PNNL

Abstract

Purge and pump sampling results from screened wells can be affected by the processes of concentration averaging and contaminant redistribution by wellbore flow. The effects due to concentration averaging were assessed in a screened well at the Hanford Site by investigating the vertical profile of a technetium-99 plume in the aquifer. Nitrate is a co-contaminant with technetium-99 and the dominant anion in the aquifer water, which enabled measurements of specific conductance to be used as an analog for technetium-99. Time-series measurements were collected during purging from three specific conductance probes installed in the well at 1.2 m, 3.1 m, and 4.9 m below the water table in a 7.7 m deep screened well. The vertical contaminant profile in the aquifer was calculated using a wellbore flow and mixing model. The highest concentrations occurred primarily in the upper part of the screened interval, and the purge and pump sample results were 41% to 58% of the calculated peak concentration in the aquifer due to concentration averaging. Plume stratification became more pronounced as pumping continued. Extended pumping may have reversed the effect of contaminant redistribution by wellbore flow. It was also found that the vertical profile of contamination in the wellbore under static (i.e., non-pumping conditions) was not representative of the profile in the aquifer. Thus, passive or micro-purge sampling techniques, which essentially sample the wellbore water, will not necessarily yield results representative of the aquifer.