Agenda - 11-01-1993 - VIII-A - Laserfiche WebLink

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Pollutant loading rates for failing septic systems were developed from a review of septic tank leachate monitoring studies. The mean concentrations of total-P and total-N assumed based upon literature values are as follows: Total-P Total-N Low 1.0 mg/L 7.5 mg/L Medium 2.0 mg/L 15.0 mg/L High 4.0 mg/L 30.0 mg/L Annual "per acre" loading rates for septic tank failures from low density residential land uses were then estimated assuming 50 gpcd wastewater flows. The loading rates were applied to residential land uses with 5-, 2-, and 1-acre lot sizes. The septic tank loading factors were added to the runoff pollution loading factors summarized in Table 3-2. The percent increase in annual per acre loadings attributed to failing septic tanks is: Total-P Total-N Low 130%-180% 120%-150% Medium 160%-250% 140%-200% High 220%-400% 180%-310% Despite the large increase in annual loading rates, septic tank failures have only a limited impact on overall .nonpoint pollution discharges into Lake Michie and Little River Reservoir. This is because the increased annual loading rates were applied only to the fraction of low density development located within the "impact zone" that are predicted to have a failing septic tank system during an average year. Based upon this methodology, failing septic tank systems occur on only about 2.5% of the watersheds. 254.4.3 AVERAGE ANNUAL NONPOINT POLLUTION LOADS Annual nonpoint pollution loadings discharged into the two reservoirs under existing land use conditions were calculated using the loading factors described in Section 3.2. The annual nonpoint pollution loadings for each jurisdiction draining to Lake Michie and Little River Reservoir are summarized in--Table 3-7 and--Table 3-8, respectively for- existing land use • 3-26