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Quantifying Groundwater Flow through a Lake Bed Down-Gradient from a Decades-Old Oil Spill near Bemidji, Minnesota
Jones, Matthew M.
Jones, Matthew M.
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Abstract
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 metabolite 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 handful of studies performed limited analysis of seepage rates in the lakebed and little is known about the spatial trends throughout. 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 seepage meter measurements and Darcy calculations based on mini-piezometer measurements. Plume presence was determined at the shore using specific conductance measurements of pore water samples. All measurements indicated flow into Unnamed Lake with some at surprisingly high rates of 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 shoreline and greater than 2.5 m at the end of the transect. This layer could potentially act as an aquitard, allowing for high rates of flow where it is not present, or when pierced by a device such as a SBPVP. Overall, the data are 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, upward flow rates appear to 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.
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2024-12-31
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University of Kansas
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Jones_ku_0099M_19766.pdf
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Keywords
Geology, Hydrologic sciences, Contaminate fate and transport, Hydrogeology, Lakebed groundwater velocity, Petroleum spill, Point Velocity Probes, Surface water-groundwater interactions