Summary:
The ARP has successfully conducted a suite of eight prospective ground water trials within a very aggressive timeframe. The analytical data confirm that under "worst case" scenarios acetochlor does not leach to ground water when used in accordance with label recommendations.
Objective:
The objective of the Prospective Ground Water (PGW) monitoring program is to investigate the potential for acetochlor transport to shallow ground water under managed test conditions resulting from the labeled use of herbicides which have acetochlor as the active ingredient. The Acetochlor Registration Partnership (ARP) is conducting this program, which was required as one of the conditions of the registration agreement between EPA and the ARP, dated March 11,1994.
Program and Study Design:
The acetochlor PGW program required a total of eight trials to be conducted across a range of soil textures and geographical locations. The distribution of soil textures within the program was weighted towards more vulnerable coarser soil types (loamy sand/sandy loam), while also including the finer textured soils found in corn growing areas. The geographic locations covered are representative of areas of acetochlor use. All eight trials have been applied with acetochlor, two in 1995 (Ohio and Wisconsin - 1995 Series), four in 1996 (Minnesota, Nebraska, Iowa and Indiana - 1996 Series), one in 1997 (Pennsylvania - 1997 Series) and one in 1998 (Delaware - 1998 series). The data generated during this program will be used to calibrate mathematical models to allow further estimates of any leaching potential to be conducted. The program is being conducted in accordance with guidelines drafted by the US Environmental Protection Agency (EPA).
The test plots at each trial site are essentially square, approximately 3.2 acres in size and are adjacent to a smaller control plot of approximately 0.4 acres. Each test plot is heavily instrumented with clusters of suction lysimeters (ceramic cup samplers) which sample soil pore water and are installed at varying depths (range 3 - 15 feet). Each trial site also contains a large number of ground water monitoring wells which are screened at varying depths within the aquifer. Ground water is sampled from the wells using dedicated pumps. Both suction lysimeters and ground water monitoring wells are sampled frequently and the suction lysimeters provide an early warning system for the potential movement of the test chemical to ground water.
Acetochlor was applied to the test plot as a single spray application to pre or post-emergent corn. The Delaware (1998) trial received a second consecutive annual application of acetochlor during 1999. Weather stations are installed at each trial site and record environmental and climatic data. To represent a worse case scenario, all trial sites receive 110% of the 30 year monthly average for precipitation during the growing season. Irrigation is applied at the appropriate intervals (by means of a linear tracking system installed at the site) to supplement natural rainfall in order to achieve the 110% target during the growing season.
After application the movement of acetochlor is studied by the collection of soil, soil pore water and ground water samples. The sampling phase of the study is two years minimum but for the six trials conducted during 1995 and 1996, has continued for at least 4 years. In addition to the acetochlor application, a separate application of potassium bromide tracer was made to the test plot, immediately after the application of acetochlor. Movement of the tracer is monitored and the information is interpreted to determine the movement of acetochlor relative to the unretained bromide tracer.
Analytical Results:
Over an eight year period, in excess of 16,000 water samples have been analyzed from this PGW program, according to Good Laboratory Practice (GLP) standards and the results have been reported to the USEPA. Despite the intensive monitoring, there has been no significant movement of acetochlor even at the highly vulnerable sites, with coarse textured soils of low organic matter. Some minor movement was observed at a silt loam site in Iowa and has been attributed to traces of preferential flow. The extensive body of analytical data available clearly demonstrate that acetochlor will not become a significant ground water contaminant when used according to label directions.
1995 Series - Ohio (clay loam) and Wisconsin (loamy sand)
The analytical results cover the period from acetochlor application to 45 months after treatment, when the trials were terminated and the land returned to the growers. Of the 1442 suction lysimeter samples analyzed 1440 (99.8%) have shown no detectable residues of acetochlor (limit of detection 0.01 PPB). Only two of the samples analyzed have shown any detectable residues of acetochlor, and both were unconnected trace detections below the limit of quantitation (0.05 PPB). No residues of acetochlor (limit of detection 0.01 PPB) have been determined in any of the 396 ground water samples analyzed from the monitoring wells at either site.
The movement of potassium bromide tracer at these sites has been followed through the analysis of soil-pore water and monitoring well samples. Significant residues of bromide tracer were detected in the shallow lysimeter units (3 foot depth) 3 months after tracer application at both sites, indicating tracer breakthrough. Significant residues of bromide were detected in ground water at 9 and 14 months after tracer application at the Ohio and Wisconsin sites respectively, providing indications of the first water recharge event since both trials commenced.
1996 Series - Minnesota (sandy loam), Iowa (silty clay loam), Nebraska and Indiana (silt loam)
The analytical results available cover the period from acetochlor application to 51 months after treatment. No residues of acetochlor have been determined in any of the 1909 suction lysimeter samples analyzed from the Minnesota and Nebraska sites. Also, no residues of acetochlor have been determined in any of the 1590 ground water samples analyzed from the monitoring wells at either site.
At the Iowa site acetochlor was detected in certain deep (9 foot) and extra deep (15 foot) lysimeter units 14 days after application. These residues subsequently declined to below detectable levels by the 6 month sampling event. All suction lysimeter samples analyzed since have shown no residues of acetochlor (834 total). At no time have any of the shallow (3 foot) and medium (6 foot) lysimeter units shown any detectable residues of acetochlor. Very low detectable residues of acetochlor were determined in seven monitoring well samples taken 1 month after treatment. No detectable residues of acetochlor have been determined in any ground water samples analyzed since this sampling event (735 total). The cause of these detections appears to be due to preferential flow of water. However, from consideration of the timing of the acetochlor detections relative to the test chemical application and the wide spatial variability of the detections, it is clear that general "sitewide" leaching of acetochlor did not occur.
At the Indiana site, 1312 of the 1321 suction lysimeter samples analyzed to date (99.3%) have shown no residues of acetochlor. Low residues of acetochlor have been observed in one single isolated lysimeter unit installed 3 foot below ground surface. This lysimeter unit was situated in an area of the test plot which was subjected to "ponding" following a very heavy thunderstorm event in the first week following acetochlor application (3.79 inches of rainfall within seven days of application). No residues of acetochlor have been determined in any of the 815 ground water samples analyzed from the monitoring wells situated at the site.
The movement of potassium bromide tracer at these sites has been followed through the analysis of soil-pore water and monitoring well samples. Significant residues of bromide tracer were detected in the shallow lysimeter units (3 foot depth) at 4, 5.5, 5 and 3 months after tracer application at the Minnesota, Nebraska, Iowa and Indiana sites respectively (indicating tracer breakthrough). Significant residues of bromide tracer were detected in ground water at 7, 17 and 15 months after application at the Minnesota, Nebraska and Indiana sites respectively, providing indications of the first water recharge event since these trials commenced. At the Iowa site, at one month after application, bromide residues were detected in the wells where trace acetochlor detections were also observed. At 12 months after application significant bromide tracer residues were seen in the other monitoring wells installed in the test plot, providing the first indications of general sitewide water recharge.
1997 Series - Pennsylvania (loam)
The analytical results available cover the period from acetochlor application to 39 months after treatment. To date, 1217 of the 1245 suction lysimeter samples analyzed (98%) have shown no residues of acetochlor. Very low residues of acetochlor (maximum 0.16 PPB in a 3 foot unit) have been observed in nine different suction lysimeter units, with the majority of these determinations below the limit of quantitation (0.05 PPB) of the analytical method. All residues observed declined to below detectable levels by the 6 month sampling event. No residues of acetochlor have been determined in any of the 609 ground water samples analyzed from the monitoring wells situated at the site.
The movement of potassium bromide tracer at this site has been followed through the analysis of soil-pore water and monitoring well samples. Significant residues of bromide tracer were detected in six shallow lysimeter units (3 foot depth) at 1 month after application providing the first indications of sitewide tracer breakthrough. Significant residues of bromide tracer were detected in a number of shallow ground water monitoring wells at 10 months after application, providing indications of the first water recharge event since the trial commenced.
1998 Series - Delaware (sandy loam)
An application of acetochlor was made at this site during May 1998 with a second consecutive application during May 1999. Analytical results available cover the period from the first acetochlor application through to 29 months later (17 months after the second application). No detectable residues of acetochlor have been measured in any of the 1253 lysimeter samples or 414 ground water samples analyzed to date.
The movement of potassium bromide tracer has been followed through the analysis of soil pore water and monitoring well samples. Significant residues of bromide tracer were detected in the shallow lysimeter units (3 foot depth) at around 8 months. Significant residues of the bromide tracer were detected in some of the well samples at around the same interval but with residues in all wells by 16 months, providing indications of the first general sitewide water recharge event since trial commencement.
PGW Analytical Results for OXA and ESA Soil Degradates of Acetochlor
As expected from monitoring conducted by USGS and others, the OXA and ESA soil degradates of acetochlor have been found to be significantly more mobile than parent acetochlor. Between the two soil degradates, Ac-ESA appears to be more mobile and/or persistent than Ac-OXA, reaching shallow ground water at slightly higher concentrations and more frequently. Maximum Ac-ESA concentrations in ground water among the eight sites have ranged from only trace levels at three sites (less than 1.0 PPB) to as high as 13 PPB at Indiana. Detected Ac-OXA residues were lower, with the highest ground water concentration observed at the Minnesota site (1.4 PPB). These results are both qualitatively and quantitatively consistent with what has been observed by USGS for the soil degradates of acetochlor and other chloro-acetanilide herbicides.
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This page was last updated on 13-Jan-03.
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