ALDICARB RESIDUE DEGRADATION RATES AT 3 DEPTHS OF A WISCONSIN CENTRAL SAND PLAIN AQUIFER

First-order oxidation and hydrolysis rate constants for aldicarb sulfoxide and aldicarb sulfone were measured in laboratory studies representing three depths of a Wisconsin central sand plain aquifer with aerobic groundwater at pH 7.6. Aldicarb sulfoxide or aldicarb sulfone were added to incubations containing sediment and groundwater from 0.5, 5.0, and 10.0 m below the water table, and their concentrations followed with time. Rate constants were similar for all three depths, averaging 0.0018, 0.0004, and 0.0029 day-1 for aldicarb sulfoxide hydrolysis, aldicarb sulfoxide oxidation and aldicarb sulfone hydrolysis, respectively. Hydrolysis values were close to those of abiotic aqueous systems, indicating chemical and biological catalysis were minimal. Assuming typical initial total aldicarb sulfoxide and aldicarb sulfone concentrations in groundwater were 100-mu-g liter-1 beneath treated fields and groundwater velocity was 0.5 m day-1, 1000 days and 500 m groundwater travel distance would be required for residue levels to attain the Wisconsin regulatory standard of 10-mu-g liter-1 at the studied location.