Abstract

Removing several casing volumes of groundwater from monitoring wells before sampling (i.e., purging) has long been standard operating procedure in the environmental industry and is presently required by most regulatory oversight agencies. The general intent of well purging is to provide data on dissolved contaminant concentrations that are of adequate quality for making prudent environmental decisions. Historically, it has been believed that purging of wells before sampling will yield more meaningful data on the concentrations of organic contaminants dissolved in groundwater. However, there is increasing evidence that this is not always the case. Furthermore, the practice of well purging produces substantial volumes of wastewater that must be properly treated and also has a major impact on monitoring costs. Approximately 50% or more of routine monitoring costs are attributable to well purging.

To evaluate the net effect of standard well purging practices, a study of monitoring wells at gasoline-impacted underground storage tank sites in southern California was conducted. At each well, paired samples were collected. Sampling consisted of first collecting a groundwater sample immediately before well purging and then collecting a second, “normal” sample immediately following well purging. The unpurged and purged samples from a given well were analyzed by the same laboratory by using standard U.S. Environmental Protection Agency methods for benzene, toluene, ethylbenzene, xylenes, and total petroleum hydrocarbons.

Study results show that samples collected after standard well purging do not systematically have higher contaminant concentrations, as might be expected if loss of dissolved contaminants by volatilization within the well was significant. In fact, mean concentrations of all five constituents were actually higher for unpurged samples than for purged samples. Statistical analysis of the data shows there is a high probability that, on average, unpurged sample concentrations actually exceed purged sample concentrations. When groundwater data are considered from the perspective of how they are used in decision making at typical underground storage tank sites, these study results demonstrate a clear need for reevaluation of regulatory policy relating to well purging.