ADVANCEMENT OF THE SCIENCE

water elevations and velocities are higher than during other months of the year when groundwater elevations recede. The annual loads estimated using MEANSS were pro- portionally divided on a monthly basis (the sum of the monthly loads remained equal to the MEANSS annual load) to match the monthly variation of streamflow at the USGS streamflow gage. Adjusting the MEANSS results for seasonal variation produced bet- ter calibration results. Results The validation results of each site or method compared with MEANSS are summarized in Table 3. The absolute average percent dier- ence between MEANSS estimated loads and the loads estimated via other methods (not including STUMOD results) in Table 3 is 19%. Site 1 (Lolo, Montana) MEANSS was used to calculate a nitrate reduction of 41.5%, which provides a total nitrate load in groundwater of 5,312 kg/year from the 558 homes. The MEANSS load is 76% of the load estimated by Boer (2002), which was 7,008 kg/year. Site 2 (Missoula, Montana) MEANSS was used to calculate a nitrate reduc- tion of 43.7%, which provides a groundwater nitrate load of 37,767 kg/year from the 4,315 homes. The MEANSS load is 76% of the amount estimated by Miller (1996), which was 49,551 kg/year. Sites 3 and 4 (Jordan Acres and Village Green Developments, Wisconsin) MEANSS was used to estimate a 19.6% nitrate reduction at Jordan Acres (Site 3), which is equal to a groundwater load of 289 kg/ year. The MEANSS estimated load is 122% of the measured load, which was 237 kg/ year. MEANSS was used to estimate a nitrate reduction of 18.4% at Village Green (Site 4), which is equal to a groundwater load of 508 kg/year. The MEANSS estimated load is 80% of the measured load, which was 637 kg/year. ArcNLET (Site 2) The ArcNLET program estimated the nitrate load into the Bitterroot River as 19,023 kg/ year. The MEANSS estimated load is 17,956 kg/year, which is 94% of the ArcNLET cal- culated load. The MEANSS estimated load

FIGURE 3

Comparison of the Soil Treatment Unit Model (STUMOD) and the Method for Estimating Attenuation of Nutrients From Septic Systems (MEANSS) Nitrogen Reduction in the Vadose Zone Below the Drainfield

0 10 20 30 4 0 5 0 6 0 7 0 8 0 9 0 100

B

C

A

C

B

B

C

D

C

C

D

D

B

A

A

B

C

n

A

C

C

n

C

C

C

C

n

n

n

S oil D es crip tion and E s timated H ydrologic S oil G roup

STU MOD (f rigid/cryic @ 120 cm )

ME A NSS

FIGURE 4

Soil and Water Assessment Tool (SWAT) Model Calibration Results for Total Nitrogen (TN) and Ortho-Phosphate (Ortho-P) Using the Method for Estimating Attenuation of Nutrients From Septic Systems (MEANSS) to Estimate TN and Ortho-P Loading From Onsite Wastewater Treatment Systems to Prickly Pear Creek

1 x 10 3

75th Percentile Maximum Median 25th Percentile Minimum

1 x 10 2

1 x 10 1

1 x 10 0

1 x 10 -1

1 x 10 -2

1 x 10 -3

Note. Based on 23 dates of observed instream concentrations and 19 dates of observed instream loads in Prickly Pear Creek.

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Volume 86 • Number 8

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