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

On-site septic systems are a source of both local and regional groundwater contamination in the United States and elsewhere in the world. Shallow grou ndwater and permeable soils increase the vulnerability of certain geographi c 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 P inelands Commission, a regional land use planning and regulatory agency, pe rmits the use of subsurface pressure dosing septic systems as an alternativ e to standard septic systems on undersized residential lots in the Pineland s. This policy was based on the assumption that pressure dosing systems rem ove a significant amount of wastewater nitrogen. To test this assumption, w e completed a field study comparing nitrogen removal in subsurface pressuri sed and standard gravity flow septic systems on sandy soils. All systems se rved single family homes. We found no significant difference in nitrogen re moval between system types. Average nitrogen removal rates of 40 and 48% we re found for eight pressure dosing and 11 standard systems, respectively In both types of systems, most nitrogen removal occurred between the septic t ank 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 wi thin the sand fill or 31 cm below the sand fill/native soil interface (bott om zone). The results of this study can provide the basis for reassessing l and use policy in the Pinelands and may be applicable to regions of similar geologic conditions. (C) 1999 Academic Press.