GEOPHYSICAL DELINEATION AND GROUNDWATER CHARACTERIZATION OF A CONTAMINANT PLUME FROM A RESIDENTIAL ON-SITE WASTEWATER SYSTEM IN PITT COUNTY, NORTH CAROLINA

On-site wastewater treatment systems (OWS) disperse effluent containing elevated concentrations of nutrients, pathogens, and dissolved salts into the subsurface. Physical, chemical and biological processes in the vadose zone beneath OWS reduce wastewater constituent concentrations. However, groundwater beneath and within the flow-path of the OWS may still contain elevated contaminant concentrations relative to background conditions. Geophysical technologies such as ground penetrating radar (GPR) and an electrical resistivity mapper (ER) may be useful in detecting and delineating groundwater that is impacted by OWS effluent (plume). The objective of this project was to determine if GPR and ER surveys could be used to detect and delineate a residential-scale OWS plume.

GPR and ER surveys were conducted in July 2012 at a residential site in Pitt County North Carolina. The site was instrumented with 15 piezometers installed between the drainfield trenches and up and down gradient of the OWS. Groundwater environmental readings including depth to water, electrical conductivity, pH, temperature, and dissolved oxygen were measured using field meters. Water samples from the piezometers were collected and analyzed for total dissolved nitrogen (TDN), chloride, and phosphate. Geophysical surveys, groundwater readings, and water sample collection and analyses were conducted during the same week. Geophysical surveys indicated an area (plume) with different electrical properties (90 ohms less resistant) than background conditions. Groundwater within this plume area had elevated mean electrical conductivity (194 µS/cm), nitrogen (7.5 m/L), phosphate (0.59 mg/L), and chloride (13.8 mg/L) concentrations relative to background groundwater (electrical conductivity 147 µS/cm; nitrogen 4.7 m/L; phosphate 0.31 mg/L; and chloride 9.7 mg/L). Additional surveys and groundwater characterization will occur in the Fall and Winter and at other sites to better quantify the findings.