Hydrogeologic controls on lake level: a case study at Mountain Lake, Virginia, USA Jeanne M. Roningen & Thomas J. Burbey Abstract Mountain Lake in Giles County, Virginia, USA, has a documented history of severe natural lake-level changes involving groundwater seepage that extends over the past 4,200years. The natural lake was full during a large part of the twentieth century but dried up completely in September 2008 and levels have yet to recover. The objective of the study was to understand the hydro- geologic factors that influence lake-level changes using a daily water balance, electrical resistivity, water sampling and geochemical analysis, and well logging. Results from the water balance demonstrate the seasonal response to precipitation of a forested first-order drainage system in fractured rock. The resistivity surveys suggest discrete high-permeability areas may provide pathways for lake drainage. Imagery, well logs, and field observations appear to confirm the presence of a fault which crosses the Eastern Continental Divide to the east of the lake that had not previously been discussed in literature on the lake; the position of the lake within local and intermediate groundwater flow systems is considered. Historical data suggest that either significant precipitation or artificial intervention to mitigate seepage would be required for lake-level recovery in the near future. Keywords Groundwater recharge/water budget . Groundwater/surface-water relations . Valley and Ridge Physiographic Province, USA Introduction Mountain Lake in Giles County, Virginia, USA (Fig. 1) has a documented history of severe natural lake-level changes involving groundwater seepage that extend over the past 4,200 years (Cawley et al. 2001). Featured in the 1986 movie Dirty Dancing, the natural lake was full during a large part of the twentieth century but dried up completely in September 2008 and levels have yet to recover. In upland catchments where numerous intermit- tent low-volume surface streams make stream gauging challenging, the presence of a larger body of water provides a way to measure the integrated hydrologic response from the surrounding basin. The interaction between lakes and groundwater sys- tems has been studied both with numerical models and in the field (Van Everdingen 1967; Winter 1984; Winter and Carr 1980), with attention paid to the place of a lake within a local, intermediate, or regional groundwater system. At Mirror Lake, NH which overlies crystalline bedrock, groundwater recharge to the lake was found to occur largely through glacial till rather than bedrock (Rosenberry and Winter 1993). At Lake Lucerne, FL, a lake on mantled karst, local flow systems were found to discharge into the lake, and recharge to the underlying unconnected Floridian aquifer occurred both through the lakebed and through upper parts of the uplands surround- ing the lake (Lee and Swancar 1997). Lakes and wetlands with no streamflow in or out such as prairie pothole wetlands in North Dakota and mantled karst lakes in Florida, are especially vulnerable to drought (LaBaugh et al. 1996; Winter and Rosenberry 1998). Hydrologic studies of small watersheds often rely on the presumption that the groundwater basin is coincident with the topographic watershed, but this may be the case only if a surface watershed is at a high ridge and sufficiently far from major rivers (Winter et al. 2003). The position of this natural lake in a first-order drainage system invites a study of watershed response in a forested upland catchment on fractured rock. Unlike well-instrumented research basins, the privately owned site presents constraints that may be commonly encoun- tered by practicing hydrogeologists. The history of lake drainage and recovery and the presence of a fault that appears to cross the Eastern Continental Divide combine to make this site, to the knowledge of the authors, unique. Received: 16 July 2011 /Accepted: 3 April 2012 Published online: 17 May 2012 * Springer-Verlag (outside the USA) 2012 J. M. Roningen ()) Cold Regions Research and Engineering Laboratory, US Army Corps of Engineers, 72 Lyme Road, Hanover, NH 03766, USA e-mail: jeanne.m.roningen@usace.army.mil Tel.: +1-603-6464138 Fax: +1-603-6464768 T. J. Burbey Department of Geosciences, Virginia Polytechnic and State University, 4044A Derring (0420), Blacksburg, VA 24061, USA e-mail: tjburbey@vt.edu Hydrogeology Journal (2012) 20: 1149–1167 DOI 10.1007/s10040-012-0859-x