PRELIMINARY ASSESSMENT OF GROUNDWATER VELOCITIES IN THE LAKEBED OF UNNAMED LAKE, NEAR BEMIDJI, MN

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 downgradient of the contamination source. Previous studies have indicated that the lake is receiving crude oil metabolites in the discharging groundwater. However, little is known about the spatial distribution of discharge, hence contaminant delivery rates to the lake bottom. This study quantified flow through the bed of Unnamed Lake along two transects perpendicular to the shore on the western edge of the lake where contaminants were expected to be entering the lake. Spatial variation of groundwater discharge and hydraulic conductivity were documented along the transects and pore water samples were collected in vertical profiles along the shore at each of the 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 Darcy calculations based on mini-piezometer measurements. Plume presence was inferred at two near-shore locations using specific conductance measurements of pore water samples. All measurements indicated flow into Unnamed Lake. In some places, flow was surprisingly high with rates up to 3 m/d. The lake was found to have a layer of very fine, 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 a leaky aquitard, allowing for high rates of flow where it is not present or where pierced by a device such as a SBPVP. Overall, the data is in agreement that there are high discharge rates into the lake along the western shore and as far into the lake as 30 m; however, flow rates decline moving into the lake. More data are needed to provide a broader and more accurate understanding of spatial changes in discharge rates, as well as to identify where the lake water is discharging into the aquifer.