HYDROGEOLOGIC CONTROLS ON LAKE LEVEL AT MOUNTAIN LAKE, VIRGINIA
A geophysical survey was conducted using two-dimensional electrical resistivity profiling with dipole-dipole arrays to investigate possible faulting and/or karst features. Three wells were logged with optical televiewer and gamma tools in an effort to correlate subsurface geology with known stratigraphy. Water levels were also monitored at all available well locations. A new bathymetric map of the lake bottom was created with a sonar survey to obtain lake volume as a function of lake height. A pressure transducer in the lake allows for the calculation of hourly changes in storage, and a rain gauge was installed on-site to correlate precipitation with lake storage. Major ions were analyzed in springs, streams, wells, and the lake.
The resistivity surveys show a highly heterogeneous subsurface. Features include: low-resistivity areas that suggest drainage pathways from the lake, a clear example of a small recharge area outside of the topographic watershed, and deeper sources for perennial and former springs. Preliminary results from a water balance indicate steady lake drainage during the growing season and recharge when vegetation is dormant, particularly after snowmelt. Notably, groundwater outflow from the lake does not appear to be head-dependent over the range of lake levels recorded from May 2009 to present.