Onsite Wastewater System Nitrogen Treatment Efficiency in Response to Groundwater Fluctuations.

Onsite wastewater systems (OWS) are a common method of wastewater treatment in coastal regions across the world. OWS rely on aerobic soil beneath drainfield trenches for pollutant attenuation and transformations. Sea level rise and climate change are projected to increase groundwater table elevations in many coastal areas. The vadose zone thickness beneath existing OWS drainfield trenches may be reduced as a result, potentially influencing pollutant treatment. The objective of this study was to determine if the nitrogen treatment efficiency of an OWS serving a school in the Coastal Plain of North Carolina was influenced by water table fluctuations. Monitoring wells (6) were installed between the drainfield trenches of a large OWS and in one background location. Groundwater readings including depth to water, pH, temperature, specific conductance, and dissolved oxygen were recorded 8 times over a 15 month period during periods of relatively “low” and “high” water tables. Septic tank effluent and groundwater from the monitoring wells were collected for analyses of total dissolved nitrogen (TDN) during each field visit for comparison and to determine if the nitrogen treatment efficiency was influenced by the water table elevation. The TDN treatment efficiency of the OWS was inversely correlated to water table elevation (r = -0.754; p = 0.031). Significant differences in groundwater TDN concentrations were observed across the drainfield suggesting dilution was more prevalent on the up-gradient section of the dispersal field. Results suggest that OWS should be installed with the maximum depth to groundwater and oriented (long trenches installed perpendicular to groundwater flow direction) to maximize wastewater mixing with groundwater beneath the drainfield to accommodate for expected future increases in groundwater elevations.