ONSITE WASTEWATER TREATMENT SYSTEMS: MOVING MODELING TO THE PLANNING STAGE

Individual and clustered onsite wastewater treatment systems (OWTSs) have been implicated in a wide range of groundwater problems and surface water impairments caused by excess loadings of nitrogen, phosphorus, and pathogens. Modeling impacts for existing OWTSs has been difficult due to the wide range of system types, soil treatment mechanisms, and load assignment ranges for systems sited in varying densities and environmental settings. Despite these difficulties, modelers have produced impact estimates for the three primary pollutants cited above at dozens of locations across the nation. These studies have been conducted primarily to quantify pollutant loads for TMDL and other analyses that seek to estimate and reduce current problems; rarely have they been used to prevent problems – i.e., to provide system design guidance and/or performance standards for new development.A review of the development histories and planning needs of ex-urban and growing rural communities demonstrates a high need for modeling support for wastewater planning in areas beyond the “big pipe” of regional centralized wastewater treatment facilities. Shrinking resources for centralized sewage plants and regional plant overloads – resulting in sanitary sewage overflows and combined sewage overflows – indicate that modeling-enhanced planning for decentralized wastewater management will grow in importance, as local communities seek to both accommodate new development and protect valued water resources.