BEDROCK AND FRACTURE PATTERNS CONTROL HIGH NATURAL GROUNDWATER ARSENIC CONCENTRATIONS IN THE PIEDMONT OF NORTH CAROLINA

Naturally occurring arsenic (As) is present above EPA regulation levels in many natural bedrock wells throughout the Piedmont of North and South Carolina. High As levels are particularly concentrated in the Carolina Slate Belt (CSB) also known as the Carolina Terrane. As levels vary widely throughout the CSB with the highest overall levels found in North Carolina along the border with South Carolina, but even within this area As levels vary between undetectable and levels well above 8 ppm within less than a km. Finding a possible cause for the naturally high As levels could help inform future well placement.

While numerous studies have looked at overall controls on As levels in bedrock wells, including rock type, location, pH, and local pollutants, these studies have been unable to identify why certain areas of the CSB have higher natural As levels. In this chemically complicated system, two basic components likely control natural As levels in the groundwater; groundwater flow and the concentration of natural As in the bedrock.

We used ARC-GIS, 10 m DEMs, existing geologic maps, and the NCWELL groundwater database of almost 63,000 wells to establish a strong spatial correlation with high groundwater As levels and bedrock formations. The southern Gold Hill Shear Zone and the southern portion of the meta-mudstone tuff member (Zcm) of the Late Proterozoic El Cid formation clearly have the highest percentage of groundwater As in excess of EPA guidelines compared to the rest of the state. While gold mines can be found throughout western NC, the shear zone and southern Zcm have the largest concentration of gold mines in the state. Because gold and As have long been recognized as geologically similar in bedrock concentration mechanisms, this correlation should be expected. Further, a dramatic change in the fracture pattern from dominantly N-S to NE-SW trending in the southern portion of the ZCM layer could indicate both a tectonic and a groundwater flow mechanism for changing concentrations of As within the meta-mudstone.