Chapter 16
The Environmental Impact of Pesticide Degradates in Groundwater
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Michael R. Barrett Office of Pesticide Programs, U.S. Environmental Protection Agency, 401 M Street, Southwest, Washington, DC 20460
Pesticide environmental fate and ground-water monitoring studies often only analyze for residues of parent. Yet available environmental fate data for many pesticide degradates indicate a higher propensity to leach in soil than the respective parent compound. Even in cases where no mobile degradate has been found to accumulate in soil to any great extent (generally true with s-triazine and acetanilide herbicides, for example), the existence of a chemically stable molecular "core" is an indicator that some degradates could reach ground water. Recent studies with atrazine and alachlor have demonstrated that quantities of degradates in ground water may exceed (sometimes by a great amount) the quantities of the parent compound. Ground-water monitoring for newer, low-rate pesticides such as sulfonylurea herbicides and their degradates is almost nonexistent. These low-rate herbicides have environmental fate properties indicating they may be more mobile in soil than some pesticides that have already been found to impact ground water at numerous locations. In cases where degradates are found to be of toxicological significance, use limitations might be needed to mitigate impact of the degradates on ground water.
Until recently, little research has been available on residues of pesticidedegradatesin ground water. This has limited our understanding of the scope of ground-water contamination by pesticides. Most pesticides havedegradatesthat have little apparent biological activity on target pests but thesedegradatesoften do have environmental fate characteristics such that the potential for leaching to ground water appears to be significant. This is particularly true for the majority of the major soil-applied herbicides
This chapter not subject to U.S. copyright Published 1996 American Chemical Society Meyer and Thurman; Herbicide Metabolites in Surface Water and Groundwater ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
16. BARRETT Environmental Impact of Pesticide Dégradâtes in Groundwater
201
currently registered in the United States. This paper examines the available data on the occurrence of some of these degradates in ground water and estimates from basic environmental fate data the likely potential for leaching to ground-water of degradates of selected pesticides. The data are also briefly compared to toxicological endpoints, including established or proposed lifetime Health Advisories (HAs) or Maximum Contaminant Levels (MCLs) in drinking water.
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Overview Of Environmental Concerns For Pesticide Dégradâtes Toxicological Concerns. Except for propesticides (pesticides designed to be converted after application to a new compound with pesticidal activity), activity ofdegradateson target pests is generally significantly lower than the parent compound activity. Indeed, a major selection criterion for prospective pesticides is toxicity to pests at low doses which do not adversely affect crops or other non-target organisms. These factors may reduce, but do not preclude the possibility of significant activity ofdegradateson various nontarget species. With respect to occurrence in water that may be used for drinking, there is also the human health issue. Regulations are designed to ensure safety to humans even with the many unknown factors about the effects of relatively low-level (but often long-term) exposure to pesticides. Often the toxicological endpoints upon which drinking water standards are based may not be related to the mode of action of the pesticide on the target organism. The mode of action of herbicides, for example, is usually not applicable to mammalian species. Some pesticides are readily converted to other compounds which are toxic to the target organisms. Organophosphate and organosulfur insecticides commonly have initial degradation products with well-established insecticidal activity, often of greater potency than the compound originally applied. In some cases, rapidly formeddegradatesmay be relied on to provide the bulk of the pest control for which the chemical is applied. Maloxon is such a product formed by oxidative desulfuration of the thione moiety (a common activation pathway for organophosphates) of malathion. A very common reaction observed in many sulfide-containing pesticides is oxidation to sulfoxides and sulfones which are usually quite active on a spectrum of pests similar to the parent compound. Formation of aldicarb sulfoxide and sulfone is an example of this. Toxicologically activedegradatesof insecticides tend to be more mobile in soils than the respective parent compound (2). However, the efficacy and toxicity of insecticides in the form applied tends to be inversely correlated with water solubility (3). Even though few insecticides have previously been found to extensively impact ground water, the potential for persistent and mobile degradates to impact ground water may be significant. Ground-Water Residues. Overall, monitoring data for pesticide degradates are relatively sparse. The U.S. E P A Pesticides in Ground Water Database (1) includes reports on analyses of 29 degradates of currently used pesticides, with, except for a handful of compounds, very little evidence of a significant impact of these compounds on ground water (Table I). Exceptions to the general lack of research emphasis on degradates have been for a few pesticides which rapidly transform into compounds that are intended to be biologically active on target pests. For example, extensive
Meyer and Thurman; Herbicide Metabolites in Surface Water and Groundwater ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
202
HERBICIDE METABOLITES IN SURFACE WATER AND GROUNDWATER
Table I. Detections of metabolites included in EPA Pesticides in Ground Water Database (1) 1
Compound
2,6-diethylaniline (alachlor, etc.)
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3-hydroxy-carbofuran
No. of wells 305
No. with detec tions
Range of detections ug L*
MCL or HA*
1
0
—
2P 40P 40P
22314
42
0 - 10.0
3 -keto-car bofu ran
839
3
0.03 - 0.03
3,5-dichlorobenzoic acid (pronamide)
126
0
—
50P
3-hydroxy-dicamba
87
0
—
200P
aldicarb sulfone
37652
5070
0 - 153
2
aldicarb sulfoxide
37593
4991
0 - 1030
4
carbofuran phenol
126
0
DCPA acid
40P
118
59
0.2 - 431
4000P
demeton sulfone
188
0
—
None
deethyl atrazine
689
27
0.05 - 2.9
3P
deisopropyl atrazine (= deethyl simazine)
689
24
0.1 - 3.5
3P
endosulfan sulfate
1969
6
0.05 - 1.4
None
ethylene thiourea (maneb, mancozeb, etc.)
183
1
0.7
None
fenamiphos sulfone
180
0
—
2P
fenamiphos sulfoxide
180
0
...
2P 200P 2P
1
0
—
methyl paraoxon (methyl parathion)
125
0
...
molinate sulfoxide
malaoxon (malathion)
196
1
0.8
None
pliorate sulfone
12
0
...
None
phorate sulfoxide
12
0
...
None
phoratoxon (phorate)
9
0
...
None
phoratoxon sulfone (phorate)
9
0
...
None
phoratoxon sulfoxide (phorate)
9
0
...
None
phosmet oxygen analog
3
0
...
None
pirimicarb sulfone
1
0
—
None
rotenolone (rotenone)
4
0
...
None
terbufos sulfone
13
0
...
0.9
...
None
157 0 thiobencarb sulfoxide ^Full chemical names available from the author upon request.
^Values followed by a Ρ are for parent compound only; no MCL or (lifetime) HA exists for the degradate.
Meyer and Thurman; Herbicide Metabolites in Surface Water and Groundwater ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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16.
BARRETT Environmental Impact of Pesticide Dégradâtes in Groundwater
203
monitoring has been conducted for aldicarb and its sulfoxide and sulfonedegradatesby the registrant and some state agencies (1). The need for inclusion of these degradates in the analyses for aldicarb was obvious because all three forms (parent, sulfoxide, and sulfone) are known to be highly active on target pests and toxic to many nontarget organisms. In recent years, studies of several herbicide degradates (particularly of atrazine) have begun to provide insight into the impact that the consideration of degradates may have on pesticide regulation, and are discussed in detail below. As monitoring efforts have increased and analytical methods have become more sensitive, there have been many more detections of pesticides in ground water and more public concern about the possible health effects of these residues. Concerns will increase further i f it turns out that some of the most environmentally significant degradates of pesticides have adverse ecological effects or toxicological effects similar to those upon which the M C L for the parent molecule is based. Evaluation of the Leaching Potential of Pesticide Dégradâtes A comprehensive discussion of all registered pesticides and their degradation products is beyond the scope of this paper. The remainder of this paper discusses three families of herbicides with the following distinctive characteristics: • •
•
High volume use with relatively high soil persistence (the s-triazine herbicides, with specific discussion of atrazine and simazine) and moderate soil mobility. High volume use with low to moderate soil persistence but potentially persistent degradates (the acetanilide herbicides with specific discussion of acetochlor, alachlor and metolachlor) and moderate parent compound soil mobility. Very low use rates and variable soil persistence, but high soil mobility (the sulfonylurea family of herbicides).
Numerous ground-water monitoring studies over the last several years have included the s-triazine and acetanilide herbicides as analytes (except for acetochlor, which was not registered in the United States until 1994). Monitoring studies for sulfonylurea herbicides, which are much more difficult to analyze for, have not yet been published. Acetanilide and s-triazine herbicide parent compounds have been found to occur in ground water (1), and, more recently, some of theirdegradateshave been analyzed for and found in ground water (discussed below). It is no accident that these compounds are herbicides: With the lone exception of aldicarb (for which the registered uses have already been substantially reduced) a large majority of ground-water detections of currently used pesticides have been of soil-applied herbicides. The potential contribution of the degradates to the total residues occurring in ground water is discussed directly from ground-water monitoring data, if available, and also by inference from soil degradation and adsorption studies. Full chemical names as well as abbreviated names used in the text for the pesticides and degradates discussed in the following sections are given in Table II, or; for the sulfonylurea herbicides discussed, chemical structures of parent compounds and degradates are given in Figures 1 and 2, respectively.
Meyer and Thurman; Herbicide Metabolites in Surface Water and Groundwater ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
204
HERBICIDE METABOLITES IN SURFACE WATER AND GROUNDWATER
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Table II. Chemical Names of Pesticide Parent Compounds and Respective Dégradâtes Discussed in Detail in the Text Common & Abbreviated Names (Parent in bold)
Chemical Name
acetochlor
2-chloro-N-ethoxymethyl-2'-ethyl-6'methylacetanilide
acetochlor sulfonic acid
N-ethoxymethyl-2'-ethyl-6'-methyl-2-sulfoacetanilide
acetochlor sulfinyl acetic acid
{[N-ethoxymethyl-N-(2'-ethyl-6'-methyl)phenyl]-2-amino-2oxoethyl} sulfinylacetic acid
acetochlor EOM-oxanilic acid
N-ethoxymethyl-2'-ethyl-6'-methyloxanilic acid
alachlor
2-chloro-2 ' ,6 ' -di ethy l-N-(methoxy-methy l)-acetanilide
alachlor sulfinyl acetic acid
[N-methoxymethyl-N-(2,6-diethylphenyl)-2-amino-2-oxoethyl] sulfinylacetic acid
alachlor oxanilic acid
2',6'-diethyloxanilic acid
alachlor sulfonic acid (ESA)
N-methoxymethyl-2',6'-diethyl-2-sulfoacetanilide
atrazine
2-chloro-4-ethylamino-6-isopropylamino-s-triazine f
deisopropyl atrazine (DEIS)
2-amino-4-chloro-6-ethylamino-s-triazine
deethyl atrazine (DEET)
2-amino-4-chloro-6-isopropylamino-s-triazine 1
diaminochloro triazine (DIAM)
2,4-diamino-6-chloro-s-triazine
hydroxy atrazine (HYAT)
2-ethylamino-4-hydroxy-6-isopropylamino-s-triazine
metolachlor
2-chloro-N-(2-ethyl-6-methy lphenyl)-N-(2-methoxy-1 methylethyl) acetamide
metolachlor acid
(N-(2'-methoxy-1 '-methylethyl)-2-ethyl-6-methyl-oxalic acid) anilide
(de-MOME) metolachlor acid
(2-ethyl-6-methyl-N-oxalic acid) anilide
(de-MOME) hydroxy acetamide
N-(2-ethyl-6-methylphenyl)-2-hydroxy-acetamide
(demethyl) hydroxy metolachlor
2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-hydroxy-1 methylethyl)acetamide
methyl morpholin
4-(2-methyl-6-ethylphenyl)-5-methylmoφhoIin
simazine
2-chloro-4,6-bis(ethylamino)-s-triazine
hydroxy simazine (HYSI) 2-hydroxy-4,6-bis(ethylamino)-s-triazine tTHese compounds aredegradatesof both atrazine and simazine.
Meyer and Thurman; Herbicide Metabolites in Surface Water and Groundwater ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
16. BARRETT Environmental Impact of Pesticide Dégradâtes in Groundwater
Herbicide
"X" Ring
Bensulfuron
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methyl
"Bridge"
£°2 3
-
OCH 3
- CH SO NH-S-NHf
Chlorimuron ethyl
_xC0 C H \0/-
Chlorsulfuron
yrr< ( O V
2
"Y» Ring
Ο
OH
2
f
Ο -SOj-NH-C-NH-
5
/N-"
2
C
V ^~
/ θ >
" \
N
3
S0 CH CH
/oV
3
3
N
^
3
0 C H
0
C
H
3
\ Sl
^Γ< -SQfNH-C-NH 2
0
1
N—
Ο /OCHF, -SCu-NH-C-NH- - / θ Ν
a
'
C0 CH.
Triasulfuron
K 1 U
-SC^NH-C-NH^0 CH
N
Prosulfuron
„
3
C H
y ^ " "Νιβ < /C H N
N ( C H
^Γ\
N
_
