Summary:
Concentrations of acetochlor were low with about 80% of samples containing no detectable concentrations of acetochlor. Peak concentrations occurred mainly in May and June. Time-weighted annualized mean concentrations (AMCs) in water from the 175 sampling sites did not exceed the EPA-mandated trigger of 2 ppb. AMCs increased as a function of corn intensity, and are expected to be lower in areas of lower corn intensity.
The results of this study demonstrate that acetochlor does not present a health concern in surface drinking water.
The Acetochlor Registration Partnership (ARP) conducted a surface drinking water monitoring program (SWM) as part of an agreement with EPA to maintain registration of acetochlor and monitor its environmental behavior.
Objective:
The original objective of the SWM was to determine concentrations of acetochlor and other major corn (maize) herbicides in finished drinking water derived from surface water sources. This objective was expanded in late 1999 by the addition of two classes of soil degradates to the anlaytes being monitored.
Design:
A total of 175 community water systems (CWSs) with watersheds in corn (maize)-producing areas of Illinois, Iowa, Indiana, Kansas, Minnesota, Nebraska, Wisconsin, Ohio, Missouri, Pennsylvania, Maryland, and Delaware, were selected for the program. The selected CWSs use only surface water, or can discretely sample surface water. At least 150 sites continued to be sampled through December 2001.
CWSs were randomly selected and represent a wide array of watersheds covering about 600 million acres in areas where corn is planted. There is a focus on small watersheds in areas of high corn production:
Breakdown of CWS (as of 1997, varies slightly each year)
| Small | >20% corn intensity | 75 |
| Small | 11-20% corn intensity | 32 |
| Small | 5-10% corn intensity | 43 |
| Continental River | | 17 |
| Great Lakes | | 8 |
The 175 CWSs serve populations ranging from 167 to 5,100,000 people (absolute numbers may vary somewhat from year to year), the majority of systems serve small communities; 15 CWSs serve a population of 100,000 or more. The watersheds cover areas ranging from 83 to 443,533,492 acres. All of the 175 CWSs in the program use some type of conventional treatment (coagulation, flocculation, sedimentation, and filtration) for their water. There were 25-30 CWSs using a granular activated carbon (GAC) treatment and 115-120 CWSs using a powdered activated carbon (PAC) treatment. Most of the GAC units are used by systems in areas of very high corn intensity (>20% corn intensity). Approximately 120 CWS's are fed by at least one reservoir.
Finished water samples were collected from the sampling sites every two weeks from mid-March through early September, when concentrations of corn herbicides are expected to be at their highest and most variable. Two additional samples were collected in November and January to represent the autumn and winter months. In addition to the finished water samples, raw water samples are collected from all CWSs that use granular activated carbon and from several systems that use powdered activated carbon. Sampling was conducted at 175 sites from March 1995 through late 1999, and continued at more than 150 sites through December 2001.
Analytical Results:
The analytical results show that there was no acetochlor detected in approximately 80% of all individual samples of finished surface water, collected since 1995. Peak concentrations of acetochlor occur mainly in May and June and rapidly dissipate. It is noteworthy that acetochlor and alachlor time-weighted annualized mean concentrations (AMCs) in surface water have not exceeded 2 ppb, the MCL for alachlor and the annual mean concentration limit for acetochlor, as set forth in the conditions of registration for acetochlor. Until an official MCL is established for acetochlor, EPA and the ARP have agreed on a concentration limit of 2 ppb. This represents an extremely conservative benchmark that provides a wide margin of safety against any potential risk to human health.
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This page was last updated on 13-Jan-03.
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