QUANTIFYING GROUNDWATER FLOW THROUGH A LAKE BED DOWN-GRADIENT FROM A DECADES-OLD OIL SPILL NEAR BEMIDJI, MINNESOTA USING THE STREAMBED POINT VELOCITY PROBE

Over the 35 years of research at the National Crude Oil Spill Fate and Natural Attenuation Research Site near Bemidji, Minnesota, only a handful of studies have examined interactions between the contaminated aquifer and the small flow-through lake located about 400 m down-gradient of the contamination source. Previous studies have indicated that the lake is receiving metabolites from the degradation of the crude oil discharge, through analysis of stable isotope ratios, spatial and temporal changes in nonvolatile dissolved organic carbon concentrations (NVDOC), and variations in specific conductance. However, only a single study so far has performed any analysis of seepage rates in the lake bed and little is known about the overall lake hydrogeology. This study quantified flow through the bed of Unnamed Lake along two transects perpendicular to the western shoreline, where contaminants were expected to be entering the lake. Spatial variation of groundwater discharge was documented and water samples collected to permit contaminant mass discharge estimations at one location along each of the two transects. Groundwater velocity measurements were made using the Stream Bed Point Velocity Probe (SBPVP), which uses a mini-tracer test to determine in situ groundwater velocities at the groundwater-surface water interface. These values were corroborated with seepage meter measurements and Darcy calculations based on mini-piezometer measurements. Contaminant mass discharge was estimated using NVDOC concentrations. All measurements indicated flow into Unnamed Lake. The lake was found to have a layer of very fine, loose, organic-rich sediment that was thin to non-existent near the edges and greater than 2.5 m near the center. This layer could potentially act as an effective aquitard, trapping plume solutes near the lake bottom and leading to high discharge rates where the organic layer was absent, or pierced by instrumentation, such as a SBPVP. More data are needed to provide a broader and more accurate understanding of spatial changes in discharge rates, and to verify the role of the organic sediment layer in the lake hydrogeology.