42-15 Characterizing Groundwater Hydrochemistry in a Karst Aquifer System, Rockbridge County, Virginia

Abstract:

     In Rockbridge County, Virginia, about 80% of the residents obtain their drinking water from private water wells and springs. Located primarily within the Valley and Ridge Province, Rockbridge County is underlain predominantly by carbonate bedrock that hosts an extensive karst aquifer system, with interbedded shale and fewer occurrences of siliciclastic units, that form the bounding ridges along the county margins. Its extreme eastern edge lies within the Blue Ridge Province. Systematic data on the county’s groundwater system remains limited, while the karst setting complicates the characterization of hydrogeochemical patterns, groundwater flow paths, and contaminant transport. This study aims to characterize the hydrogeochemical signatures in the groundwater system across the county. A total of 106 groundwater samples were collected from unfiltered sources, comprising 88 wells and 18 springs across the county.  

     Preliminary results indicate that the groundwater is predominantly of the Ca-Mg-HCO₃ type, reflecting carbonate bedrock control, with a few samples displaying mixed types marked by elevated Na and/or K. This is consistent with the elevated total dissolved solids (TDS; mean = 375 mg/L), near-neutral to basic pH (mean = 7.29), and relatively high alkalinity (mean = 257 mg/L as CaCO₃) across the groundwater system. The strong positive relationship between TDS and alkalinity (R² = 0.824) further establishes that carbonate mineral dissolution, and the resulting increase in dissolved bicarbonate, is a primary control on both parameters. pH and alkalinity exhibited a moderate inverse relationship (R² = 0.381), potentially reflecting consistency with CO₂-driven dissolution of carbonate bedrock. Well depth showed little to no relationship with either DO or TDS, suggesting that rapid conduit flow and groundwater mixing may obscure depth-dependent hydrochemical patterns within the karst system. Ongoing analyses of groundwater-age tracers and isotopic compositions will help to further constrain recharge sources, residence times, and flow paths. 

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