OCCURRENCE AND GEOCHEMISTRY OF ARSENIC IN GROUNDWATER IN SELECTED AREAS OF THE NORTH CAROLINA PIEDMONT

Groundwater arsenic (As) levels exceed the USEPA MCL of 10 µg/L in some areas of the NC Piedmont. Studies have identified geologically-controlled hot spots of As that occur more frequently in the Carolina terrane (CT) than in other units. In this study, we evaluated county-level data across CT, conducted periodic sampling of monitoring wells at stations representing two lithologies – meta-mudstones in CT (area 1) and mica-schists in the Kings Mountain sequence (area 2), interpreted to be part of CT, along with random sampling of nearby private wells. Also, 16 high-As wells were sampled 3 to 5 times over 5 years.

Elevated groundwater As, ranging from <2 to 800 µg/L, is more prevalent in the Albermarle Group of CT, with the Mudstone Member of the Cid Formation being the main As-bearing lithology. In contrast, the older sequences of CT rocks display sparse detects >10 µg/L.

Private well sampling showed total As concentrations of <5 to 77 µg/L in area 1 and <5 to 9 µg/L in area 2. In area 1, elevated (>10 µg/L) As generally occurred with low levels (<100 µg/L) of Fe, (<50 µg/L) Mn, (<20 mg/L) SO₄, (<1 mg/L) DO, and at pH>7. In area 2, elevated As occurred at variable levels of target parameters. Targeted sampling of high As (10 to 150 µg/L) wells showed higher temporal variations in area 2 than in area 1, which may be related to well pumping and redox conditions. Well pumping slightly increased As levels in monitoring wells at both areas, but notably increased in samples collected at water levels below well casing.

A thin regolith and highly fractured bedrock provide organic matter and facilitate oxic conditions in an unconfined aquifer, rich in sulfidic minerals at area 1. Following initial release of As from arsenical pyrite, desorption of As from Fe/Mn-oxyhydroxides under mixed oxic–anoxic conditions and increased pH may be the main mechanism for release of As in area 1, although bacterial reduction of hydroxides under reducing conditions may occur in deep bedrock. Fewer occurrences of elevated groundwater As in area 2 may be due to higher levels of Fe in the thicker regolith-bedrock aquifer, which would constrain the release of As to groundwater through adsorption onto Fe/Mn-oxyhydroxides. In the CT, wells with shallow casings are thus particularly vulnerable to water-level drawdown and repeated redox cycling and to release of As to groundwater.