HYDROGEOLOGIC CONTROLS ON LAKE LEVEL AT MOUNTAIN LAKE, VIRGINIA

Mountain Lake in Giles County, Virginia, has a documented history of severe natural lake-level changes from groundwater seepage that extend over the past 6000 years, and as of December 2010 the lake was about 2% full by volume. Mountain Lake is situated in the Valley and Ridge physiographic province and lies on a plunging anticline, straddling contacts between three upper Ordovician and lower Silurian formations. The lake is one of two natural lakes in Virginia, and at an elevation of 3887 feet is the highest-altitude named U.S. lake south of New England and east of westernmost Kansas. Geophysical surveying and an hourly water balance have shed light on the nature of groundwater outflow from the lake, including: (1) the response to precipitation of a forested first-order drainage system in fractured rock, (2) the relationship between the geologic catchment and the topographic watershed, and (3) information pertaining to potential geotechnical approaches for mitigating the draining lake problem.

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.