Nutrient exports from watersheds with varying septic system densities in the North Carolina Piedmont.

Septic systems (SSs) have been shown to be a significant source of nitrogen and phosphorus to nutrient-sensitive coastal surface and groundwaters. However, few published studies have quantified the effects of SSs on nutrient inputs to water supply watersheds in the Piedmont region of the USA. This region consists of rolling hills at the surface underlain by clayey soils. There are nearly 1 million SSs in this region, which accounts for approximately 50% of all SSs in North Carolina. The goal of this study was to determine if significant differences in nutrient concentrations and exports exist between Piedmont watersheds with different densities of SSs. Water quality was assessed in watersheds with SSs (n = 11) and a sewer and a forested watershed, which were designated as controls. Stream flow and environmental readings were recorded and water samples were collected from the watersheds from January 2015-December 2016. Additional samples were collected from sand filter watersheds in April 2015-March 2016 to compare to septic and control watersheds. Samples were analyzed for total dissolved nitrogen (TDN) and orthophosphate (PO4-P). Results indicated that watersheds served by a high-density (HD) of SSs (4.9 kg-N yr-1 ha-1; 0.2 kg-P yr-1 ha-1) exported more than double the median masses of TDN and PO4-P, respectively, relative to low-density (1.0 kg-N yr-1 ha-1; <0.1 kg-P yr-1 ha-1) and control watersheds (1.4 kg-N yr-1 ha-1; <0.1 kg-P yr-1 ha-1) during baseflow. Isotopic analysis indicated that wastewater was the most likely source of nitrate-N in HD watersheds. In all other watersheds, isotopic results suggested non-wastewater sources as the dominant nitrate-N provider. These findings indicated that SS density was a significant factor in the delivery of septic-derived nutrients to these nutrient-sensitive, water supply watersheds of the North Carolina Piedmont.

Meteorological influences on nitrogen dynamics of a coastal onsite wastewater treatment system.

On-site wastewater treatment systems (OWTS) can contribute nitrogen (N) to coastal waters. In coastal areas with shallow groundwater, OWTS are likely affected by meteorological events. However, the meteorological influences on temporal variability of N exports from OWTS are not well documented. Hydrogeological characterization and seasonal monitoring of wastewater and groundwater quality were conducted at a residence adjacent to the Pamlico River Estuary, North Carolina, during a 2-yr field study (October 2009-2011). Rainfall was elevated during the first study year, relative to the annual mean. In the second year, drought was followed by extreme precipitation from Hurricane Irene. Recent meteorological conditions influenced N speciation and concentrations in groundwater. Groundwater total dissolved nitrogen (TDN) beneath the OWTS drainfield was dominated by nitrate during the drought; during wetter periods, ammonium and organic N were common. Effective precipitation (precipitation [P] minus evapotranspiration [ET]) affected OWTS TDN exports because of its influence on groundwater recharge and discharge. Groundwater nitrate-N concentrations beneath the drainfield were typically higher than 10 mg/L when total biweekly precipitation was less than evapotranspiration (precipitation deficit: P < ET). Overall, groundwater TDN concentrations were elevated above background concentrations at distances >15 m downgradient of the drainfield. Although OWTS nitrate inputs caused elevated groundwater nitrate concentrations between the drainfield and the estuary, the majority of nitrate was attenuated via denitrification between the OWTS and 48 m to the estuary. However, DON originating from the OWTS was mobile and contributed to elevated TDN concentrations along the groundwater flowpath to the estuary.

Onsite wastewater system nitrogen contributions to groundwater in coastal North Carolina.

The objective of the study described in this article was to evaluate the nitrogen contributions from two onsite wastewater systems (sites 1 and 2) to groundwater and adjacent surface waters in coastal Beaufort County, North Carolina. Groundwater levels and water quality parameters including total nitrogen, nitrogen species, temperature, and pH were monitored from October 2009 to May 2010. Nitrogen was also tested in groundwater from deeper irrigation or drinking water wells from the two sites and six additional neighboring residences. Mean total nitrogen concentrations in groundwater beneath onsite wastewater systems 1 and 2 were 34.3 +/- 16.7 mg/L and 12.2 +/- 2.9 mg/L, respectively, and significantly higher than background groundwater concentrations (< 1 mg/L). Groundwater in the deeper wells appeared not to be influenced by the onsite systems. Groundwater nitrogen concentrations typically decreased with distance down-gradient from the systems, but were still elevated relative to background conditions more than 15 m from the systems and near the estuary. This was a pioneering effort to better understand the link of onsite systems, the fate of nitrogen in the environment, and public health.

Evaluation of on-site wastewater system Escherichia coli contributions to shallow groundwater in coastal North Carolina.

The study goal was to determine if on-site wastewater systems (OSWWS) installed in coastal areas were effective at reducing indicator bacteria densities before discharge to groundwater. Groundwater Escherichia coli (E. coli) densities and groundwater levels adjacent to 16 OSWWS in three different soil groups (sand, sandy loam, and sandy clay loam) were monitored and compared to background groundwater conditions on four occasions between March 2007 and February 2008 in coastal North Carolina. Groundwater beneath OSWWS had significantly (p≤0.05) lower densities of E. coli than septic tank effluent, but significantly higher densities of E. coli than background conditions for each soil type. Twenty three percent of all groundwater samples near OSWWS had E. coli densities that exceeded the EPA freshwater contact standards (single sample 235 cfu/100 mL) for surface waters. Groundwater E. coli densities near OSWWS were highest during shallow water table periods. The results indicate that increasing the required vertical separation distance from drainfield trenches to seasonal high water table could improve shallow groundwater quality.

Controls on groundwater nitrogen contributions from on-site wastewater systems in coastal North Carolina.

The goal of this study was to evaluate the influence of soil type and separation distance to water table on dissolved inorganic nitrogen concentrations in groundwater adjacent to on-site wastewater systems. Groundwater nitrogen species (NO3--N and NH4+-N) and groundwater levels adjacent to 16 on-site systems in three different soil groups (group I- sand, group II- coarse loams and group III -sandy clay loams) were monitored for 15 months (January 2007-March 2008) in coastal North Carolina. On-site systems in soil group I had the highest concentrations of dissolved inorganic nitrogen (median of 18.9 mg/L) in groundwater, and most frequently (mean 61%) exceeded 10 mg/L, followed by systems in soil group II (11.0 mg/L, 50%) and soil group III (2.6 mg/L, 9%), respectively. Groundwater NH4+-N concentrations near on-site systems in soil groups I and II that maintained a 60+cm separation to the seasonal high water table were 4 mg/L lower in relation to systems that had <60 cm separation, but median NO3--N concentrations were 6.5 mg/L higher. On-site systems in group I and II soils are prone to groundwater nitrogen loading with separation distance often controlling the nitrogen speciation in groundwater near on-site systems.