ADVANCEMENT OF THE SCIENCE
single-family homes that existed at the time of the study: 558 homes. The analysis was run without using the second nitrate scoring category (soil type within 30.5 m of surface water) because the groundwater data was primarily from wells not within the 30.5-m riparian buer. Site 2 (Missoula, Montana) A study to estimate nitrate loading to the Bitterroot River from OWTS in and around the city of Missoula (Miller, 1996) was used for the second site. The study estimated the groundwater flux based on a groundwater model as 144,432 m 2 /day (Miller, 1991). Groundwater concentrations in the study were based on 8 groundwater wells and 11 groundwater seep samples collected in August 1995. Only the groundwater seeps were used in this analysis because they were closer to the river and therefore provided a better estimate of the nitrate concentrations near the river. The average nitrate + nitrite-N concentration of the 11 seeps was 1.04 mg/L (for the purpose of this analysis, the nitrate + nitrite concentration is assumed to consist entirely of nitrate). After accounting for the natural background concentration of nitrate in the groundwater (estimated as 0.1 mg/L nitrate-N), the total calculated OWTS-related nitrate groundwater load near the river was 49,551 kg/year. The MEANSS analysis used the 2008 data- base provided by the Missoula Valley Water Quality District to extrapolate the number of single-family OWTS that were contributing treated wastewater to the river in 1995: 4,315 homes. The analysis was run without using the second nitrate scoring category in Table 1 (soil type within 30.5 m of surface water) because the groundwater data was primar- ily from groundwater seeps not within the 30.5-m riparian buer. Sites 3 and 4 (Jordan Acres and Village Green Developments, Wisconsin) Two separate subdivisions in Wisconsin were monitored for nitrate impacts to ground- water (Shaw et al., 1993). Several multiport groundwater wells were used to measure the three-dimensional extent of nitrate impact on groundwater from selected portions of the subdivisions. The study used phosphorus and fluorescence in the multiport monitoring wells to separate the groundwater being impacted
FIGURE 2
Prickly Pear Watershed Used for Soil and Water Assessment Tool (SWAT) Model
Note. OWTS = onsite wastewater treatment system; USGS = U.S. Geological Survey.
from upgradient sources (i.e., deeper water) versus groundwater impacted by the subdivi- sions. The study calculated low, medium, and high groundwater flow rates beneath the sub- divisions to determine loading rates. Using the medium flow rates in the study, the calculated nitrate groundwater load from the Jordan Acres (26 homes) and Village Green (45 homes) OWTS was 240 and 616 kg/year, respectively.
MEANSS was run without using the sec- ond nitrate scoring category in Table 1 (soil type within 30.5 m of surface water) because the groundwater data was not collected within the 30.5-m riparian buer. ArcNLET (Site #2, Missoula, Montana) An analysis with the ArcGIS-Based Nitrate Load Estimation Toolkit (ArcNLET) was
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Volume 86 • Number 8
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