GEOPHYSICAL AND WATER QUALITY CHARACTERIZATION OF ON-SITE WASTEWATER TREATMENT SYSTEM EFFLUENT PLUMES AT TWO SCHOOLS IN THE NORTH CAROLINA COASTAL PLAIN

On-site wastewater treatment systems (OWTS) are the predominant means of wastewater disposal in many rural areas. When functioning properly, OWTS are effective at treating wastewater and discharging it into the subsurface in a manner that does not compromise human or environmental health. However, insufficient setback distances or inadequate subsurface treatment of wastewater can contribute significant non-point source pollution to ground and surface waters. Monitoring wells provide access to groundwater for characterization of physical, chemical, and biological properties at discrete points in the subsurface. However, the process of installing a network of monitoring wells is labor intensive, costly, and intrusive. As a result the number of wells installed is often limited by these constraints. Therefore, methods are needed that can help reduce the number of monitoring wells installed, or site wells more efficiently while providing sufficient groundwater quality data. OWTS discharge effluent with high concentrations of dissolved ions that alter groundwater quality. It may be possible to detect these changes in groundwater quality directly beneath and immediately down-gradient of an OWTS using geophysical instrumentation such as electrical resistivity (ER) and ground penetrating radar (GPR). To test this theory, 2D and 3D ER and GPR surveys were conducted bi-monthly over a one year period at two school sites utilizing OWTS, both located in Craven County, North Carolina. Additionally, 48 monitoring wells were installed at the study sites within and outside of the effluent plume boundaries, as determined by the geophysical surveys. Depth to water, temperature and specific conductance measurements were collected at both sites on the same days as the geophysical surveys. The geophysical data are compared to the water quality data to determine if there is a correlation, which will test the accuracy of these geophysical approaches. Preliminary results suggest that geophysical surveys can provide information on the extent of wastewater plumes.