Multi-Faceted Approach to Assess Contaminant Flux and Focus Remedial Efforts in Fractured Metamorphic Bedrock

Geologic heterogeneity results in spatially variable groundwater flow and contaminant flux in aquifers and there is growing recognition that quantifying contaminant flux supports more accurate conceptual site models and more reliable and cost-effective remedial strategies.  Multiple tools, including single-well tracer tests, pumping tests, and rock core analyses were completed at a site in the Piedmont of South Carolina in two hydrostratigraphic units (partially weathered/ fractured bedrock and competent biotite gneiss bedrock) to evaluate vertical mass distribution and quantify groundwater and contaminant flux. The results clearly demonstrate that groundwater flux in the competent bedrock unit is very low; for example, the bulk average hydraulic conductivity of the bedrock is one to two orders of magnitude lower than the partially weathered bedrock, and significant tracer concentrations persisted in monitoring wells for more than 60 days in bedrock wells completed in the competent biotite gneiss bedrock (indicating groundwater flux ranges from <0.02 to 0.08 ft/year).

The combination of investigation approaches provided a better understanding vertical mass distribution and the transport mechanisms in the weathered/ fractured portion of the bedrock.  The characterization of the contaminant flux demonstrated that some elevated concentrations measured in bedrock wells were actually caused by leakage from the overlying fractured bedrock due to improper well construction.  In addition, the improved understanding of the two hydrostratigraphic units allowed the remedial strategy to be focused on the partially weathered / fractured bedrock unit, which is the primary pathway for contaminant transport.