Estimating Natural Attenuation of Nitrate and Phosphorus From Onsite Wastewater Treatment Systems

Estimating the attenuation of nutrients (e.g., nitrogen, phosphorus) in groundwater from onsite wastewater treatment systems (OWTS) is difficult due to the costs and uncertainty associated with determining site -specific degradation rates, groundwater flow paths, and aquifer hydraulic properties. Some available methods allow users to specify natural degradation rates for nutrients from OWTS but provide inadequate guidance on how to determine those rates. Other methods use a mechanistic approach with numerous variables and equations that are known to affect nutrient attenuation but are necessarily complex and difficult to use for assessing hundreds or thousands of septic systems, as is often needed for regulatory applications. A spreadsheet analysis, the Method for Estimating Attenuation of Nutrients from Septic Systems (MEANSS), has been developed within the Montana Department of Environmental Quality to provide a relatively simple method of estimation with minimal data requirements. MEANSS is an empirical method designed to estimate the load of nutrients that will migrate to groundwater and surface water from OWTS sources. MEANSS provided comparable estimates of nitrogen and ortho-phosphorus attenuation below and downgradient of OWTS when evaluated against several field studies, a GIS-based nitrogen loading model, a mechanistic model, and a watershed model.