Representative Groundwater Sampling Using Low-Flow Methods, 1997 Environmental Conference Proceedings

Increasingly, regulatory agencies are requiring analysis of unfiltered groundwater samples from waste sites. Potlatch faced this requirement for analysis of metals and dioxins in groundwater during a remedial investigation / feasibility study (RI/FS) of their industrial solid waste landfill in Cloquet, MN. Initial groundwater samples were collected using traditional bailing methods, which yielded very turbid samples and correspondingly high contaminant concentrations, To obtain samples that were more representative of the groundwater, Potlatch decided to use low-flow rate sampling methods, also called micro-purge. Although pump selection, field methods development, and increased sampling costs were a challenge, the results indicated that dioxins are not migrating in the groundwater and that metals are a concern in only a limited area. Typically, the samples collected by the low-flow rate method had contaminant concentrations two to four orders of magnitude lower than those collected by the traditional bailed method. This methodology was controversial when proposed, and extra research was necessary to demonstrate its validity. The low- flow method is gaining acceptance and is being written into the Minnesota Pollution Control Agency’s solid waste site sampling guidance document. The method continues to be used and refined at the Potlatch facility.