A comparison of nitrogen removal by subsurface pressure dosing and standard septic systems in sandy soils

Abstract

On-site septic systems are a source of both local and regional groundwater contamination in the United States and elsewhere in the world. Shallow groundwater and permeable soils increase the vulnerability of certain geographic areas to nitrogen contamination by septic systems. The Pinelands region, located on the Atlantic Coastal Plain in New Jersey, USA, is characterised by sandy soils, and is underlain by an extensive water-table aquifer. The Pinelands Commission, a regional land use planning and regulatory agency, permits the use of subsurface pressure dosing septic systems as an alternative to standard septic systems on undersized residential lots in the Pinelands. This policy was based on the assumption that pressure dosing systems remove a significant amount of wastewater nitrogen. To test this assumption, we completed a field study comparing nitrogen removal in subsurface pressurised and standard gravity flow septic systems on sandy soils. All systems served single family homes. We found no significant difference in nitrogen removal between system types. Average nitrogen removal rates of 40 and 48% were found for eight pressure dosing and 11 standard systems, respectively. In both types of systems, most nitrogen removal occurred between the septic tank and the first 15 cm (top zone) of the 1·2 m layer of sand fill. In the majority of both system types, no additional change in nitrogen occurred within the sand fill or 31 cm below the sand fill/native soil interface (bottom zone). The results of this study can provide the basis for reassessing land use policy in the Pinelands and may be applicable to regions of similar geologic conditions.