Nitrogen and carbon dynamics beneath on-site wastewater treatment systems in Pitt County, North Carolina

Abstract

On-site wastewater treatment systems (OWS) are a potentially significant non-point source of nutrients to groundwater and surface waters, and are extensively used in coastal North Carolina. The goal of this study was to determine the treatment efficiency of four OWS in reducing total dissolved nitrogen (TDN) and dissolved organic carbon (DOC) concentrations before discharge to groundwater and/or adjacent surface water. Piezometers were installed for groundwater sample collection and nutrient analysis at four separate residences that use OWS. Septic tank effluent, groundwater, and surface water samples (from an adjacent stream) were collected four times during 2012 for TDN and DOC analysis and pH, temperature, electrical conductivity, and dissolved oxygen measurements. Treatment efficiencies from the tank to the groundwater beneath the drainfields ranged from 33 to 95% for TDN and 45 to 82% for DOC, although dilution accounted for most of the concentration reductions. There was a significant positive correlation between nitrate concentration and separation distance from trench bottom to water table and a significant negative correlation between DOC concentration and separation distance. The TDN and DOC transport (>15 m) from two OWS with groundwater saturated drainfield trenches was significant.

Spatial Distribution of Fecal Indicator Bacteria in Groundwater beneath Two Large On-Site Wastewater Treatment Systems

“…FIB data were log transformed and a Pearson Correlation test was used to determine if there were significant correlations between the different FIB concentrations, and between FIB concentrations and SC. Correlations between FIB and SC were performed because SC is often elevated in fresh groundwater influenced by effluent [7,25,26].…”

Nitrogen Treatment Efficiency of a Large Onsite Wastewater System in Relation to Water Table Dynamics

“…Groundwater and septic effluent TDN concentrations were lowest in July 2013 (27.2 and 41.7 mg L −1 , respectively) and highest in March 2014 (64.8 and 144.7 mg L −1 , respectively) (Figure A). The school operates on a traditional school calendar (late August to early June), thus the lower concentrations of TDN in July could be associated with relatively low flows because of the summer break. The lower concentrations of TDN in groundwater in July may also have been related to an abnormally wet period when rainfall exceeded the long term average by 100%, potentially diluting groundwater TDN concentrations .…”

“…O'Driscoll et al reported that during periods of increased precipitation and recharge, the hydraulic gradient near an OSW increased, and thus transport of nitrogen toward the estuary increased. Other studies have shown that when the separation distance between OSW and groundwater decreases, treatment efficiency of wastewater phosphorus, bacteria, carbon, and pharmaceuticals decreases. Prior studies have shown that groundwater levels in coastal North Carolina respond quickly to precipitation events because of the relatively shallow depths to groundwater, and high permeability rates of the sandy soil .…”

“…If precipitation rates in North Carolina increase as a result of climate change and sea level rises 30 cm or more over the next 50 years as projected, then groundwater levels and OSW treatment may be affected. For example, studies conducted by Humphrey et al, Del Rosario et al, and O'Driscoll et al have shown that OSW nitrogen transformations and transport are influenced by separation distances from OSW drainfield trenches to groundwater. Humphrey et al in a field study of 16 OSWs showed that NH 4 ‐N concentrations were elevated in groundwater beneath OSW with less than 45 cm of separation to groundwater in comparison to OSW with more than 45 cm of separation.…”

“…Humphrey et al in a field study of 16 OSWs showed that NH 4 ‐N concentrations were elevated in groundwater beneath OSW with less than 45 cm of separation to groundwater in comparison to OSW with more than 45 cm of separation. Del Rosario et al reported elevated concentrations of dissolved nitrogen more than 15 m down‐gradient from OSWs that had less than 30 cm separation distance from OSW trenches to groundwater. O'Driscoll et al reported that during periods of increased precipitation and recharge, the hydraulic gradient near an OSW increased, and thus transport of nitrogen toward the estuary increased.…”