ARSENIC OCCURANCE IN THE UNCONFINED FRACTURED BEDROCK AQUIFER SYSTEM OF THE NORTH CAROLINA PIEDMONT

A study to assess the distribution of arsenic in North Carolina was initiated in 2000. Data collected from historically and recently sampled domestic water supply wells (DWSW) have been combined to produce a database of over 24,000 groundwater samples, of which ~10,000 samples have been geolocated based on address information. Probability analysis, using indicator kriging, revealed a high probability zone for detectable arsenic in groundwater that trends northeast from Union County to Person County. This zone is spatially correlative with volcanic and volcaniclastic rock bodies of the Carolina Zone (CZ).

Veined and disseminated sulfides have been observed in cores from the CZ. Laboratory analyses of respective sulfide minerals and their host rocks suggest that they are a naturally occurring source of arsenic. In addition, field-based geochemical studies of naturally occurring iron-manganese boulder and fracture coatings, ceramic streak plate experiments, along with additional sampling of DWSW, soils, stream water and stream sediment are being used to understand arsenic fate and transport in the unconfined fractured bedrock aquifer system, characteristic of the CZ.

Our interim conclusions suggest that chemical weathering of the upper bedrock results in the dissolution of arsenic from sulfide bearing minerals and, depending on groundwater chemistry, precipitation onto fracture surfaces. The fraction that is not precipitated is then flushed from the groundwater system via discharge to surface waters where transport through a greater oxidation front (i.e., moving from a groundwater system to a surface water system) forces precipitation of iron and manganese oxyhydroxides with which arsenic is co-precipitated.

Of particular interest to our study is the role DWSW have on the geochemical system. For example, historically elevated arsenic levels have been recorded in Public Water Supply Wells (PWSW) from many areas in the state. These levels typically attenuate over a few years. The limited temporal data we have for DWSW does not indicate similar reductions in arsenic levels. We hypothesize that the greater production volumes of PWSW changes the local geochemistry resulting in either the depletion of the source material or the prevention of conditions favorable for sulfide dissolution.