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Chapter 6
Pesticide Storage Dissipation in Surface Water Samples Chang Sook Lee Peoples Department of Pesticide Regulation, California Environmental Protection Agency, Sacramento, California 95812, United States E-mail:
[email protected].
Ideally, surface water samples should be analyzed immediately after sampling. However, such rapid sample processing is often not possible. Therefore, preserving surface water samples until they can be analyzed becomes a critical part of the pesticide monitoring and analysis process. The objective of this chapter is to determine the integrity of the sample to ensure proper quantification of pesticide residues in surface water while the samples are stored. No significant difference in recoveries was observed for most of the pesticides stored up to 14 days except for some pyrethroids. Resmethrin, deltamethrin, and trans-permethrin break down rapidly by day 4; however, adding hexane as a keeper solvent to surface water samples improves the preservation of these pesticides. The effects of water pH on the stability of organophosphate and carbamate pesticides were also evaluated. Organophosphate and carbamate pesticides are very stable in acidic conditions except for diazinon, which rapidly degrades in water with a low pH.
© 2019 American Chemical Society Goh et al.; Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment, and Management ACS Symposium Series; American Chemical Society: Washington, DC, 2019.
Introduction The California Department of Pesticide Regulation (CDPR) Surface Water Protection Program (SWPP) protects human health and the environment by preventing pesticides from adversely affecting our surface water. The goals of the SWPP are to characterize pesticide residues, identify the source of the contamination, determine the mechanisms of off-site movement of pesticides to surface water, and develop site-specific mitigation strategies. Prevention of and response to water contamination are both intricate parts of the SWPP, and such strategies are implemented primarily through surface water monitoring and research to identify the causes of off-site movement. The SWPP addresses both agricultural and nonagricultural sources of pesticide residues in surface waters. The program collects and tests approximately 800+ surface water samples per year throughout California. Most of the surface water samples are analyzed by the California Department of Food and Agriculture (CDFA) Center for Analytical Chemistry (CAC) in Sacramento. Ideally, surface water samples should be analyzed within the first 24 hours of sampling, but it is often very difficult or impossible to do so due to, for example, the distance between sampling sites and testing labs, time restrictions, and various logistical constraints. In most instances, samples are stored for a period of time prior to their analysis. Therefore, to ensure the integrity of collected water samples, stability studies of several pesticides need to be performed. To determine the optimal storage conditions for surface water samples prior to analysis, different storage periods and conditions were studied.
1. The Storage Dissipation of Various Pesticides in Surface Water Hydrolytically stable compounds are usually stable under long-term storage conditions. However, storage stability must be evaluated on a case-by-case basis. In general, the test should be run for the longest anticipated holding period. Some of the main factors affecting the stability of analytes in water samples include hydrolysis, photolysis, physical adsorption/desorption with suspended particulates, chemical degradation, and water solubility (1). The CAC conducted storage dissipation studies for pesticides that are commonly used in California in high quantities. Three spiked replicate samples were stored in amber bottles and refrigerated at 4 °C. At select intervals (i.e., at 0, 4, 7, 14, and 28 days), each spiked surface water sample was analyzed for recovery. After 28 days of storage, no significant difference in the recoveries was observed between day 0 and day 28 for all compounds listed in Table 1. The results clearly indicate that all the listed pesticides are stable for up to 28 days of storage.
90 Goh et al.; Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment, and Management ACS Symposium Series; American Chemical Society: Washington, DC, 2019.
Table 1. Storage Stability of Selected Pesticides % Recovery
Pesticide
Day 0
Day 4
Day 7
Day 14
Day 28
Ethalfluralina
85.1
86.2
87.0
93.2
92.9
Trifluralina
78.8
84.6
87.5
94.2
91.4
Benfluralina
85.6
84.6
86.9
93.7
93.6
Prodiaminea
89.2
87.4
89.9
98.0
94.8
Pendimethlina
87.2
83.0
84.9
91.8
91.6
Oxyfluorfena
84.0
80.9
86.4
91.5
89.5
Oryzalina
94.0
92.8
88.9
98.2
85.8
87.7
87.8
87.7
92.6
100.5
86.5
83.9
86.4
91.4
97.7
99.3
80.8
95.5
96.3
87.9
Desulfinyl Fipronil
Fipronilb
Sulfideb
Fipronilb Amideb
105.0
98.0
93.9
102.1
101.0
Fipronil
Sulfoneb
94.3
89.5
90.8
99.2
99.5
Fipronil
Amideb
105.8
106.3
102.7
109.6
95.7
74.6
72.3
75.0
Desulfinyl Fipronil
84.4
-
Alachlor
OXAc
89.3
-
90.8
85.5
79.8
Alachlor
ESAc
92.0
-
94.9
93.2
91.4
Alachlorc
84.3
-
77.2
77.8
80.7
Metolachlor
OXAc
86.1
-
84.8
88.6
96.2
Metolachlor
ESAc
97.5
-
101.5
81.9
95.7
Azoxystrobind
85.4
89.1
78.3
93.8
92.6
Kresoxim Methyld
84.4
86.0
72.1
87.2
78.7
Pyraclostrobind
83.0
85.9
73.8
91.4
89.5
Trifloxystrobind
75.4
74.9
63.4
79.9
71.6
Methoxyfenozidee
87.9
85.8
87.6
84.1
85.0
Tebufenozidee
87.4
85.1
87.5
83.7
84.1
Chlorothalonilf
73.7
79.4
87.1
92.0
91.8
Metolachlorc
CDFA analytical methods used: a EMON-SM-05-006 (2). b EMON-SM-05-013 (3). c EMON 38.0 (4). d EMON-SM-05-024 (5). e EMON-SM-05-026 (6). f EMON-SM-05-020 (7). Note: Day 4 recovery data for alachlor, alachlor OXA, alachlor ESA, metolachlor, metolachlor OXA, metolachlor ESA were not collected.
91 Goh et al.; Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment, and Management ACS Symposium Series; American Chemical Society: Washington, DC, 2019.
2. The Stability of Pyrethroids in Water Samples The CDFA carried out a study on the stability of pyrethroids in water samples that were stored in amber glass bottles and refrigerated at 4 °C. The study evaluated surface water samples spiked at 100 parts per trillion (ppt) over a 28-day period. The three replicates of spiked surface water samples were stored in the refrigerator until analysis at 0, 4, 7, 14, and 28 days. Pyrethroids were recovered at a rate of 87–104% at 0 days; however, the study showed rapid degradation for resmethrin, deltamethrin, and trans-permethrin by day 4 (see Table 2). The rate of recovery for these three compounds decreased to 51–65% by day 4 and 49–57% by day 7. Other pyrethroid compounds showed more gradual degradation. As such, water samples should be analyzed within 24 hours of collection to maximize the recovery of pyrethroids.
Table 2. Storage Stability of Pyrethroids in Surface Water Pesticide
% Recovery Day 0
Day 4
Day 7
Day 14
Day 28
Bifenthrin
86.7
77.6
66.2
61.0
54.8
Fenpropathrin
98.9
96.6
86.1
75.5
77.6
λ cyhalothrin epimer
98.1
87.3
73.7
68.0
54.2
λ cyhalothrin
96.6
83.5
74.7
67.7
52.8
Cis-permethrin
96.6
80.5
70.4
63.4
42.9
Trans-permethrin
104.3
65.3
48.7
34.2
14.6
Cyfluthrin
102.3
79.2
67.3
58.9
42.7
Cypermethrin
101.3
77.3
64.7
61.1
44.8
Fenvalerate/ Esfenvalerate
104.3
77.8
68.3
64.5
44.5
Deltamethrin
93.5
59.4
51.3
43.3
32.7
Resmethrin
91.6
50.6
56.9
45.1
30.3
CDFA analytical method used: EMON-SM-05-003 (8).
The effects of adding a keeper solvent to the samples to improve storage stability were also studied. The same procedures were repeated except that 900 ml of surface water were spiked and 10 ml of hexane were added as a keeper solvent to each water sample. The amount spiked was kept the same; however, the spiking level was increased to 111 ppt due to the change of the total sample volume. The samples with the keeper solvent added showed little to no degradation for all compounds up to 28 days. With the keeper solvent added, recoveries were at 95–108% at 0 days and 66–100% after 4 days, with most compounds yielding increased recoveries compared to the samples without the hexane keeper solvent (Table 3). 92 Goh et al.; Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment, and Management ACS Symposium Series; American Chemical Society: Washington, DC, 2019.
Table 3. Storage Stability of Pyrethroids in Surface Water with Hexane Pesticide
% Recovery Day 0
Day 4
Day 7
Day 14
Day 28
Bifenthrin
95.2
87.7
79.0
75.3
83.4
Fenpropathrin
101.1
100.1
96.4
90.4
95.5
λ cyhalothrin epimer
103.0
95.2
88.2
83.5
90.0
λ cyhalothrin
101.3
91.9
87.3
80.5
86.8
Cis-permethrin
100.3
91.4
89.7
83.6
91.2
Trans-permethrin
108.3
93.7
92.2
85.8
93.8
Cyfluthrin
105.0
86.0
89.2
78.5
86.6
Cypermethrin
105.7
81.3
89.3
77.3
82.3
Fenvalerate/Esfenvalerate
109.7
79.5
88.6
77.5
81.5
Deltamethrin
89.9
65.7
74.5
65.5
69.3
Resmethrin
102.4
78.6
90.0
88.1
85.9
CDFA analytical method used: EMON-SM-05-003 (8).
3. Effect of Water pH on the Stability of Pesticides One of the main factors affecting the stability of pesticides in water samples is hydrolysis. Some pesticides, particularly the commonly used organophosphates and carbamates, are more susceptible than others to chemical reactions in alkaline water due to alkaline hydrolysis. Table 4 shows how different pH values affect the half-life of selected pesticides (9–11). To determine the effect of pH in actual surface water samples, several organophosphates were spiked at a level of 0.25 parts per billion (ppb) with pH levels at 3, 7, and 8.5 in surface water and stored up to 28 days. Each spiked sample was analyzed for recoveries. With the exceptions of profenofos, malathion, and diazinon, most compounds stayed somewhat stable throughout the 28-day period with pH having a minimal effect on degradation (Table 5). Figures 1 and 2 show profenofos and malathion rapidly degrading under alkaline conditions. The recoveries for profenofos and malathion at day 0 with a pH of 8.5 were 69.2 and 70.2%, respectively, but by day 2 the recoveries drastically dropped to 49.8 and 32.0%, respectively. However, for samples with a pH of 7 and 3, both profenofos and malathion were relatively stable up to 21 days. Figure 3 clearly shows that diazinon behaves very differently from other organophosphates. Under acidic conditions (pH of 3), diazinon experiences rapid degradation. The recovery at day 0 started at 81.6% but decreased to 37.6% by day 2. By day 7, the recovery decreased to less than 3%. It is clear that diazinon goes through acidic hydrolysis rather than alkaline hydrolysis. As such, diazinon should be analyzed separately from other acidified organophosphate samples. 93 Goh et al.; Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment, and Management ACS Symposium Series; American Chemical Society: Washington, DC, 2019.
Table 4. The Half-Lives of Selected Pesticides at Different pH Values Pesticide
Half-Life Time at Different pH Solutions 5
7
6
8
9
Acephate
40 d
46 d
16 d
Azinphos methyl
17 d
10 d
12 h
4d
45 m
Bendiocarb Carbaryl
125 d
27 d
2-3 d
1-3 d
Carbofuran
200 d
40 d
5d
3d
Chlorpyrifos
63 d
35 d
22 d
Diazinon
14 d
70 d
21 d
Dimethoate Disulfoton
60 h
29 d 1h
32 h
7h
8d
3d
19 h
Methomyl
54 w
38 w
20 w
Parathion methyl
690 d
120 d
Phosmet
7d
12 h
Propargite
7d
12 h
Malathion
331 d
10
2h
29 h 1m
4h 1d
w = weeks, d = days, h = hours, m = minutes.
94 Goh et al.; Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment, and Management ACS Symposium Series; American Chemical Society: Washington, DC, 2019.
Table 5. Storage Stability of Organophosphate at pH 3, pH 7, and pH 8.5 Pesticide
Ethoprop
Diazinon
Disulfoton
Chlorpyrifos
Malathion
Methidathion
Fenamiphos
Methyl Azinphos
Dichlorvos
Phorate
% Recovery Day 0
Day 2
Day 4
Day 7
Day 14
Day 28
pH 3.0
83.6
78.7
75.3
65.2
76.2
69.9
pH 7.0
86.6
77.7
85.3
67.0
70.8
69.0
pH 8.5
70.5
70.1
73.9
75.4
73.2
70.7
pH 3.0
81.6
37.6
13.8
2.81
0.1
0.0
pH 7.0
85.3
77.8
83.5
66.4
71.0
70.4
pH 8.5
70.4
80.4
73.3
75.1
74.0
72.5
pH 3.0
87.5
76.4
79.7
70.9
82.8
62.6
pH 7.0
91.8
72.3
88.0
71.9
72.1
48.2
pH 8.5
76.2
67.8
77.7
82.0
75.4
61.5
pH 3.0
83.7
79.6
72.7
65.5
80.3
69.1
pH 7.0
84.0
77.8
80.3
63.8
71.6
70.2
pH 8.5
69.7
71.9
69.7
72.0
71.7
68.9
pH 3.0
88.5
83.3
75.6
67.3
82.5
67.6
pH 7.0
87.9
80.2
82.1
67.8
70.1
64.9
pH 8.5
70.2
32.0
18.1
15.5
6.5
6.3
pH 3.0
91.1
87.3
76.4
66.2
81.4
64.6
pH 7.0
87.1
84.2
83.7
66.8
72.0
66.7
pH 8.5
73.3
78.6
70.2
72.0
69.3
65.9
pH 3.0
88.3
82.6
72.0
64.0
80.1
57.9
pH 7.0
83.7
80.0
81.2
67.8
77.0
66.7
pH 8.5
70.6
77.6
70.3
75.5
77.4
70.1
pH 3.0
88.5
92.5
77.2
74.8
78.6
63.8
pH 7.0
82.3
88.6
85.1
71.3
73.3
68.4
pH 8.5
66.8
88.9
65.8
74.9
65.2
60.6
pH 3.0
71.6
62.9
67.1
58.8
67.4
72.2
pH 7.0
71.7
58.3
68.6
60.6
56.2
59.0
pH 8.5
66.8
58.8
57.4
52.4
48.2
50.4
pH 3.0
70.7
59.1
63.6
55.4
63.0
63.5
pH 7.0
72.8
55.9
65.6
58.6
57.4
48.4
pH 8.5
67.6
61.0
63.3
60.7
60.2
63.7
Continued on next page.
95 Goh et al.; Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment, and Management ACS Symposium Series; American Chemical Society: Washington, DC, 2019.
Table 5. (Continued). Storage Stability of Organophosphate at pH 3, pH 7, and pH 8.5 % Recovery
Pesticide
Fenofos
Dimethoate
Methyl Paration
Tribufos (DEF)
Profenofos
Day 0
Day 2
Day 4
Day 7
Day 14
Day 28
pH 3.0
75.3
67.1
70.4
64.1
73.8
90.9
pH 7.0
74.1
64.3
72.4
67.4
67.3
82.2
pH 8.5
70.4
66.7
72.1
67.3
70.9
90.4
pH 3.0
78.5
71.8
72.0
65.8
72.7
83.2
pH 7.0
75.5
71.3
73.5
66.6
66.5
81.8
pH 8.5
71.9
67.2
68.8
64.3
63.4
70.9
pH 3.0
79.3
72.3
72.1
65.3
72.6
80.4
pH 7.0
76.7
70.7
73.7
66.2
65.8
78.0
pH 8.5
70.9
70.8
73.0
66.8
68.9
78.7
pH 3.0
76.8
71.3
71.4
68.4
74.9
68.8
pH 7.0
73.5
64.1
70.7
60.7
66.8
66.6
pH 8.5
69.8
67.8
70.1
68.1
68.6
67.2
pH 3.0
78.0
72.8
72.4
67.2
76.1
73.3
pH 7.0
74.6
69.8
73.1
66.2
69.2
73.6
pH 8.5
69.2
49.8
43.1
36.4
26.0
17.9
CDFA analytical method used: EMON-SM-46 (12).
Figure 1. Profenofos dissipation at pH levels of 3, 7, and 8.5. 96 Goh et al.; Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment, and Management ACS Symposium Series; American Chemical Society: Washington, DC, 2019.
Figure 2. Malathion dissipation at pH levels of 3, 7, and 8.5.
Figure 3. Diazinon dissipation at pH levels of 3, 7, and 8.5. 97 Goh et al.; Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment, and Management ACS Symposium Series; American Chemical Society: Washington, DC, 2019.
Table 6. Storage Stability of Carbamates at pH 3 and pH 8.5 % Recovery Pesticide
Day 0
Day 5
Day 14
Day 28
Day 42
Day 56
Day 70
Day 84
pH 3.0
88.0
76.0
80.0
56.0
46.0
52.0
40.0
39.0
pH 8.5
90.0
82.0
92.0
88.0
73.0
91.0
72.0
85.0
pH 3.0
97.0
92.0
101.0 101.0 92.0
101.0 97.0
96.0
pH 8.5
95.0
75.0
74.0
49.0
62.0
60.0
58.0
57.0
pH 3.0
97.0
93.0
87.0
93.0
87.0
93.0
95.0
90.0
pH 8.5
89.0
52.0
45.0
41.0
35.0
30.0
28.0
25.0
pH 3.0
99.0
95.0
98.0
93.0
87.0
92.0
92.0
87.0
pH 8.5
94.0
93.0
94.0
96.0
83.0
97.0
92.0
96.0
pH 3.0
94.0
91.0
70.0
82.0
81.0
87.0
93.0
77.0
pH 8.5
88.0
48.0
42.0
34.0
34.0
23.0
28.0
20.0
Methiocarb Sulfoxide
pH 3.0
103.0 96.0
108.0 85.0
83.0
89.0
89.0
84.0
pH 8.5
43.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
Methiocarb Sulfone
pH 3.0
86.0
101.0 57.0
43.0
35.0
35.0
36.0
34.0
pH 8.5
12.5
0.0
0.0
0.0
0.0
0.0
0.0
0.0
pH 3.0
100.0 91.0
94.0
91.0
91.0
89.0
95.0
93.0
pH 8.5
86.0
34.0
28.0
20.0
17.0
17.0
13.0
9.0
pH 3.0
86.0
69.0
71.0
75.0
57.0
60.0
49.0
56.0
pH 8.5
67.0
44.0
74.0
70.0
62.0
57.0
60.0
68.0
Aldicarb Carbofuran 3-OH Carbaryl
Methomyl
Methiocarb
Oxamyl Aldicarb Sulfoxide
CDFA analytical method used: EMON-SM-11 (13).
Carbamate pesticides are also susceptible to hydrolysis in alkaline water. To determine the effects of pH on carbamate hydrolysis, the CDFA completed a storage dissipation study to test for the potential breakdown of carbamates in surface water over 84 days at two different pH conditions. Each carbamate was spiked at a level of 2.0 ppb, replicated twice, and stored for 84 days in amber glass bottles at 4 °C. Aldicarb, aldicarb sulfoxide, and methomyl were fairly stable up to 84 days at 4 °C with a pH of 8.5; however, carbaryl, oxamyl, and methiocarb and its metabolites were extremely unstable under the same conditions but at a pH of 8.5 (Table 6). Their recoveries on day 5 dropped to half or as little as 0%. The results also indicated that most of the carbamates were stable for up to 14 days if adjusted to a pH of 3 and refrigerated at 4 °C.
98 Goh et al.; Pesticides in Surface Water: Monitoring, Modeling, Risk Assessment, and Management ACS Symposium Series; American Chemical Society: Washington, DC, 2019.
Conclusion The most effective way to handle surface water samples is to analyze them immediately after sampling; however, as previously stated, this is not always possible. In reality, it is sometimes necessary to store surface water samples before analysis. As shown by the CAC’s storage dissipation studies, most pesticide compounds are quite stable in storage for up to at least 14 days when the samples are refrigerated and/or acidified, except for diazinon. Unlike other organophosphate pesticides, diazinon is highly unstable when exposed to acidic conditions, and should be stored without the addition of acid. Resmethrin, deltamethrin, and trans-permethrin rapidly break down within 4 days, so pyrethroid surface water samples should be extracted and analyzed as soon as possible after sampling. If immediate analysis is not possible, hexane should be added as a keeper solvent to slow down the degradation process. Aldicarb and methomyl are fairly stable in alkaline conditions, but carbaryl, oxamyl, methiocarb, and its metabolites are extremely unstable at a pH of 8.5. Acidifying carbamate samples to a pH of 3 will increase the recoveries for all the carbamates in the study, so carbamate samples should be acidified before storage. As the storage of samples is an integral part of pesticide analysis, pesticide storage dissipation studies cannot be overlooked in understanding the integrity of each pesticide and ensuring proper quantification of pesticide residues in surface water.
Acknowledgments I would like to express my gratitude towards Jane White, Jean Hsu, Stephen Siegel, and other chemists at the CDFA CAC Environmental Monitoring Section for their hard work in conducting the studies and providing all the analytical chemistry data.
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