Submitted to: Ground Water Monitoring and Remediation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/29/1998
Publication Date: N/A
Interpretive Summary: Stagnant well water must be removed from groundwater monitoring wells in order to obtain a representative aquifer sample. A representative water sample is assumed when a measured chemical parameter in the purge effluent has stabilized. Periodically, this exercise must be conducted on wells when determining the quality of groundwater. We conducted a study to determine the amount of water to pump from three wells in order to obtain a water sample that resembled typical groundwater. We found that the stagnant well water did not resemble typical groundwater. Water collected after the stagnant water was removed did resemble groundwater in the aquifer. Our study showed that determination of the suitability of groundwater for human consumption requires that stagnant water be removed from the well to insure that a sample truly resembles groundwater in the aquifer.
Technical Abstract: Groundwater monitoring wells are usually purged to remove stagnant well water in order to obtain a representative aquifer sample. We investigated the influence of well bore volume removal (Vn) on the concentration of two pesticides -- alachlor and prometon -- in groundwater obtained from three monitoring wells installed in the Coastal Plain region of North Carolina. 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 stagnant well water. 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. We conclude that a protocol for improved accuracy in pesticide analyses can be obtained by establishing a pesticide concentration-purging (well bore volume) relationship for each well.