Methomyl

Biodegradation in water and sediment: simulation tests

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

biodegradation in water: simulation testing on ultimate degradation in surface water

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 309 (Aerobic Mineralisation in Surface Water - Simulation Biodegradation Test)

Deviations:

no

Qualifier:

according to guideline

Guideline:

EPA OPPTS 835.3190 (Aerobic Mineralisation in Surface Water - Simulation Biodegradation Test)

Deviations:

no

GLP compliance:

yes

Specific details on test material used for the study:

Radiolabeled Methomyl
Lot #: 3652007
Purity: 99.8%
Specific Activity: 42.00 μCi/mg

Radiolabelling:

yes

Oxygen conditions:

aerobic

Inoculum or test system:

other: surface water

Duration of test (contact time):

62 d

Initial conc.:

10 µg/L

Based on:

act. ingr.

Initial conc.:

100 µg/L

Based on:

act. ingr.

Parameter followed for biodegradation estimation:

radiochem. meas.

Reference substance:

benzoic acid, sodium salt

Test performance:

The test substance did not significantly mineralize over the duration of the test. Since no significant mineralization occurred, no kinetic analysis of mineralization was performed.

Compartment:

other: Natural surface water

% Recovery:

97.1

Remarks on result:

other: 10 µg/L

Compartment:

other: Natural surface water

% Recovery:

95.3

Remarks on result:

other: 100 µg/L

Remarks on result:

other: No significant degradation in the non-sterile samples

Key result

Compartment:

other: Surface Water (Non-Sterile)

DT50:

284 d

Type:

other: SFO

Temp.:

20 °C

Key result

Compartment:

other: Surface Water (Sterile)

DT50:

> 1 yr

Type:

other: SFO

Temp.:

20 °C

Transformation products:

yes

Remarks:

IN-X1177 in the sterile samples. No major transformation products were observed in the non-sterile samples.

Evaporation of parent compound:

no

Volatile metabolites:

no

Residues:

no

Results with reference substance:

The reference compound was mineralized up to a mean of 97.5% AR after 30 days, confirming the viability of the system

Validity criteria fulfilled:

yes

Conclusions:

DT50 (surface water non-sterile): 284 days
DT50 (surface water sterile): > 1 year
Based on the results of this study, the test substance would not be expected to hydrolyze in surface water or further mineralize to CO2.

Executive summary:

The aerobic mineralization of [14C]-test substance was studied in a natural surface water at 20 ± 2°C in the dark according to the guidelines OECD 309 and U.S. EPA OPPTS 835.3190. The water was collected from a pond at Southeast Ag Research located in Chula, Georgia. The pH, biological oxygen demand, and total suspended solids were 7.1, 11.9 mg-O2/L, and 36 ppm, respectively.

A total of twenty bioreactors (1-liter capacity) containing 700 mL of surface water were used. To four of the twenty bioreactors, [14C]- test substance was applied at nominal rates of 10 μg a.i./L and 100 μg a.i./L. Prepared in duplicate for each concentration, these samples were used for mineralization testing at zero time, 1, 2, 3, 7, 14, 30, 45, and 62 days after dosing. Twelve of the twenty bioreactors, also prepared in duplicate for each concentration, were used for mass balance sampling at zero time, 30, and 62 days after dosing. To compare biotic and abiotic processes of the surface water with respect to mineralization, two of the twenty bioreactors were sterilized and prepared at a nominal rate of 100 μg/L. These samples were analyzed in parallel at 7, 14, 30, 45, and 62 days after dosing. To confirm microbial viability of the surface water, two of the twenty bioreactors were prepared with a reference compound ([14C]-sodium benzoate) at a nominal rate of 100 μg/L. These samples were analyzed in parallel at each interval for mineralization.

For mineralization testing, a sub-sample of the surface water was removed and assayed for radioactivity followed by acidification (pH 2-3 using concentrated hydrochloric acid) and purging with air to expel any CO2. Decreases in radioactivity following acidification were evaluated as possible mineralization. The potential for biotransformation was determined by HPLC analysis of the surface water prior to acidification.

The results indicated that test substance did not significantly mineralize (<5% applied radioactivity (AR) over the study duration. Due to the lack of mineralization, no mineralization rate constants were derived for this study. The reference compound (sodium benzoate) was mineralized up to a mean of 97.5% AR after 30 days, confirming the viability of the system.

The amount of [14C]-test substance in the surface water (non-sterile samples) showed a mean of 97.6% AR at time zero that decreased to 82.9% AR by Day 62. In the sterile samples, the amount of [14C]-test substance in the surface water decreased slightly to a mean of 88.3% AR by Day 62. No major transformation products (≥5% AR) were observed in the non-sterile samples. One major transformation product was observed as IN-X1177 in the sterile controls.

The DT50 values for the degradation of [14C]-test substance in non-sterile and sterile surface water are 284 days and >1 year respectively. The DT90 values for the degradation of [14C]-test substance in both non-sterile and sterile surface water is >1 year.

In surface water, [14C]-test substance did not degrade via abiotic and/or biotic processes, forming only minor amounts of known hydrolytic components. Based on the results of this study, [14C]-test substance would not mineralize from surface waters in the environment.

Reference 2

Endpoint:

biodegradation in water and sediment: simulation testing, other

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 308 (Aerobic and Anaerobic Transformation in Aquatic Sediment Systems)

Deviations:

no

Qualifier:

according to guideline

Guideline:

EPA OPPTS 835.4400 (Anaerobic Aquatic Metabolism)

Deviations:

no

GLP compliance:

yes

Specific details on test material used for the study:

Test substance Name: [14C]-Methomyl
Lot#: 3652007
Radiochemical Purity: 99.8%
Specific activity: 42 µCi/mg

Radiolabelling:

yes

Oxygen conditions:

anaerobic

Inoculum or test system:

natural water / sediment

Duration of test (contact time):

92 d

Initial conc.:

5 other: µg/mL

Based on:

act. ingr.

Parameter followed for biodegradation estimation:

radiochem. meas.

Reference substance:

other: IN-X1177

Compartment:

natural sediment

Remarks on result:

other: Overall, the mean material balance was 96.5 ± 9.5% and 76.9 ± 20.2% of the applied radioactivity (%AR) for the [14C]-labelled test substance treated Tift and Wabasha samples, respectively

Key result

Compartment:

water

DT50:

20.5 d

Type:

(pseudo-)first order (= half-life)

Temp.:

20 °C

Remarks on result:

other: Sediment type: Tift (Sandy loam)

Key result

Compartment:

water

DT50:

2.5 d

Type:

(pseudo-)first order (= half-life)

Temp.:

20 °C

Remarks on result:

other: Sediment type: Wabasha (Loam)

Other kinetic parameters:

first order rate constant

Transformation products:

yes

Remarks:

14CO2

Volatile metabolites:

yes

Remarks:

14CO2

Residues:

yes

Results with reference substance:

IN-X1177 was not detected in the water and sediment extracts during 30-Day incubation period.

Conclusions:

92 Day DT50 (Tilt, sandy loam): 20.5 days
92 Day DT50 (Wabasha, loam): 2.5 days

Executive summary:

The study was conducted to according OECD guideline 308 to evaluate the anaerobic aquatic metabolism of [14C]-labelled test substance in two water-sediment systems. The degradation of [14C]-labelled test substance was investigated in two water/sediment systems for up to 92 days under anaerobic conditions in the dark at 20±2°C. The water/sediment systems were from Chula, Tift County, Georgia, USA and Wabasha County, Minnesota, USA. The Tift water had a pH of 7.8, while the sediment was characterized as loamy sand with a pH of 8.0 and organic matter content of 0.6%. The Wabasha water had a pH of 7.4, while the sediment was characterized as a loam with a pH of 5.9 and organic matter content of 10.8%.

The water phase of the test systems were treated with [14C]- labelled test substance at a nominal rate of 5 μg a.i./mL water. The test systems were incubated in darkness at approximately 20 ± 2°C. Due to the volatility of many of the test substance degradates, anaerobic incubations were performed in static test systems and measurements of pH, dissolved oxygen (DO), and redox potentials were obtained using representative controls. Prior to separation of water and sediment layers, duplicate test systems from each sediment/water system were connected to a series of traps to collect volatile radioactivity. The gases exiting the test vessels were passed through a set of traps consisting of an Orbo™ tube followed by ethylene glycol, four organic solvent cold-traps in dry ice (two methanol and two acetonitrile), two aqueous KOH traps, ethanolamine:2-ethoxyethanol, and finally a second Orbo™ tube. The volatile traps were all analyzed by LSC, and the Orbo™ tubes were extracted with methanol and radioassayed by LSC.

Duplicate samples were analyzed at 0, 1, 3, 7, 14, 21, and 30 days (Wabasha) or 0, 1, 3, 7, 14, 21, 30, 62, and 92 days (Tift) after application. The water and sediment layers were first separated by decanting. The water layer was analyzed by LSC followed by direct injection onto reversed-phase HPLC. The sediment layer was extracted with 2:1 methanol: water and centrifuged to separate the extract. This extraction was repeated twice for a total of three extractions. Sediment extracts were analyzed by LSC. For representative sediment extracts containing >5% of the applied radioactivity, the pH was adjusted to >10.4 to retain any 14CO2 and 14C-acetic acid that may have been present, and the extract was rotary evaporated and analyzed by LSC before being analyzed by HPLC. The material lost upon concentration of the sediment extracts was presumed to be 14C-acetonitrile as all other degradates would not have volatilized during this concentration.

For both sediment systems, the average material balance was >90% until parent had degraded to <40% of the applied radioactivity (%AR). Once the parent had degraded to <40% AR, the production of volatile radiolabeled degradates led to a loss of mass balance at these later sampling intervals. Overall, the mean material balance was 96.5 ± 9.5% and 76.9 ± 20.2% of the applied radioactivity (%AR) for the [14C]- labelled test substance treated Tift and Wabasha samples, respectively.

Radioactivity in the water layer generally decreased over the course of the study from a mean of 101% and 95.0% AR at Day 0 to a mean of 44.8% and 16.2% AR at termination for the Tift and Wabasha systems, respectively. At Day 0, the mean extractable radioactivity from the Tift and Wabasha sediments was 3.4% and 1.5% AR, respectively. Extractable residues in the Tift sediment increased to a maximum of 9.2% (14 days) and were 6.2% at termination (92 days). Extractable residues in the Wabasha sediment increased to a maximum of 9.1% AR (7 days) and were 4.7% at termination (30 days). Non-extractable residues increased during the study, reaching a mean maximum of 27.5% (92 days) and 8.7% AR (14 days) for the Tift and Wabasha samples, respectively. No radioactivity above 4.1% AR was observed in the Tift volatile traps. The Wabasha volatile traps yielded a maximum of 22.9% AR (14 days), followed by a decline to 20.8% AR at 30 days. In the Wabasha traps, 14CO2 reached a maximum of 8.7% AR (14 days) followed by a decline to 1.5% AR at 30 days.

In the water layer of the Tift system, average levels of test substance decreased from 95.9% AR at day 0 to 2.4% AR at the end of the 92-days incubation. The major transformation products in the water layer were 14C-acetonitrile and dissolved 14CO2 which reached maximums of 29.7% and 22.3% AR at 62 and 92 days, respectively. 14C-Acetonitrile declined slightly to 20.2% of AR at 92 days. Three minor degradates, IN-X1177, 14C-acetamide, and 14C-acetic acid were also detected. IN-X1177 (<1% AR) was only observed at 7 days, and 14C-acetic acid was only observed in the 7 and 14 day samples with a maximum of 2.6% AR at 14 days. 14C-Acetamide reached a maximum concentration of 4.4% AR (14 and 21 days) before declining to 0% AR at 62 days. The largest single unassigned radioactive residue was 3.5% AR at 30 days.

Analysis of the Tift sediment extracts was complicated by the presence of volatile radioactivity. Direct evaporation of these extracts led to the loss of the majority (>70%) of the radioactivity. The extracts were adjusted to a basic pH with KOH in order to retain dissolved 14CO2 or 14C-acetic acid, if present. Neither test substance nor 14C-acetamide is appreciably volatile, so they were retained as well. Thus, volatile radioactivity lost upon concentration was presumed to be 14C-acetonitrile since this compound forms azeotropes with both methanol and water, and would be very volatile. After addition of base, selected sediment samples were concentrated by rotary evaporation and analyzed by HPLC. The major residue observed in the sediment extracts, based on volatility from base, was 14C-acetonitrile, which reached a maximum of 6.4% AR at 21 days before declining to 3.4% at 92 days. 14C-Acetamide increased gradually to 2.0% AR at 21 days and declined to <1% at 92 days. Traces of test substance (≤0.4% AR) and IN-X1117 (≤0.6% AR) were observed. No other radioactive residue exceeded 1.2% AR.

Non-extractable residues in the Tift sediment increased steadily throughout the study and reached a maximum of 27.5% AR at 92 days. Fractionation using a humic/fluvic acid extraction clearly showed that the majority of these bound residues reside within the humin fraction. At 92 days, approximately 7.6% AR was associated with the fulvic acid fraction, 0.3% AR was associated with the humic acid fraction, and approximately 19.7% AR was associated with the residual solids or humin fraction.

In the water layer of the Wabasha system, levels of test substance rapidly decreased from 88.6% AR at Day 0 to 0% AR at Day 14. The major transformation products in the water layer were 14C-acetamide, 14C-acetonitrile, and dissolved 14CO2, which reached maximums of 18.4%, 21.3%, and 20.3% AR at 7, 7, and 14 days, respectively. Both 14C-acetamide and 14C-acetonitrile declined to 0% AR by 30 days, and dissolved 14CO2 declined to 12.5% AR at 30 days. A minor degradate, 14C-acetic acid, was also detected at a maximum of 2.6% AR at 7 days. The largest single unassigned radioactive residue was 4.7% AR at 1 day.

Evaporation of the Wabasha sediment extracts led to the loss of the majority of the radioactivity, so the same process was followed as indicated above for the Tift sediment. The major residue observed in the sediment extracts, based on volatility from base, was 14C-acetonitrile, which reached a maximum of 6.7% AR at 7 days before declining to 2.3% at 30 days. 14C-Acetamide was observed in the sediment extracts as a minor component and ranged from 1.5 to 2.5% of AR in the sediment. No other radioactive residue, including test substance, was observed in the Wabasha sediment extracts.

Non-extractable residues increased to a maximum of 8.7% AR at 14 days before declining slightly to 7% of AR at 30 days.

The majority of the radioactivity in both test systems was found in the water layer until after the majority of the parent had degraded, and test substance was not observed to any significant extent in any sediment extracts. Therefore, degradation kinetics for [14C] labelled test substance was performed using the radioactive residues from only the water layer. The DT50 and DT90 values for [14C] labelled test substance was calculated by a single first-order (SFO) kinetic fit using ModelMaker® 4.0. The DT50, DT90 value and rate constants for [14C]-labelled test substance in the water layer of Tift (Sandy loam) sediment/water system was 20.5 days, 68.2 days and 0.034 respectively. The DT50, DT90 value and rate constants for [14C]-labelled test substance in the water layer of Wabasha (Sandy loam) sediment/water system was 2.5 days, 8.3 days and 0.278 respectively.

Description of key information

Study Type	Study Details	Value	Guideline	Reliability
Biodegradation in water	surface water non-sterile surface water sterile	284 days > 1 year	OECD 309 EPA OPPTS 835.3190	1
Biodegradation in water and sediment	Tilt, sandy loam Wabasha, loam	20.5 days 2.5 days	OECD 308	1