Summary
In December 1999, the U.S. Environmental Protection Agency (EPA) requested that the ARP analyze water samples collected during 1999 in the State Ground Water and State Surface Water monitoring programs for two soil degradates of acetochlor. The EPA also requested that such sampling continue through December 2001, thereby providing three years of monitoring data for these materials. The monitoring is now complete, which suggests the degradates are detected somewhat more frequently than parent acetochlor. However, a detailed discussion of these results is provided here, demonstrating that the presence of these trace concentrations in surface and ground water poses no significant risk to public health or the environment.
The Acetochlor Registration Partnership (ARP) analyzed water samples collected during the years 1999 through 2001 for the presence of two tertiary amide soil degradates of acetochlor: sulfonic acid (Ac-ESA) and oxanilic acid (Ac-OXA). For comparative purposes, the ARP has used an analytical method (liquid chromatography with MS/MS detection) which accurately quantifies both these acetochlor degradates and the corresponding ESA and OXA degradates of two other widely-used chloro-acetanilide herbicides, alachlor (Al) and metolachlor (Me).
When acetochlor and other chloro-acetanilide herbicides are sprayed on a farmer’s field, natural processes in the soil convert these herbicidal chemicals to a variety of water soluble degradates. ESA and OXA are the two major water soluble degradates. These products tend to move more readily through soil than the herbicidal parent compounds from which they are formed. Highly polar and water soluble compounds such as these are generally of low toxicity. To confirm that this is the case with ESA and OXA, the ARP conducted a series of toxicological studies to evaluate any potential human or ecological effects that may be associated with these materials. The results from these studies confirm that ESA and OXA are poorly absorbed, rapidly excreted and exhibit a low degree of toxicity to mammals and aquatic organisms. Although low levels of these degradates may be found in surface and ground water supplies in areas where corn is grown, ESA and OXA are not considered to be of toxicological concern and will not pose a significant risk to human health or the environment.
The discussion below is limited to samples collected during the first year of degradate monitoring (1999). Very similar results were observed during the final two years of degradate monitoring (2000 and 2001).
Ground Water Results
Samples from the quarterly sampling rounds of the State Ground Water Monitoring (GWM) Program were analyzed for the six soil degradates. As was seen for the corresponding parent compounds, concentrations of the degradates are generally a bit higher in June, likely a response to late spring runoff to very shallow monitoring wells. The 95th percentile concentrations are shown in the table below. The rank order of the analytes is the same in all four rounds of sampling as follows: Me-ESA > Al-ESA > Ac-ESA > Me-OXA > Ac-OXA ~ Al-OXA. These findings are similar to those reported by Mike Thurman of the United States Geological Society (USGS), who uses different analytical methods.
95th Percentile Concentrations of Acetanilide Soil Degradates
in the GWM Wells (reported in parts per billion)
| Analyte | Q1 ‘99 | Q2 ‘99 | Q3 ‘99 | Q4 ‘99 |
| Ac-ESA | 1.98 | 2.59 | 1.94 | 1.79 |
| Ac-OXA | <0.50 | <0.50 | <0.50 | <0.50 |
| Al-ESA | 3.63 | 3.95 | 3.27 | 4.45 |
| Al-OXA | <0.50 | <0.50 | <0.50 | <0.50 |
| Me-ESA | 6.27 | 6.59 | 6.16 | 5.35 |
| Me-OXA | 0.94 | 0.78 | 0.75 | 0.73 |
The higher levels of metolachlor degradates provide an upper-bound estimate of the concentrations which might be seen for acetochlor if it were used as widely and for as long a period as the more persistent metolachlor herbicide.
The lower frequency of detection of the OXAs in ground water is consistent with a conceptual model of acetanilide herbicide degradation in which both OXA and ESA are initially formed in similar quantities in soil, but OXA subsequently degrades more rapidly than ESA. This model is consistent with the laboratory and in situ field degradation studies previously conducted, and is corroborated by these new field monitoring data.
Surface Water Results
All 1999 samples from the 175 surface water monitoring sites were analyzed for the six soil degradates. In addition, the January 2000 analytical results were used to better estimate the annualized mean concentrations during 1999. Per ARP protocol, finished drinking water was collected at all 175 sites. Ac-ESA and Ac-OXA were detected in only 20 percent and 36 percent of the finished drinking water samples, respectively. The maximum and 95th percentile annualized mean concentrations in finished drinking water are given in the table below.
Annualized Mean Concentrations (reported in parts per billion)
of Acetanilide Soil Degradates in Finished Surface Water
| Analyte | Maximum | 95th Percentile |
| Ac-ESA | 0.70 | <0.50 |
| Ac-OXA | 1.26 | 0.56 |
| Al-ESA | 1.75 | <0.50 |
| Al-OXA | 0.57 | <0.50 |
| Me-ESA | 1.67 | 1.01 |
| Me-OXA | 0.92 | 0.66 |
The magnitude of the concentrations seen in the finished surface water are similar to those observed in shallow ground water, except that they are generally somewhat lower for ESAs and somewhat higher for OXAs. The trend for relatively higher concentrations of Ac-OXA than Ac- ESA in surface water vs. ground water is consistent with the conceptual model already presented, with the clarification that initial formation rates of Ac-OXA appear to be a slightly greater than those for Ac-ESA. The time series data for surface water generally show Ac-OXA declining more rapidly than Ac-ESA during the season. Looking at the 95th-percentile annualized mean concentrations, the overall rank order of 1999 levels in finished drinking water is as follows: Me-ESA > Me-OXA > Ac-OXA > Ac-ESA ~ Al-ESA ~ Al-OXA.
Use of Data for Ac-ESA and Ac-OXA Risk Assessment
Exposure via Ground Water: A comparison of the ARP’s shallow monitoring well results for alachlor, atrazine, and metolachlor has been made with published findings for these three compounds (Holden, L.R., J.A. Graham, R.W. Whitmore, W.J. Alexander, R.W. Pratt, S.K. Liddle, and L.L. Piper, 1992, Environ. Sci. Technol., 26, 935-943.). The monitoring results for these three compounds in drinking water wells (which are generally much deeper), demonstrate that the 95th percentile concentration measured in the ARP wells exceed the 99th percentile of concentrations in drinking water wells in every round of monthly ARP sampling. It is therefore reasonable to use the highest measured 95th percentile values for the two degradates as a conservative estimate of the lifetime (chronic) exposure concentration for individuals living in acetochlor-use areas and using wells for their drinking water, i.e. 2.59 PPB for Ac-ESA and < 0.50 PPB for Ac-OXA.
Exposure via Surface Water: The ARP is directly measuring concentrations in drinking water derived from surface sources. However, a key question is whether results observed in 1999 may be used to estimate lifetime (chronic) exposure. Analysis of the parent concentrations in the ARP data set has demonstrated that the annual variation in 95th percentile concentrations across the acetochlor use region is only about a factor of 2 to 3. This would suggest that the highest mean annualized values for the soil degradates of acetochlor measured this year in finished water could be used for chronic exposure estimates with only minimal error, i.e. 0.70 PPB for Ac-ESA and 1.26 PPB for Ac-OXA.
Hazard Evaluation: A series of toxicological studies has been initiated to evaluate any potential human or ecological effects that may be associated with the ESA and OXA degradates. Initial results from these studies indicate that these compounds are poorly absorbed, rapidly excreted and exhibit a low degree of toxicity to mammalian and aquatic organisms. The No-Observable-Adverse-Effect-Levels (NOAELs) in 90-day rat studies with both compounds ranged from 225 to 268 mg/kg/day. No acute mammalian toxicological hazards were identified, and no effects on aquatic organisms were observed at even the highest potential environmental concentrations of the two degradates.
Risk Assessment: The highest concentrations of Ac-ESA were found in ground water. Thus, the potential human risk from long-term exposure to Ac-ESA was estimated using the highest 95th percentile quarterly value (2.59 ppb in June 1999) as a conservative estimate of the potential long-term exposure concentrations for people using wells as their sole source of drinking water. For Ac-OXA, the highest concentrations were detected in surface water. Thus, the potential human risk from long-term exposure to Ac-OXA was estimated based on the maximum annualized mean concentration of 1.26 ppb that was detected in drinking water derived from finished surface water. Given these scenarios, a 70 kg adult (154 lb.) could drink more than 18 million 8-ounce glasses of water per day and still not reach the level of exposure causing no adverse effect in the toxicity studies of these materials.
Conclusion: Based on this analysis, it is clear that the presence of low levels of these degradates in surface and ground water poses no significant risk to public health or the environment. Under recently proposed EU guidelines (Guidance Document on Relevant Metabolites, October 1999), the acetochlor degradates would be considered "not relevant" for human risk assessment.
This page was last updated on 13-Jan-03.
Copyright © 1995-2003 Acetochlor Registration Partnership. See notice on Home Page.
Please direct Web site questions and comments to:
Dave Gustafson
Monsanto Company
800 N. Lindbergh Blvd
St. Louis, MO 63167