WASTEWATER NITROGEN SOURCES

In general, the most important source of nitrogen to the estuaries of southern New England is human waste. Wastes are processed by either sewer systems or on-site wastewater disposal systems, mostly septic tanks and associated leaching fields. Waste disposal in many communities is primarily by septic tank methodology, in which raw wastewater first enters a holding, or septic, tank where sedimentation and microbial breakdown of organic matter occurs. Ammonium (NH₄⁺) is the major nitrogenous breakdown product, and is carried off in the effluent that enters the unsaturated sub-surface soil layer of the leaching field. Ideally, the ammonium would be removed by adsorption to soil particles, or converted to nitrate, then to nitrogen gas by bacteria, and escape to the atmosphere before entering the groundwater. Unfortunately, these removal processes are not very efficient, and 50-60% of the nitrogen (mostly as nitrate) is likely to escape the leaching field and percolates into the groundwater. With the rapid growth of housing and proliferation of septic tank disposal systems there has been a large increase in groundwater nitrogen concentration. The problem is exacerbated by housing developments sited immediately adjacent to the estuary, where the short travel distance to the water's edge further limits the effectiveness of nitrogen removal processes. Generally, those houses within 200 meters (about two football fields) of the shoreline will contribute disproportionately to the nitrogen load entering the estuary. Waste from some houses flows into cesspools, that are septic holding tanks without leaching fields. This older and largely illegal technology results in higher amounts of nitrogen entering the groundwater, but cesspools are increasingly rare in southeastern Massachusetts. Sewer systems offer one solution to the diffuse nitrogen loading of groundwater caused by on-site septic systems. However, unless tertiary treatment is practiced or effluent directed away from the estuary (e.g., the Massachusetts Bay sewage outfall), sewering focuses nitrogenous waste input into estuaries as a "point source", and does not necessarily reduce the total nitrogen load to the estuary.