Concentration profiles in screened wells under static and pumped conditions

Abstract

Purge and pump samples from screened wells reflect concentration averaging and contaminant redistribution by wellbore flow. These issues were assessed in a screened well at the Hanford Site by investigating the vertical profile of a technetium-99 plume in a conventional well under static and pumped conditions. Specific conductance and technetium-99 concentrations were well correlated, and this enabled measurement of specific conductance to be used as a surrogate for technetium-99 concentration. Time-series measurements were collected during purging from three specific conductance probes installed in the well at 1.2, 3.1, and 4.9 m below the static water level in a 7.7-m-deep screened well. The vertical contaminant profile adjacent to the well in the aquifer was calculated using the concentration profile in the well during pumping, the pumping flow rate, and a wellbore flow and mixing model. The plume was found to be stratified in the aquifer - the highest concentrations occurred adjacent to the upper part of the screened interval. The purge and pump sample concentrations were 41% to 58% of the calculated peak concentration in the aquifer. Plume stratification in the aquifer adjacent to the well screen became more pronounced as pumping continued. Extended pumping may have partially reversed the effect of contaminant redistribution in the aquifer by wellbore flow and allowed the stratification of the plume to be more observable. It was also found that the vertical profile of contamination in the well under static (i.e., nonpumping conditions) was not representative of the profile in the aquifer. Thus, passive or micropurge sampling techniques, which sample the wellbore water at different depths, would not yield results representative of the aquifer in this well. © 2009 National Ground Water Association.