42-1 Assessing Health Risk of Groundwater from Microbial and Geochemical Contamination in the Karst Aquifer of Rockbridge County, Virginia

Abstract:

In Rockbridge County, Virginia, about 80% of residents rely on private wells and springs for their drinking water, underscoring the need for routine water-quality testing to protect public health. Rockbridge County is underlain by a carbonate-dominated karst aquifer system, making groundwater highly vulnerable to surficial contamination. This community-focused study assesses groundwater quality and suitability for drinking by characterizing microbial, nutrient, and trace metal contamination in the groundwater system. A total of 106 groundwater samples were collected from unfiltered sources (88 wells and 18 springs) at 91 residences across Rockbridge County during the summers of 2025 and 2026. 

Microbial contamination was more prevalent in spring samples; coliform bacteria were detected in all springs and E. coli in 66.7%, compared with 46.6% and 3.4%, respectively, in wells. Of the E. coli-positive samples, 81.8% were from springs or wells less than 7 m deep, and all but two samples with coliform counts exceeding 5 CFU per 5 ml sample originated from groundwater sources within 70 m of the ground surface. Of the samples, 13% of Total Dissolved Solids measurements exceeded the EPA secondary maximum contaminant level (SMCL) of 500 mg/L. In 2025 and 2026, samples collected from shallow groundwater sources (0–50 m deep) had the highest median nitrate concentration (1.27 mg/L), with 59.09% exceeding 1 mg/L. Median nitrate concentrations decreased with source depth, from 0.85 mg/L at 50-100 m to 0.51 mg/L at 100-150 m and 0.21 mg/L at 150-250 mg/L. These depth-related patterns are consistent with the microbial data, suggesting that shallow groundwater sources are more vulnerable to surficial contamination from agricultural runoff, septic systems, and other near-surface inputs. 

In 2025, 31% of samples had lithium concentrations exceeding the USGS health-based screening level of 0.01 mg/L, potentially reflecting the natural occurrence of lithium-bearing geologic materials in the Appalachian region. Two samples exceeded the SMCL for iron (0.3 mg/L), and two exceeded the SMCL for manganese (0.05 mg/L); however, the two metals were not significantly correlated to the dataset. Collectively, these findings highlight the vulnerability of Rockbridge County’s karst aquifer to contamination and demonstrate the need for continued monitoring of private water sources to protect public health.  

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.