Fate of Aldicarb Residues in a Groundwater Basin Near Plover, Wisconsin

The aldicarb residues aldicarb sulfoxide (ASO) and aldicarb sulfone (ASO2) are persisting in a 6-km2 Wisconsin central sandplain (WCSP) basin even though aldicarb use there ceased in 1981.Groundwater monitoring in 1986-88 indicated that a residue plume4-8 m thick plume underlaid most of the basin. The highest detected concentration in 1986-88 was 51 pg L; similar concentrations were still detected in a water supply well in 1990.This persistence is long compared to most of the U.S.A. and WCSP. The long persistence relative to other U.S.A. locations is likely due to cold groundwater temperature (10C) and relative to’I other WCSP locales is due to atypical groundwater chemistry.

Basin ground water chemistry type is Ca-Mg-HCO -NO3 with pH about 5.6 at the water table rising to 7.2 10-m below the water table. DO is about 8 mg L at the water table decreasing to <1img L at 10 m below the water table. The pH of shallow groundwater is at the lower extreme of WCSP values. ASO and ASO2 half-lives were measured in incubations simulating basin conditions at three depths below the water table (0.5, xiii5, and 10 m) with pH 5.8, 6.5, and 7.2 and pO2 0.20P 0.20, and0.015 for the three depths, respectively. ASO2 half-lives were3.3, 2.0, and 0.05 y for the three depths. ASO half-lives at 5 and10 m were 3.0 and 0.1 y with no degradation observed in the 0.5 incubation.

The rapid degradation observed under pO2 0.015 was catalyzed by microorganisms. Half-lives measured for a second location with aerobic, pH 7.6 groundwater averaged 1.0 and 0.6 for ASO and ASO . These conditions are probably more common to the potato-growing regions of the WCSP.

Residues have persisted in the basin because the plume is in the upper part of the aquifer where chemistry is not conducive to degradation. If the plume advects downward- and evolves chemically to higher pH and lower pO2, conditions more amenable to degradation will develop.

Final report was submitted as a thesis: Kraft, G. (1990). Fate of aldicarb residues in a groundwater basin near Plover, Wisconsin. [Unpublished doctoral thesis]. University of Wisconsin-Madison.