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Marine resistivity as a tool for characterizing groundwater discharge zones at Lake Lacawac, PA
Heaney, Matthew J.
Heaney, Matthew J.
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Thesis/Dissertation
Date
2007
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Earth and Environmental Science
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Heaney-Thesis-2007.pdf
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https://doi.org/10.34944/fqsj-1s74
Abstract
Understanding the groundwater-surface water exchange zones of lakes and streams is a key element in predicting water supply sustainability and dealing with water contamination issues. Spatial variability makes seepage zones hard to locate using traditional point sampling methods. The goal of this project was to test the use of marine electrical resistivity as a method for identifying potential zones of groundwater discharge and recharge, providing a better understanding of subsurface heterogeneities and guiding the use of point measurements. Multiple resistivity surveys were conducted at Lake Lacawac, a small, glacially formed lake in northeastern Pennsylvania. A continuous resistivity survey circumnavigating the lake was collected by towing a floating multi electrode cable parallel to shore. This was followed by a series of resistivity surveys run perpendicular to the shoreline using a multi-electrode cable deployed on the lake bottom. These surveys mapped lithology changes both along the shoreline and with distance from shore. Analysis of sediment samples taken from Lake Lacawac identified three sediment types, an electrically resistive (-300 Qm), hydrologically permeable (10'^ cm/s) fine grained sand; an electrically resistive (-300 Qm), less permeable (lO''* cm/s) silty sand; and an electrically conductive (-150 Qm), hydrologically impermeable (10'^ cm/s) silty clay layer. Although the resistivity measurements could not discriminate between the fine sand and the silty sand, the impermeable clay was distinguishable from the more permeable sandy sediments. Direct seepage measurements yielded results consistent with a conceptual model of groundwater discharge at locations where a continuous, permeable lake bottom sediment layer connected with the shore, and minimal seepage where the resistivity showed this layer was interrupted. Although the measured seepage rates (-0.2 cm/day) were too low for this interpretation to be conclusive, the initial indication is that marine resistivity can provide valuable information about sediment heterogeneities that can be used to guide studies of groundwater-surface water interaction.
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