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Dependence of Aldicarb Residue Degradation Rates on Groundwater Chemistry in the Wisconsin Central Sands

Central Wisconsin Groundwater Center, University of Wisconsin-Stevens Point, Stevens Point, WI 54481;

Philip A. Helmke

Dep. of Soil Science, Univ. of Wisconsin-Madison, Madison, WI 53706.

^* Corresponding author.

ABSTRACT

Aldicarb sulfoxide (ASO) (2-methyl-2-(methylsulfinyl)propanal O-[(methylamino)carbonyl]oxime) and aldicarb sulfone (ASO₂) (2-methyl-2-(methylsulfonyl)propanal O-[(methylamino)carbonyl]oxime) degradation rates were measured in incubations simulating three depths (0.5, 5, and 10 m) of Wisconsin central sand plain (WCSP) aquifer. The incubations differed mainly in pH (5.8, 6.5, and 7.2, respectively, for the three depths) and pO₂ (20 kPa [0.2 atm] for the 0.5- and 5-m depths, 1.5 kPa [0.015 atm] for the 10-m depth). Degradation rates corresponded to half-lives ranging from years to less than a month. Aldicarb sulfone (ASO₂) degraded faster than ASO at the same depth. No ASO degradation was measured under aerobic, pH 5.8 conditions; no ASO oxidation was observed in any incubation. The shortest half-lives, 19 and 32 d for ASO and ASO₂, respectively, were obtained under the low pO₂ conditions. The results of this and a previous experiment provide degradation rates for a cross-section of WCSP groundwater chemistries. Thus, with knowledge of the groundwater chemistry at a given WCSP location, degradation rates can be estimated. Degradation rates of common groundwater contaminants as functions of chemical and physical controlling factors could prove valuable to groundwater managers for endeavors such as wellhead protection.