GEOLOGIC ASPECTS OF NATURALLY OCCURING ARSENIC IN GROUNDWATER IN THE CHAPEL HILL, HILLSBOROUGH AND EFLAND 7.5-MINUTE QUADRANGLES, CAROLINA TERRANE, NORTH CAROLINA

Naturally occurring dissolved arsenic (As) in groundwater is a growing concern in the North Carolina Piedmont. The study area is underlain by weakly metamorphosed Late Proterozoic volcaniclastic and intrusive rocks. Rocks exhibiting the effects of hydrothermal alteration and sulfide mineralization are common. The units have been folded into an anticlinorium. Generally, primary pyroclastic rocks and lavas dominate the core and volcaniclastic sedimentary lithologies dominate the flanks of the anticlinorium. Models of volcanic massive sulfide and disseminated gold-pyrite deposits indicate that favorable horizons for economic mineral deposits occur at the transition from an environment dominated by primary pyroclastic rocks and lavas to an environment dominated by volcaniclastic sedimentary rocks. This model was applied to the origin of groundwater As in Orange County, NC. Detailed geologic data of the Chapel Hill, Hillsborough and Efland 7.5-minute quadrangles were integrated by the Duke University researchers with groundwater As concentration data, well construction data and tax parcel data for Orange County, NC into a GIS. A statistical model was developed to determine if any spatial patterns in groundwater As detections were present. Detailed geologic mapping separated the study area into over 45 geologic units. For the statistical analysis, the geologic units were simplified into six generalized categories based on rock type and interpreted mode of deposition/emplacement: 1) plutons, 2) felsic lavas and tuffs, 3) mafic lavas and tuffs, 4) welded tuffs and hydrothermal quartz bodies, 5) air fall tuffs and 6) volcaniclastic sedimentary rocks. Triassic sediments, Jurassic diabase and surficial units were not included in the classification scheme. The geologic transition from rocks of a primary pyroclastic origin to rocks of volcaniclastic sedimentary origin (“transition zone”) was designated as polylines in the GIS. Statistical model results indicate: 1) wells close to the “transition zone” are more likely to contain detectible As, 2) welded tuffs and hydrothermal quartz bodies are associated with relatively higher As concentrations and 3) welded tuffs proximal to a pluton appear to exhibit even higher As concentrations.