Pesticide concentration variations correlated with well bore volume removal in shallow coastal plain ground water

The effects of well bore volume removal (Vn) on the concentration of alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxy methyl) acetamide] and prometon (6-methoxy-N,N'-bis(1-methylethyl)-1,3,5-triazine-2,4-diamine] in ground water obtained from three monitoring wells installed in the Coastal Plain region of North Carolina was investigated. Seasonal effects were also investigated by conducting the exercise in February and May. In the majority of cases, the lowest pesticide concentrations occurred in the initial well bore volume (V1 = stagnant water). Removal of additional well bore volumes (V2 to V10) from two of the wells resulted in pesticide concentrations that did not vary substantially. This indicates that a representative aquifer sample was obtainable, in most cases from these wells, after removal of the initial well bore volume. In contrast, a third well required the purging of two well bore volumes before a stable alachlor concentration was achieved. Seasonal effects of bore volume removal vs. pesticide concentrations for the three wells were not significant (P > 0.05). It was concluded that a protocol for improved accuracy in pesticide analyses of ground water can be obtained by establishing a pesticide concentration-purging (well bore volume) relationship for each well.