Abstract
Wastewater may be a source of nitrogen (N) to groundwater and surface waters if not effectively treated. In North Carolina, onsite wastewater systems (OWSs) are used by 50% of the population for wastewater treatment, but most OWSs are not routinely monitored. There is a lack of information regarding the N contributions from OWSs to water resources. Four sites with OWSs were instrumented with groundwater wells near their drainfield trenches to compare N concentrations in groundwater to concentrations in wastewater and to determine the N treatment efficiency of the systems. Two OWSs (Site 200 and 300) were less than 1 year old, and two (Site 100 and 400) were more than 10 years old at the start of the study. Two OWSs (Site 100 and 200) used pressure dosing, while two OWSs (Site 300 and 400) used gravity distribution. The mean N treatment efficiency of the four OWSs was 77%. The new OWSs were more efficient (92%) relative to the older OWSs (62%) at reducing N concentrations. Similar N treatment efficiencies were observed when pooling data for the pressure dosed (77%) and gravity (79%) OWSs. Each OWS influenced groundwater by causing increases in N concentrations. It is important that new OWSs are installed at a shallow depth and with sufficient separation to groundwater to promote the aerobic treatment of wastewater. Remediation strategies including the installation of permeable reactive barriers or the use of media filters may be needed in some areas to reduce N transport from existing OWS.
Highlights
The mean Total dissolved N (TDN) treatment efficiency of the four onsite wastewater systems (OWSs) was 77%, but differences in efficiencies were observed between systems
Lower efficiencies were observed at the older sites including Site 400 (63%), and Site 100 (61%)
Prior studies and reports [34,37] suggest that biomat formation may take a few years to develop and since biomat development is expected to improve N treatment [20,32], it was expected that older systems with biomats may perform better than new OWSs without developed biomats
Summary
Additions of N-based fertilizers to agricultural fields and forests have increased crop yields [3] and timber production [4] These increases are important as the global population has doubled in the last 50 years, and the demand for food and fiber has grown [5]. More research was suggested to gain a better understanding of the treatment efficiency of OWSs and their impact with regard to N contributions to water resources. If these data suggest OWSs can be a potentially significant N source, additional efforts will be necessary to develop mitigation strategies for reducing N loads from OWSs to water resources
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