| Summary |
In North Carolina, wastewater-derived nutrients and harmful bacteria threaten public health and reduce water quality for many surface water bodies. North Carolina uses ~ 2 million on-site wastewater treatment systems, yet the state monitors [much less than] 1% of these systems annually. This is largely due to the time/work intensive and invasive techniques required to monitor these systems. One technique, low-induction number (LIN) electromagnetic induction (EMI), could provide a means to quickly monitor these systems without intruding the ground surface. Studies employing LIN EMI show that this technique can track wastewater-impacted groundwater in some instances, however these studies were not conducted in North Carolina nor the Atlantic Coastal Plain. The goals of this study were to test EMI's ability to detect wastewater-impacted groundwater, to detect on-site wastewater treatment systems' (OWTS) components and to illuminate OWTS efficacy in the North Carolina Coastal Plain (NCCP). Three sites (100, 200, 300) were selected for this study, located in the Neuse River Basin (100) and Tar-Pamlico River Basin (200, 300). EMI surveys and capacitively-coupled resistivity (CCR) transects were conducted at each drainfield for spring and summer periods (March/April 2014 and August/September 2014). Water samples were collected from groundwater monitoring wells at each site and tested for nutrients (NO 3 and NH 4 ), chloride and enterococci bacteria concentration to help determine wastewater-impacted groundwater migration. In-well measurements of specific conductance, depth to the water table, pH, dissolved oxygen and temperature were used to help identify wastewater-impacted groundwater migration. Hand-auguered cores from site 100 were used to classify sediment grain size at this site and compared to previously published data for the other sites (200, 300). Data loggers were used to measure specific conductance and pressure (depth) for background and drainfield monitoring wells (from 7/21/14-9/10/2015). Log transformed dissolved inorganic nitrogen (DIN) related well with log transformed chloride, specific conductance, and EMI apparent conductivity for the spring surveys (r 2 = 0.714, 0.694, 0.499 respectively) but not as well for the summer surveys (r 2 = 0.417, 0.176, 0.181 respectively). Enterococci bacteria did not correlate well with any of these parameters. Geophysical transects displayed zones of low resistivity corresponding to high apparent conductivity. Some surveys displayed an increase in depth of low resistivity zones corresponding to decreases in apparent conductivity, suggesting water table influence on EMI. Wilcoxon-Mann-Whitney tests revealed that apparent conductivity was statistically greater (p [much less than] 0.001) within the drainfield than outside of it. Where depth to the water table was [greater than] ~5 m, changes in groundwater specific conductance had less influence on apparent conductivity, likely due to reduced sensitivity for the EMI device for depths [greater than]5 m. Most EMI surveys roughly outlined drainfield extent, presumably due to wastewater and not drainfield characteristics. EMI proved to be poor for identifying individual OWTS components; drainline/trench locations were not apparent. Drainfield groundwater specific conductance remained elevated above background groundwater for long-term data logging (7/21/14-9/10/2015) at all sites. Overall, EMI more effectively tracked wastewater-impacted groundwater for the spring survey (low rainfall, low evapotranspiration) than the summer survey (high rainfall, high evapotranspiration). Note that these sites are schools, and therefore use OWTS less during the summer months. Due to the temporal/spatial variation among the depth to the water table and groundwater specific conductance, the timing of the surveys may be important to confidently identify wastewater-impacted groundwater using EMI. This technique seems appropriate for monitoring nutrient migration from OWTS with a shallow water table ([less than]~5 m) and strong wastewater-groundwater electrical conductivity contrasts. Further research is necessary to assess EMI for use in other NCCP soils and with OWTS of different sizes. |
