Capacitively-Coupled Resistivity Surveys to Delineate Subsurface Wastewater Migration in Coastal Surficial Aquifers.

Nutrient exports from conventional onsite wastewater systems to surface waters are not well-constrained. There is a growing need to better quantify non-point source nutrient inputs to surficial aquifers and surface waters, particularly in nutrient-sensitive coastal watersheds. Characterizing subsurface wastewater migration and associated nutrient transport in coastal surficial aquifers can be challenging and resource-intensive due to the need for site access, invasive soil and hydrogeological site characterization, in-situ groundwater monitoring, and water quality analyses. Since wastewater typically has elevated specific conductivity, shallow aquifers that receive wastewater may respond by becoming more conductive (or less resistive) to the flow of electrical current. Capacitively-Coupled Resistivity (CCR) surveying is a technique that quantifies subsurface electrical resistivity. This technique can be useful in onsite wastewater studies because it can potentially detect changes in groundwater conductivity that can be used to quantify the extent of subsurface wastewater migration in shallow aquifers. During the course of three groundwater investigations, we evaluated if CCR surveys could help quantify the extent of wastewater transport in the subsurface. The onsite wastewater study sites included schools, an environmental education center, and private residences in Pitt, Beaufort, and Craven Counties, located in the Coastal Plain of North Carolina. Electrical resistivity surveys were conducted with an OhmMapper (Geometrics, Inc.) and apparent resistivity data was inversely modeled using RES2DINV and RES3DINV software. Soil and sediment cores were collected adjacent to resistivity transects. Groundwater environmental measurements taken at the survey 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 system, 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. Overall, the results showed that CCR surveys were sensitive to the presence of wastewater in sandy surficial aquifers with shallow water tables (< 5 m deep) and were able to detect changes in groundwater specific conductivity at depths of up to approximately 8m. Resistivity data can help delineate the orientation and extent of subsurface wastewater plumes if the contrast between background groundwater and wastewater-affected groundwater is greater than approximately 200 mS/cm. Interpretation is most straightforward when the sandy surficial aquifer sediments are relatively homogeneous because clay layers can also result in lower resistivity values.