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A Mass-Balance Approach for Recommending Septic Tank Soil-Absorption System Density/Lot-Size Requirements Based on Potential Water-Quality Degradation Due to Nitrate; Examples from Three Utah Valleys
Septic tank soil-absorption systems are the primary means of wastewater disposal in many Utah valleys where development is proceeding at a rapid rate and ground water from unconsolidated valley-/basin-fill aquifers is the primary source of drinking water. Land-use planners have long used septic-tank-suitability maps to determine where these systems will likely percolate within an acceptable range. However, they are only now becoming aware that percolation alone does not remediate many constituents found in wastewater. Nitrogen in the form of nitrate is one of the principal indicators of pollution from septic tank soil-absorption systems.
We use a mass-balance approach to determine the potential impact of projected increased numbers of septic-tank systems on water quality in valley-/basin-fill aquifers and recommend appropriate lot-size requirements. In this approach, the nitrogen mass from the projected additional septic tanks is added to the current nitrogen mass and then diluted with the ground-water flow available for mixing plus the water added by the septic-tank systems themselves. Mass-balance analyses for Ogden Valley in Weber County, Tooele Valley in Tooele County, and Cedar Valley in Iron County, indicate the amount of ground-water flow available for mixing is the major control on projected aquifer nitrate concentration. Assuming an allowable degradation in water quality due to nitrate of 1 mg/L, the average lot size in Ogden Valley for development based on septic-tank systems could be as small as 3 acres (0.012 km2), while in Tooele Valley and Cedar Valley average lot size should be no smaller than 53 acres (0.21 km2) and 27 acres (0.11 km2), respectively. Intermontane Ogden Valley has an average ground-water flow of 166 ft3/s (4.7 m3/s), while average ground-water flow in the more arid Tooele Valley and Cedar Valley is 46.96 ft3/s (1.3 m3/s) and 52.5 fr3/s (1.49 m3/s), respectively. While there are many caveats to applying this mass-balance approach, we think it is useful in valley-wide land-use planning. We also provide guidelines for one method of assessing the site-specific impact of septic tank soil-absorption systems on ground-water quality for proposed subdivisions in valley-/basin-fill aquifer settings.
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