Long-term monitoring of aldicarb residues in groundwater beneath a Canadian potato field.

Four shallow monitoring wells beneath a potato field to which Temik (active ingredient--aldicarb) had been applied were sampled up to 19 times over six years. These samples were regularly analyzed for aldicarb sulfoxide, aldicarb sulfone and nitrate and less frequently for major ions and for pH, EH, dissolved oxygen and alkalinity. Pesticide residue concentration in groundwater was related both to time after application and to the elevation of the water table, a measure of groundwater recharge. This dependence on recharge indicates storage in the unsaturated zone. The presence of relatively high residues four years after the last application is due to this storage phenomenon and to water and soil conditions, particularly low temperature, that inhibit the hydrolysis of the active species.