PESTICIDE CONCENTRATION VARIATIONS CORRELATED WITH WELL BORE VOLUME REMOVAL IN SHALLOW COASTAL-PLAIN GROUND-WATER

The effects of well bore volume removal (V-n) 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-d iamine] in ground water obtained from three monitoring wells installed in the Coastal Plain region of North Carolina was investigated. Seaso nal effects were also investigated by conducting the exercise in Febru ary and May. In the majority of cases, the lowest pesticide concentrat ions occurred in the initial well bore volume (V-1 = stagnant water). Removal of additional well bore volumes (V-2 to V-10) from two of the wells resulted in pesticide concentrations that did not vary substanti ally. This indicates that a representative aquifer sample was obtainab le, in most cases from these wells, after removal of the initial well bore volume. In contrast, a third well required the purging of two wel l bare volumes before a stable alachlor concentration was achieved. Se asonal effects of bore volume removal vs. pesticide concentrations for the three wells were not significant (P > 0.05). It was concluded tha t a protocol for improved accuracy in pesticide analyses of ground wat er can be obtained by establishing a pesticide concentration-purging ( well bore volume) relationship for each well.