Geophysical characterization of onsite wastewater treatment system effects on groundwater quality, eastern North Carolina

Abstract

Onsite wastewater treatment systems (OWS) are a potential non-point source of pollution that can result in water quality degradation in groundwater and surface water. Locating OWS wastewater plumes in the subsurface often requires extensive site instrumentation and monitoring. The application of capacitively coupled resistivity (CCR) surveys offers a time efficient method to image wastewater plumes and thus may help in the design of groundwater monitoring networks. Additionally, while most OWS permits provide the general location of the system components, the exact locations of the OWS drainfield trenches are not always displayed relative to a benchmark, and modifications to the system (and location) are not always recorded. Ground penetrating radar (GPR) can be used to identify drainfield trenches in real time in the field. The current study utilized seven OWS in Pitt and Craven Counties, located in the Inner Coastal Plain of North Carolina. CCR and GPR surveys were conducted in conjunction with laboratory analysis of water samples and environmental water quality readings collected in the field on three survey dates. The field study period was from June 2012 – May 2013. CCR survey drainfield resistivity values ≤ 250 Ω.m and corresponding groundwater specific conductivity values ≥ 200 µS/cm were indicative of wastewater plumes and the influence of wastewater on groundwater quality detected up to approximately 15 m from the drainfield. Ultimately, the results from this study provide further insight for CCR applicability to detect onsite wastewater treatment system effects on shallow groundwater quality. GPR 3D surveys and 2D transects were successfully used to identify active and de-activated drainfield trenches that matched with the locations determined in the field using a tile drain probe and on the OWS permit. Additional structures identified at a residential site using the surveys and transects included a French drain and two low attenuation structures not listed on the permit. This research demonstrates the use of GPR to efficiently locate OWS components. The dual application of both geophysical methods creates an opportunity to reduce costs and time spent at a site as well as provides a non-intrusive approach to better quantify the extent of the influence onsite wastewater inputs have on shallow groundwater quality.