Geophysical Methods for Locating Karst Conduits in Cane Run Watershed, Central Kentucky

Cane Run watershed in central Kentucky was listed by the Kentucky Division of Water as one of four focus watersheds for clean-up under the State’s nonpoint-source pollution program. This watershed is degraded by pathogens, nutrients, siltation, and organic enrichment. The sources of pollution include both municipal point sources and nonpoint agricultural and nonagricultural sources. The relative contribution of different parts of the watershed to the contamination is not well understood, however. The geology of Cane Run watershed consists of Ordovician thin-bedded limestone with sparse interbeds of shale. The landscape is dominated by karst features such as sinkholes and springs. Cane Run only flows during times of significant rainfall, usually in the spring of the year. The remainder of the year, most water is recharged to a karst conduit system that leads from Lexington, Ky to Royal Spring, as demonstrated by groundwater tracing. Royal Spring is the major water supply for Georgetown in Scott County, Ky. We attempted to locate the karst conduit so that groundwater flowing through the conduit could be monitored. These monitoring data are essential for assessing the effectiveness of remediation plans. In 2008, based on geology, karst features, and hydrogeology, an initial round of electrical-resistivity and spontaneous-potential geophysical surveys were conducted to help pinpoint the location of the conduit at three sites. Fifteen exploratory boreholes were drilled on the basis of the geophysical results. The boreholes confirmed the geophysical surveys had located minor mud-filled conduits that were interpreted as tributaries to the main conduit. Another round of 2D and 3D electrical resistivity surveys were conducted in 2009 to search for the main conduit. The analysis of this round of surveys resulted in one promising site that is suspected to be in close proximity to the conduit. A time-lapse 2D electrical resistivity survey in conjunction with calcium chlorite solution injection was performed to pinpoint the conduit location. Microgravity measurements were also taken at the site to check whether the gravity anomaly signals are associated with conduits. The geophysical results will be further tested by exploratory boreholes. This work is being carried out in cooperation with the University of Kentucky’s College of Agriculture and Department of Earth and Environmental Sciences.