SPATIAL ANALYSIS OF WELL YIELDS 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 due to the limited interconnectivity among these features. The purpose of this study was to identify the structural and lithological controls on well productivity within the fractured bedrock aquifer of the Piedmont of northwestern South Carolina. Groundwater well data (e.g., well depth, well yields, static water level) of 1,070 wells, geologic data (e.g., lithology, mapped structural features), and topographic data (e.g., surface elevation, slope) were integrated within a GIS database for a spatial analysis of well yield distribution. Borehole geophysical logs of 7 selected wells were also analyzed for a better vertical understanding of the depth and extent of fracture zones. Wells dug in alluvium had the highest median yield (15 gal/min), while those dug in schist, amphibolite, and gneisses had lower median yields (9, 8.5, and 8 gal/min, respectively). Surprisingly, non-parametric statistical analyses indicated that no geologic or topographic variables considered were strongly correlated with reported well yields. Spearman’s coefficients for well depth (0.24), static water level (0.19), proximity to waterbodies (-0.10), and proximity to lithologic contacts (-0.08) were statistically significant, but only moderately to weakly correlated with well yield. Topographic variables and proximity to mapped faults were not statistically significant. Interpretation of geophysical logs indicated that high-yielding wells had increased fracturing at depths greater than 150 feet below the land surface. Wells dug in alluvium had the highest yields due to the higher porosity and permeability compared to the bedrock. The lower median yields of other lithologies are attributed to lack of fracture development in amphibolite and low degree of weathering within gneiss foliation planes. To maximize yields, wells should be dug in alluvium close to waterbodies and lithologic contacts. Wells in other lithologies should be dug greater than 150 feet to reach productive fractures.