tional) treatment (Heatwole & McCray, 2007; Howarth et al., 2002; Montana Department of Environmental Quality, 2015; Morgan & Everett, 2005; Morgan et al., 2007; Toor et al., 2020). As nitrate is the primary form of OWTS nitrogen that impacts groundwater and surface water, MEANSS is designed to estimate the denitrification rate after waste- water has been treated and discharged from the drainfield. While there could be some minor denitrification (approximately 10%) immediately below a properly operating drainfield (Costa et al., 2002; Lowe, 2007; Rosen et al., 2006), denitrification primarily occurs after the treated wastewater migrates away from the drainfield. For denitrification to occur beyond the drainfield, a suitable environment must exist. The necessary factors include: 1) tem- perature near or above 10 ºC; 2) an adequate carbon source that serves as food for the bacteria (available carbon is related to the soil organic matter content); 3) an anoxic environment with an oxygen range of <1–2 mg/L; and 4) the correct bacteria to utilize the oxygen atoms in the nitrate compound. A riparian zone with shallow groundwater is the most common natural environment that has these conditions (Gilliam, 1994; Gold & Sims, 2000; Harden & Spruill, 2008; McDowell et al., 2005; Rosenblatt et al., 2001). Although a literature review did not provide any specific lower limit of carbon concentration below which denitrification does not occur, an adequate carbon source is cited as the most common limiting factor for denitrification (Gold & Sims, 2000; Kobus & Kinzelbach, 1989; Rivett et al., 2008). Due to this finding, MEANSS accounts for the carbon limiting factor using site-specific soil characteristics. As fine-grained soils are more likely to con- tain two of the conditions necessary for deni- trification—anoxic conditions and carbon— they typically provide better denitrification conditions than coarse-grained soils (Briar & Dutton, 2000; Mueller et al., 1995; Tesoriero & Voss, 1997; Umari et al., 1993). Ander- son (1998) used results from several studies to show a Pearson correlation coecient ( r ) of .91 between denitrification rates and soil organic content. Another study by Ricker et al. (1994) used this relationship to estimate the amount of denitrification beneath drain- fields as 15% for sandy soils and 25% for finer
FIGURE 1
Plan View Schematic of the Method for Estimating Attenuation of Nutrients From Septic Systems (MEANSS) Parameters
Stream
30.5 m
Hydrologic Soil Group B
100 m
Hydrologic Soil Group C (>15% CaCO 3 )
Drainfield
Not to scale
Note. CaCO 3 = calcium carbonate.
rate such a factor into the results of MEANSS in areas where information on the groundwa- ter and surface water interaction is available. Also, groundwater dilution eects on concen- trations are not accounted for because total loads reaching surface waters are sucient for most applications. Separate groundwater dilution calculations can be used in conjunc- tion with MEANSS to estimate groundwater concentrations.
temperature climates than Montana, however, the nitrogen attenuation values might need to be adjusted and validated for use in MEANSS. The factors that aect the natural attenuation of nitrogen and phosphorus are described here to provide the basis for MEANSS. Nitrogen Attenuation Factors Nitrogen in raw domestic wastewater that is discharged to a septic tank is in the form of ammonia primarily (Lowe et al., 2010). Dis- posal of untreated wastewater in a properly constructed and sized drainfield typically will provide sucient oxygen and naturally occurring bacteria to convert the ammonia to nitrite and then quickly to nitrate. Conven- tional OWTS are not designed to complete the final step of nitrogen treatment—denitri- fication—which is the conversion of nitrate to nitrogen gas. Studies and water quality regulations com- monly assume that most or all the ammonia in the raw wastewater is converted to nitrate after septic tank and drainfield (conven-
Factors Controlling Nutrient Attenuation
Although many variables control the natu- ral attenuation of nitrogen and phosphorus that are discharged from OWTS, the variables that control attenuation the most—based on a search of the literature—were used in MEANSS (i.e., soil type, soil calcium car- bonate [CaCO 3 ] content, and distance to surface water). MEANSS was developed and validated for cold climates similar to Mon- tana (Supplemental Figure 1, www.neha.org/ jeh-supplementals). In significantly dierent
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April 2024 • Journal of Environmental Health
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