METEOROLOGICAL CONTROLS ON ONSITE WASTEWATER TREATMENT AND GROUNDWATER NITROGEN DYNAMICS IN COASTAL NORTH CAROLINA

Onsite wastewater treatment systems (OWTS) have been documented as a source of nitrogen (N) inputs to surface water and groundwater in coastal areas with sandy soils and shallow water tables. The spatial and temporal variability of N inputs from OWTS to surficial aquifers and nutrient sensitive surface waters is not well constrained. It is known that loading rates, time of day, and the use of personal care and household cleaning products can cause significant temporal variability in wastewater effluent quality. Most of the N treatment by OWTS occurs in the drainfield, biomat, and vadose zone, but these components are influenced by weather events. However, the influence of meteorological controls on the temporal variability of N loadings from OWTS to surficial aquifers has received minimal study. We performed a two-year field study (October 2009-2011) to evaluate the N inputs from OWTS to a coastal surficial aquifer. Soil/hydrogeological characterizations and seasonal monitoring of septic tank performance were conducted at a residential site adjacent to the Pamlico Estuary in Beaufort County, North Carolina. Groundwater and soil water samples were collected seasonally at 29 piezometers and 6 suction lysimeters, respectively. Rainfall was above average during the first year of the study. In contrast, drought conditions occurred during the second year followed by extreme precipitation inputs from Hurricane Irene in August 2011. Drainfield groundwater was dominated by nitrate during the drought in 2011, whereas during wetter periods ammonium and dissolved organic N were commonly found. Groundwater nitrate concentrations below the drainfield were typically higher than 10 mg/L when the total precipitation two weeks prior to sampling was less than 2 inches. N concentrations in groundwater decreased with increasing distance downgradient from the OWTS, which is probably due to dilution and denitrification. Overall, groundwater quality data suggested that N transported from the OWTS could migrate at least 40 m from the drainfield to the adjacent estuary. Extreme weather events and meteorological controls influenced N speciation and concentrations in drainfield groundwater. More work is needed to explain potential climate change implications at event and longer time scales.