USING TEMPERATURE AND SPECIFIC CONDUCTANCE DATA TO EVALUATE GROUNDWATER FLOW DIRECTIONS AND DIVIDES, AND WELL CAPTURE ZONES IN A GLACIAL OUTWASH AQUIFER

Over 200 monitoring wells have been installed and are being used to characterize and evaluate the groundwater flow system and contaminated groundwater within a large outwash aquifer adjacent to the Mississippi River. Potentiometric contour maps and output from a six-layer groundwater model have been used to evaluate groundwater flow directions and velocities, direction of contaminant plume migration, and capture zones for recovery wells. However, uncertainty still remains whether the capture wells are inducing flow from the river or are drawing water upward from the underlying dolomite bedrock aquifer, which would reduce the effectiveness of the wells to capture contaminated groundwater. The flow system includes river water, deep bedrock groundwater, and shallow groundwater from upgradient of the site. The site is located near Minneapolis, Minnesota, so there is typically a large temperature difference between the river water and groundwater in winter and late summer. In addition, it was assumed that specific conductance could be used to discriminate between the three end members in the flow system. Temperature and specific conductance data from December 1999 were mapped in three dimensions. The highest temperatures (13 to 19 degrees C) occurred in the shallow outwash in the immediate vicinity of the recovery wells and downgradient of the large industrial building and the source of organic contaminants. The plant may impart an elevated temperature to the groundwater, in which case the elevated temperatures show flow direction under the building toward the recovery wells. The higher temperatures may also be due in part to the exothermic biodegradation of trichloroethene. The coldest temperatures (10 to 13 degrees C)were found in the deeper groundwater and the monitoring wells located closest to the river. Thus, the temperature data suggest that the wells are effectively capturing contaminated groundwater from beneath the plant and minimizing capture from the river and the underlying bedrock aquifer. The evaluation of the specific conductance data resulted in similar conclusions.