The Fate of Onsite Septic System Nitrogen Discharges in Groundwater of the Hood Canal Basin

Abstract

Nitrogen loading from onsite septic systems (OSSs) may degrade the ecological health of aquatic systems by causing or accentuating eutrophication leading to hypoxia or anoxia. OSSs do not typically remove nitrogen within the septic tank; therefore, denitrification is the primary mechanism controlling nitrogen loading from OSSs to surface water systems. Hood Canal, located in the Puget Sound region of Washington, has been severely affected by low dissolved oxygen concentrations due to excess nitrogen loading. Onsite septic systems are the primary wastewater treatment process in this watershed and may be an important source of nitrogen to Hood Canal. Horowitz describes a study in which she determined how much denitrification occurred in OSS effluents in the Hood Canal basin, a glacially-formed, geologically complex system. Shallow sampling wells (0.5 – 2 m) were installed around five OSS drainfields and deep sampling wells (6 m) at one OSS drainfield. Well samples were analyzed for nitrogen, chloride, bromide, and dissolved organic carbon concentrations. The extent of nitrogen reduction varied among the field sites and between two years at one field site. A study site, located in a riparian floodplain, was found to have nearly complete denitrification. Shoreline sites, located near rocky intertidal zones, were found to have little denitrification. And sites located in upland areas were found to have partial denitrification.