instances, users will have to decide how best to estimate this value. The MEANSS spreadsheet for nitrate attenuation is presented in Table 1 and the spreadsheet for phosphorus attenuation is presented in Table 2. Model Performance The accuracy of MEANSS was evaluated by comparing it to: a) four site-specific ground- water OWTS nitrate studies, b) two OWTS nitrate attenuation models, and c) the Soil and Water Assessment Tool (SWAT) water- shed model. A lack of adequate existing phos- phorus studies that were of sucient quality to validate MEANSS limited the evaluation of MEANSS phosphorus attenuation estimates to the SWAT watershed model. For evaluating MEANSS performance, the estimated nitrate and ortho-phosphorus (ortho-P) loads discharged from a single-fam- ily OWTS were assumed to be 13.8 and 2.92 kg/year, respectively. These loads are based on averages of published treated wastewater char- acteristics (Montana Department of Environ- mental Quality, 2015; U.S. EPA, 2002). Validation Several locations around the U.S. were identi- fied as locations where the MEANSS model could be validated. Each is detailed in this section. For each, the percent dierence between the OWTS load and the load esti- mated by MEANSS was calculated as: (OWTS LOAD MEANSS – OWTS LOAD CASE STUDY )/ (OWTS LOAD CASE STUDY) ) × 100 Site 1 (Lolo, Montana) The study site (Boer, 2002) is a low-density residential area near Lolo, Montana. It cov- ers 6.5 km 2 and contains >500 single-family OWTS. The study used site-specific data for the hydraulic conductivity, hydraulic gradi- ent, and groundwater nitrate concentrations to estimate the amount of OWTS-related nitrate migrating from the study area. After accounting for the natural background con- centration of nitrate in the groundwater (esti- mated as 0.1 mg/L nitrate-N), the total calcu- lated OWTS-related nitrate groundwater load was 6,103 kg/year. The MEANSS analysis used a 2008 data- base provided by the Missoula Valley Water Quality District to extrapolate the number of
TABLE 2
Phosphorus Attenuation Factors for Onsite Wastewater Treatment System Discharges to Soil
% Phosphorus Reduction *
Scoring Category 1
Scoring Category 2
Hydrologic Soil Group at
Hydrologic Soil Group at Drainfield (CaCO 3 ≥ 15%)
Distance to Surface Water (m)
Hydrologic Soil Group at Drainfield (CaCO 3 ≤ 1%)
Drainfield (CaCO 3 > 1% and < 15%)
10 20 40 50 60 80
A
A
A B C
0–30.5
B
30.6–152.5
B C D
C D
D
152.6–1,525
100
≥1,526
*The total phosphorus reduction is the sum of the individual reductions for Category 1 + Category 2. For example, a drainfield in a hydrologic group C soil with >15% CaCO 3 (40% reduction) and is 100 m from the surface water (50% reduction) would reduce its phosphorus load to the surface water by 90% from the load discharged from the drainfield (Figure 1). Note. CaCO 3 = calcium carbonate.
lowing text) are inputted into spreadsheets in the MEANSS model. The distance to the nearest receiving sur- face water is often the most uncertain param- eter required in MEANSS. The NHD informa- tion can provide the locations of ephemeral, intermittent, and perennial streams—but without a detailed groundwater flow map, it can be dicult to determine the direction of groundwater movement and where shal- low groundwater will intersect surface water. When site-specific data are not available, one option is to assume the shortest distance between the OWTS and the nearest downgra- dient perennial surface water (as classified in the NHD) for the distance value (Figure 1). In MEANSS, the HSG used is based on the SSURGO classification of the predominant soil type beneath the drainfield and within 30.5 m of the receiving surface water. The 30.5-m stream buer is used as the default width to determine predominant soil types in the riparian area (Figure 1). The soil CaCO 3 content used in MEANSS is also based on values in the SSURGO data- base. In some areas, however, the CaCO 3 content is not available in SSURGO. In these
conditions are not used in the estimation of phosphorus attenuation as they are for nitrate. The review of the existing literature pro- vided three factors that are used in MEANSS to estimate phosphorus attenuation: 1) pre- dominant soil HSG in the drainfield area, 2) predominant soil CaCO 3 content in the drainfield area, and 3) distance between the drainfield and the receiving surface water. The phosphorus reduction values applied to these characteristics are presented in the Parameters subsection within the Methods section. Parameters The parameters used in MEANSS are avail- able via the following sources: GIS mapping for distance values, the National Hydrogra- phy Dataset (NHD or NHDPlus) from the U.S. Geological Survey (USGS) to determine appropriate receiving surface waters, and the Soil Survey Geographic Database (SSURGO) from NRCS to determine the HSG and soil CaCO 3 content. GIS analysis tools can be used to determine distance to surface water, soil characteristics at the drainfield, and soil HSG in the 30.5-m riparian buer (Figure 1). These data and others (described in the fol-
11
April 2024 • Journal of Environmental Health
Powered by FlippingBook