NEHA April 2024 Journal of Environmental Health


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

Eric Regensburger, MS Water Quality Planning Bureau Montana Department of Environmental Quality

tion can be complex, require significant data collection, or use average values that do not account for site-specific conditions. Quanti- fying the contributions of OWTS nutrients to groundwater and surface water is important for many facets of environmental and human health protection. To facilitate the implementation of state water quality standards, a quantitative tool to estimate nutrient loading from OWTS to surface waters was developed. The Method for Estimating Attenuation of Nutrients from Septic Systems (MEANSS) was created within the Montana Department of Environmental Quality primarily for use in OWTS permit- ting, nutrient trading, and for total maximum daily load development. MEANSS provides an option to estimate site-specific nutrient fate when more complex methods are not technically or logistically feasible.

b=>r-/> Estimating the attenuation of nutrients (e.g., nitro- gen, phosphorus) in groundwater from onsite wastewater treatment sys- tems (OWTS) is dicult 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 a‚ect nutrient attenuation but are necessarily complex and dicult 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 De- partment 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 esti- mates of nitrogen and ortho-phosphorus attenuation below and downgradi- ent of OWTS when evaluated against several field studies, a GIS-based nitro- gen loading model, a mechanistic model, and a watershed model. Keywords: wastewater, nitrogen attenuation, phosphorus attenuation, septic system, nutrients


Development MEANSS was developed to meet three cri- teria: 1) to be easy to understand and use, 2) to use accessible and existing informa- tion, and 3) to use site-specific information that incorporates factors known to control natural attenuation of nutrients. MEANSS uses steady-state conditions as a simplify- ing condition; it does not account for the lag time needed for the treated wastewater from an OWTS to migrate into the groundwater and eventually into surface water. Another simplifying condition is that all the treated wastewater is assumed to enter the user-spec- ified receiving surface water; thus, there is no factor included for wastewater loads that do not discharge to the surface water due to local groundwater and surface water interac- tions. The user can, however, easily incorpo-

Introduction In 1990, approximately 25% of the U.S. popu- lation used onsite wastewater systems (com- monly known as septic systems) to dispose of household wastewater; 1990 was the last year this information was collected by the U.S. Census Bureau (2021). In Montana, usage was 37.5%. The nutrients discharged from onsite wastewater treatment systems (OWTS) can create both groundwater and

surface water pollution that aˆects both human health and aquatic life (Dubrovsky et al., 2010). Determining the amount of these nutrients (nitrogen and phosphorus) dis- charged from OWTS that eventually impact groundwater and surface water is diŒcult for many reasons, but primarily the diŒculty is due to the complex site-specific reactions that control natural attenuation of both nutrients. Existing methods to estimate that attenua-


Volume 86 • Number 8

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