Summary:
The ARP GWM program created a unique network of ground water monitoring wells of great relevance to U.S. agriculture because they represent the range of corn agronomy and soils in each of seven key Midwest states and are positioned directly next to treated corn fields. The program is providing valuable scientific data on agricultural chemical behavior for a number of corn herbicides; this information is being shared with relevant scientists in the States and with Federal EPA.
The results to date confirm that acetochlor, as expected when the product was registered, is not a threat to ground water when used according to the label.
As required under the U.S. registration conditions for acetochlor the ARP agreed to conduct a monitoring study to investigate the likelihood of acetochlor leaching to ground water under conditions of typical farmer use.
Objective:
The objective of the GWM program was to determine concentrations of acetochlor and other major corn (maize) herbicides in U.S. ground water sampled from wells positioned immediately adjacent to fields treated with acetochlor. The EPA specified that the ARP monitor 25 wells in each of 7 specific states; Iowa, Illinois, Indiana, Kansas, Minnesota, Nebraska and Wisconsin. Furthermore, the program was required to be developed and implemented in conjunction with relevant authorities in the individual states. All 175 wells were sampled monthly through latae 1999 and more than 150 sites continued to be sampled quarterly through December 2001.
Program Design and Sampling Issues
To meet the requirements of the acetochlor agreement, the ARP installed 175 new monitoring wells. Each of the wells was within 30 to 200 feet of the treated fields and all are positioned to monitor ground water flowing from the treated areas. Each site had at least 10 acres of land upgradient of the well which is to be cropped to corn (maize) and treated with acetochlor in at least 4 seasons during the period 1995 to 2001. Each well was protected from direct overspray with acetochlor by a small buffer area which remains untreated. In each state, the wells were distributed across soil textures according to the distribution of soil textures in the counties in that state which produce a significant percentage of corn (maize). Where possible within the distribution of soil textures, the sites were selected to represent a broad range of corn (maize) agronomy and irrigation methods. Most states required that the wells be screened in the first ground water encountered. The GWM well network was developed to be ready for sampling by the 1995 corn (maize) planting season.
All seven states had an opportunity to modify these general parameters and certain states greatly influenced the program design; as a result, there were some differences between the GWM programs in each of the seven states.
For the first five years of the program, each monitoring well was sampled monthly (weather permitting) and samples shipped under refrigerated conditions to ARP analytical facilities in St. Louis. At least 150 sites continued to be sampled quarterly through December 2001. Some states required additional occasional samples to be shipped to their own laboratories for confirmatory analysis and other work of interest to that particular state.
Program Summary:
The resulting ground water monitoring network included wells with measured depths to water ranging between 1 and 80 feet below ground surface; the distribution was heavily skewed towards shallow ground water with over half the wells having depths to ground water of less than 20 feet. The distribution of soil textures was reflective of U.S. corn (maize) agriculture in each state but overestimates the significance of lighter soils within Midwest corn agriculture as a whole because the seven states are weighted equally rather than by their contributions to total US corn production.
Since initiation, the program remained stable; with only a few sites requiring replacement. Sampling success was excellent through the summer months but, due to factors such as the distance of many of the wells from the nearest road and freezing of water in pump delivery tubes, sampling success was lower in the winter months in the northern tier states.
Analytical Results:
Over the seven seasons, more than 11,000 ARP GWM samples were analyzed, verified to Good Laboratory Practice (GLP) standards, and reported to EPA, the relevant states and the SFIREG (State FIFRA Issues Research and Evaluation Group) committee on Water Quality and Pesticide Disposal. Data have been submitted to these bodies on a regular basis.
Generally, the majority of samples have no detectable residues of acetochlor, alachlor or atrazine. While detections of acetochlor and alachlor are very infrequent, atrazine is found more often.
Only seven of the 175 sites have shown acetochlor detections greater than 0.1 ppb for three or more months out of seven, the frequency which triggers further action by the ARP under the Acetochlor Registration Agreement. As specified in the agreement, in each of these cases the ARP used a Third Party to investigate the sites/detections. The samples from these sites generally contained high residues of other corn herbicides.
The independent third party investigator concluded that two of the instances (IA07 and IL24) were due to surface water containing high pesticide residues which ran off the treated fields very shortly after application and then cross contaminated the ground water by interaction through ponding near the wellhead. At the time of the runoff, the depths to ground water at these sites were <3 and <4 feet below ground surface. As expected for shallow ground water/surface water interactions, both these wells returned to non-detectable residues shortly after the original contamination. At both sites, an additional well was installed in a downgradient position not susceptible to interaction with surface water. Both of these were residue free - thus confirming there had been no general leaching of the herbicides. The original IA07 well again gave detections in 1997, 1999, and 2001 (the second, third, and fourth years the site was treated) at the same time of year, while the additional well remained residue free thus lending weight to the third party investigator's explanation.
The third site (KS14) showed a longer-lived pattern of residues in deeper ground water (approximately 18 feet below ground surface). The hydrology at this site is exceptionally complex and results of additional investigations have shown that the contamination is likely to result from a point source because the traces of herbicide residues appear to extend in a band less than 25 feet wide; general leaching has been excluded. From the beginning of the 1998 corn season, it no longer contained any detectable acetochlor residues.
Beginning in 1998, three other Kansas wells showed acetochlor detections in excess of 0.1 ppb (KS06, KS17, and KS19). Surface runoff was implicated in one of these sites (KS06), and third-party investigations point to the possibility that oil/gas drilling activities near the well site may be implicated at KS17 and KS19. In 2001, a final well (MN25) showed acetochlor detections in excess of 0.1 ppb. The report of the third-party investigator for this well implicated potential point source mixing/loading/disposal activities within several yards of the well-head as the most likely source of the residues.
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This page was last updated on 13-Jan-03.
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