OFR 07-05, The Lake Elmo Downhole Logging Project: Hydrostratigraphic Characterization of Fractured Bedrock at a Perfluorochemical Contamination Site

Abstract

The Minnesota Pollution Control Agency (MPCA) and Minnesota Department of Health are actively involved in detecting and remediating perfluorochemical contamination of groundwater in the Twin Cities Metropolitan area. A perfluorochemical plume in the Lake Elmo area of Washington County contaminated the water supply of more than 285 residents in that area. As a result, a municipal water system has been connected to more than 200 residents, and domestic wells in the area were abandoned. This report summarizes the results of a project by the Minnesota Geological Survey to collect downhole geophysical data from nearly 200 of these domestic wells before they were sealed as part of the abandonment process. Over 95 percent of these wells extract water from the lower one-half of the St. Peter Sandstone or upper part of the Shakopee Formation, with the remaining wells open to overlying unconsolidated glacial deposits. The downhole geophysical data are used to determine rock properties and flow characteristics of these bedrock aquifers. The results will be used by the MPCA to help better determine the three dimensional extent of contamination, and in particular provide insight into the role of groundwater flow through fractures. Such information will assist the MPCA in determining appropriate remedial monitoring actions in a cost effective manner.