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GROUNDWATER CHARACTERIZATION OF GRIGSBY HOLLOW, NEAR KINGSTON TN
Students in a graduate course on Field & Lab Methods in Hydrogeology (University of Tennessee) collaborated to produce a preliminary hydrogeologic model of a typical sub-watershed on Black Oak Ridge in Roane County, Tennessee. The study area encompassed more than 1,400 acres (2.2 sq. mi.) near Watts Bar Lake, where land use is undergoing a transition from forest and farmland to residential developments. The site is underlain mainly by southeast dipping Knox Group dolomite and Conasauga Group limestone. Bedrock is mantled with thick cherty residuum in a porous, fine-grained matrix. At least 6 perennial springs and several large residuum-choked sinkholes are located within the study area. Hydrologic budget calculations suggest that spring discharge is mainly derived from within the topographic boundaries of the sub-watershed. This is likely the case for most of the 10+ mile long ridge, which is dissected by spring-fed streams every 1-2 miles. Spring discharge from the 4 largest springs was measured near the end of an exceptionally dry fall and ranged from 35 to 55 gpm. Sustained dry season spring and stream flows indicate the significance of groundwater storage in the epikarst. The role of the residuum on spring and stream hydrochemistry is reflected in substantially lower pH and specific conductivity than expected for karst settings. Under low-flow conditions, fecal coliform values for springs and streams ranged from 7-115 and 0-96 CFU/100 mL, respectively, while values measured in 4 bedrock wells were below the detection limit. This suggests the thick residuum layer provides some protection against microbial contamination of the underlying aquifer. In summary, the study indicates that, in spite of the karst setting, locally-derived, diffuse flow is the primary groundwater recharge mechanism and the thick residuum plays an important role in sustaining base flow and protecting water quality.