploratory and theoretical research must be done before standard methods can be recommended and adequately interpreted.
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Kaufman et al.; Bound and Conjugated Pesticide Residues ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
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2.
There is still considerable confusion as t o what e x a c t l y is being measured when r a d i o a c t i v e m a t e r i a l s are used. I tisu n c l e a r whether one is measuring the i n t a c t bound parent m a t e r i a l o r one o f s e v e r a l metabolites. 3. There is still considerable q u e s t i o n as t o the s i g n i f i c a n c e o f soil bound r e s i d u e s in succeeding crops, on the soil ecology, and on other segments o f the e n v i r o n ment i n t o which a soil bound r e s i d u e might migrate. There may not be s u f f i c i e n t j u s t i f i c a t i o n t o merit in-depth i n v e s t i g a t i o n s unless there is evidence f o r adverse environmental e f f e c t s o f soil bound r e s i d u e s . Much o f the subsequent d i s c u s s i o n d e a l t w i t h paraquat, a h e r b i c i d e t h a t is s t r o n g l y bound t o soil components soon a f t e r a p p l i c a t i o n . Because o f the s t r o n g b i n d i n g c a p a c i t y o f paraquat, and because o f p o t e n t i a l h e a l t h - r e l a t e d problems a s s o c i a t e d w i t h the unadsorbed molecule, t h i s h e r b i c i d e has been the most e x t e n s i v e l y s t u d i e d from the standpoint o f bound r e s i d u e s . I t was p o i n t e d out t h a t because o f i t s h i g h b i o l o g i c a l a c t i v i t y when unadsorbed, b i n d i n g o f paraquat represents a r a p i d and s a f e decontamination mechanism in s o i l s . There was c o n s i d e r a b l e debate on whether p l a n t uptake of paraquat residues might be a problem in sandy s o i l s , where b i n d i n g is l i m i t e d . I t was p o i n t e d out, however, that t h e l a b e l c l e a r l y s t a t e s t h a t use o f paraquat on sandy soil is n o t recommended. I t was a l s o p o i n t e d out t h a t it is d i f f i c u l t , if not i m p o s s i b l e , t o extend f i n d i n g s on paraquat in s o i l s t o other p e s t i c i d e s because o f the unique chemical s t r u c t u r e o f t h i s h e r b i c i d e . Paraquat and the s t r u c t u r a l l y r e l a t e d analog d i q u a t , are both d i - c a t i o n s , and represent a unique c l a s s o f compounds when compared t o most other h e r b i c i d e s which are e i t h e r n e u t r a l o r a n i o n i c . Because o f the unique chemistry o f paraquat, it was cautioned t h a t e x t r a p o l a t i o n o f f i n d i n g s w i t h t h i s compound t o other h e r b i c i d e s o r i n s e c t i c i d e s could be misl e a d i n g . I t was the general o p i n i o n o f the p a r t i c i p a n t s t h a t we w i l l continue t o conduct bound r e s i d u e s t u d i e s on most new p e s t i c i d e s considered in the f u t u r e . The depth o f the study w i l l depend on the compound being considered f o r r e g i s t r a t i o n . How
should
soil
bound residues be measured?
The second c r i t i c a l q u e s t i o n on the chemistry o f soil bound r e s i d u e s is the adequacy o f current methods. B a s i c a l l y t h i s question r e s o l v e s i n t o two more s p e c i f i c q u e s t i o n s , i . e . , (a) I s the method recommended by the American I n s t i t u t e B i o l o g i c a l Sciences Environmental Chemistry Task Group s a t i s f a c t o r y ? and (b) Are there other b e t t e r methods? The c l a s s i c a l method recommended by the AIBS committee, which subsequently appeared in The Environmental P r o t e c t i o n Agency s " G u i d e l i n e s f o r R e g i s t e r i n g P e s t i c i d e s in the United S t a t e s " (Federal R e g i s t e r 40 [123]: 1
Kaufman et al.; Bound and Conjugated Pesticide Residues ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
B O U N D AND
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C O N J U G A T E D PESTICIDE RESIDUES
26802-26928, June 25, 1975) i n v o l v e s e x t r a c t i o n o f the organic matter from s o i l s w i t h c a u s t i c a l k a l i and the f u r t h e r s u b d i v i s i o n o f the e x t r a c t e d m a t e r i a l by p a r t i a l p r e c i p i t a t i o n w i t h mineral a c i d s . The proposed scheme is o u t l i n e d below:
Soil
a f t e r nonpolar organic and p o l a r extraction solvent
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E x t r a c t w i t h O.5
Ν NaOH (room temp)
ι—
Soluble
Insoluble (humin)
Treat w i t h l c o n c . HC1 t o pHl
ι
Soluble ( f u l v i c acid)
Precipitate (humic a c i d ) The method as o u t l i n e d is approximately 100 years o l d and yet it is a method t h a t has been w i d e l y accepted by soil s c i e n t i s t s f o r the f r a c t i o n a t i o n and c h a r a c t e r i z a t i o n o f soil o r g a n i c matter. The method is reasonably simple and conducive t o r o u t i n e analy t i c a l techniques. I t s u f f e r s the disadvantage o f employing r a t h e r s t r o n g agents t o separate the humic and f u l v i c a c i d f r a c t i o n s . Consequently, t h e r e has been c r i t i c i s m as t o the i n t e g r i t y o f the e x t r a c t e d components. These r i g o r o u s c o n d i t i o n s may a l s o a l t e r the bound p e s t i c i d e moiety t o the p o i n t where it no longer resembles what is i n i t i a l l y bound in the undisturbed soil organic m a t r i x . Nevertheless, due t o i t s e x t e n s i v e use in the past as a method o f c h a r a c t e r i z i n g o r g a n i c matter components in s o i l s , it was recommended t h i s method be employed u n t i l b e t t e r ones are developed and v e r i f i e d . The second s p e c i f i c question discussed under the broad general t o p i c o f methodology d e a l t w i t h new o r improved methods o f f r a c t i o n a t i n g soil bound r e s i d u e s . I t was p o i n t e d out t h a t there are many methods f o r e x t r a c t i n g soil organic matter, some more c h e m i c a l l y f a c i l e than the method o u t l i n e d above. For exam p l e , a DMF/oxalate method might y i e l d v a l u a b l e f r a c t i o n s which are l e s s d i s t o r t e d than those from s t r o n g a c i d and base e x t r a c t i o n . I t was a l s o noted t h a t pyrophosphate e x t r a c t i o n s o f f e r more f a c i l e methods o f c l e a v i n g l i n k a g e s in the complex soil humic a c i d f r a c t i o n , again y i e l d i n g f r a c t i o n s t h a t are more c h a r a c t e r i s t i c o f the n a t u r a l soil m a t r i x . I t was g e n e r a l l y concluded t h a t methodology needs considerable f u t u r e a t t e n t i o n . A f r u i t f u l area o f research would be t o examine s e v e r a l methods o f e x t r a c t i n g s o i l s and determine the magnitude and d i s t r i b u t i o n
Kaufman et al.; Bound and Conjugated Pesticide Residues ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
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o f bound p e s t i c i d e s y i e l d e d by these methods. A very important p o i n t was made t h a t the l i v i n g p l a n t might be one o f the best bioassays f o r soil bound r e s i d u e s in the unextracted soil. I t was f u r t h e r recommended t h a t methodology f o r bound soil residues proceed both w i t h chemical and b i o l o g i c a l assays t h a t would shed some l i g h t on t h e o v e r a l l question o f s i g n i f i c a n c e .
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What is t h e s i g n i f i c a n c e o f a soil bound residue? The question o f s i g n i f i c a n c e o f soil bound residues is one o f the major imponderables a t the current time. In an attempt to assess the environmental s i g n i f i c a n c e o f soil bound r e s i d u e s , a number o f environmental components were considered. Those components t h a t would be l e a s t a f f e c t e d by soil bound residues are probably higher animals, i n c l u d i n g man. Those components t h a t may be d i r e c t l y impacted by soil bound residues because o f t h e i r p r o x i m i t y would i n c l u d e agronomic p l a n t s , a q u a t i c organisms, soil processes, and soil organisms. S p e c i f i c a l l y , there is concern t h a t changes in c u l t u r a l p r a c t i c e s may l i b e r a t e bound r e s i d u e s , r e i n t r o d u c i n g them i n t o the soil s o l u t i o n and subsequent uptake and t r a n s l o c a t i o n i n t o t h e economic p o r t i o n s o f p l a n t s . I tisconceivable t h a t soil bound p e s t i c i d e residues may e n t e r i n t o the a q u a t i c environment, be r e l e a s e d , and subsequently be accumulated in a q u a t i c food chains. L i k e w i s e , there is some concern t h a t continued b u i l d u p o f bound r e s i d u e s in s o i l s may a f f e c t important p h y s i c a l and biochemical processes such as water h o l d i n g c a p a c i t y , soil s t r u c t u r e , and the processes o f n i t r i f i c a t i o n , ammonification, c e l l u l o s e decomposition, and a host o f r e l a t e d processes. F i n a l l y , there is some concern t h a t bound residues may a d v e r s e l y a f f e c t soil organisms such as the earthworm. P r e s e n t l y there appears t o be little foundation f o r these concerns, based on t h e current a v a i l a b l e l i t e r a t u r e . Nevert h e l e s s , it was p o i n t e d out t h a t experiments should be designed t o a s c e r t a i n t h e s i g n i f i c a n c e o f each o f these p o t e n t i a l problems. In summarizing the o v e r a l l philosophy o f the s i g n i f i c a n c e o f soil bound r e s i d u e s , two opposing viewpoints were presented. On the negative s i d e , it can be argued t h a t bound r e s i d u e s are r e a l l y hidden residues t h a t keep an i n t a c t molecule capable o f subsequent r e l e a s e and e x e r t i o n o f long-term b i o l o g i c a l e f f e c t s . On the p o s i t i v e s i d e , it can be argued t h a t b i n d i n g o f soil r e s i dues represents the most e f f e c t i v e and s a f e s t method o f decontami n a t i o n by r e n d e r i n g the molecule innocuous and a l l o w i n g slow degradation in the bound s t a t e t o products t h a t pose no s h o r t or long-term problems. I n defense o f the second argument, s e v e r a l examples were c i t e d where soil d i s p o s a l represented a reasonable o p t i o n f o r d i s p o s a l o f hazardous m a t e r i a l s . In these cases, b i n d i n g o f the t o x i c residue represents a mechanism o f immobili z i n g the t o x i c a n t so t h a t o t h e r soil processes can degrade the molecule.
Kaufman et al.; Bound and Conjugated Pesticide Residues ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
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CONJUGATED PESTICIDE RESIDUES
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In summary, it was recommended t h a t the D i v i s i o n o f P e s t i c i d e Chemistry, American Chemical S o c i e t y take no s t r o n g p o s i t i o n on soil bound r e s i d u e s at the current time. A s e r i e s o f recommendations now might be counterproductive t o the development o f e x p l o r a t o r y research needed t o e l u c i d a t e the t r u e s i g n i f i c a n c e and nature o r soil bound r e s i d u e s . The r e g u l a t o r y agenc i e s , p a r t i c u l a r l y EPA, expressed a w i l l i n g n e s s t o remain f l e x i b l e on requirements on bound r e s i d u e s t u d i e s . I t w i l l be the p o l i c y o f these agencies in the f u t u r e t o cons i d e r these compounds on t h e i r i n d i v i d u a l m e r i t s as t o what a d d i t i o n a l i n f o r m a t i o n is needed on soil bound r e s i d u e s .
Kaufman et al.; Bound and Conjugated Pesticide Residues ACS Symposium Series; American Chemical Society: Washington, DC, 1976.
