GEOELECTRICAL IMAGING OF WASTEWATER MIGRATION IN COASTAL SURFICIAL AQUIFERS, NORTH CAROLINA, USA

In-situ characterization of subsurface wastewater migration and associated contaminant transport in coastal surficial aquifers can be challenging. Effectiveness can be constrained by limited site access, obstructions related to infrastructure, and lack of on-site groundwater sampling networks. Wastewater typically has elevated specific conductivity. Shallow aquifers that receive wastewater may respond by becoming more conductive to electrical current. Capacitively-Coupled Resistivity (CCR) surveying is a technique that quantifies subsurface electrical resistivity. During the course of three groundwater investigations, we evaluated if CCR surveys could help characterize the extent of wastewater migration in the subsurface. The onsite wastewater study sites included two schools, an environmental education center, and two private residences in Pitt, Beaufort, and Craven Counties, located in coastal North Carolina. CCR surveys were conducted with an OhmMapper (Geometrics, Inc.) and soil and sediment cores were collected adjacent to surveys. Groundwater measurements taken at the sites included depth to groundwater, pH, specific conductivity, temperature, dissolved nitrogen, and dissolved oxygen. Water samples were collected from septic tanks, groundwater beneath the drainfield, groundwater up- and down-gradient from the onsite wastewater systems, and from nearby surface waters. We compared electrical resistivity, groundwater specific conductivity, and dissolved nitrogen concentrations in the surficial aquifer adjacent to onsite wastewater systems. Wastewater-affected groundwater could be distinguished from unaffected groundwater where the resistivity contrast was greater than approximately 200 mS/cm. CCR surveys were sensitive to the presence of wastewater in sandy surficial aquifers with shallow water tables (< 5 m deep) and may be a useful tool for delineating areas where wastewater inputs have led to elevated groundwater nitrogen concentrations, however false positives are possible if denitrification occurs. Interpretation of resistivity data may also be complicated by heterogeneous geology and the presence of clay layers.