Spatial Analysis of Hydrological Productivity in Fractured Bedrock Terrains of the Piedmont of Northwestern South Carolina

Fractured bedrock aquifers are structurally complex groundwater systems. Groundwater flow is limited to secondary porosity features such as faults and fractures on account of the low primary porosity and permeability of the native bedrock. The hydrologic productivity of wells drilled within these systems is spatially and vertically variable because of limited interconnectivity among these features. The purpose of this study was to assess potential correlations between driller-estimated well yields and the mapped lithology and structural features of the fractured bedrock aquifers of the Piedmont of northwestern South Carolina. Groundwater well data (e.g., well depth, well yields, static water level) of 1,069 wells, geologic data (e.g., lithology, mapped structural features), and topographic data (e.g., surface elevation, slope) were integrated within a geographic information system database for a spatial analysis of well yield distribution. Wells drilled in alluvium had the highest median yield (15 gal/min), whereas those drilled in schist, amphibolite, and gneisses had lower median yields (9, 8.5, and 8 gal/min, respectively). Nonparametric statistical analyses indicated that no geologic or topographic variables considered were strongly or moderately correlated with reported well yields. Spearman’s correlation coefficients for well depth (0.24), static water level (0.19), proximity to water bodies (–0.10), and proximity to lithologic contacts (–0.08) were statistically significant (at the 0.05 confidence level) but only weakly correlated with well yield. Topographic variables and proximity to mapped faults were not statistically significant. Wells drilled in alluvium had the highest yields due to the higher porosity and permeability compared to the bedrock. However, alluvium makes up less than 5% of the study area surface, and so opportunities to further tap this unit are limited and spatially constrained. The lower median yields of other lithologies are attributed to the lack of fracture development in amphibolite and the low degree of weathering within gneiss foliation planes. To maximize yields, wells should be drilled in alluvium close to water bodies and lithologic contacts where possible.