SEPTIC-TANK DENSITY ANALYSIS USING A MASS-BALANCE APPROACH FOR THE PRINCIPAL AQUIFER IN CACHE VALLEY, NORTHERN UTAH

We applied a ground-water flow model using a mass-balance approach to determine the potential impact of projected increased septic-tank systems on water quality in the Cache Valley basin-fill aquifer, and thereby recommended appropriate septic-system density requirements to limit water-quality degradation. As we have done for several other basin-fill aquifers in Utah, we used nitrate in ground water as an indicator for evaluating the dilution of constituents in wastewater that potentially reaches the aquifer. We estimated ground-water flow available for mixing (dilution) in each of the model's cells (representing a specific land-surface area ranging from 0.2 to 1 square mile) for the model's uppermost layer. We then grouped cells into 11 ground-water flow domains (geographic areas having similar characteristics of flow-volume per unit area), and defined a 12th domain in an area of Cache Valley that was not included in the ground-water flow model. For the unmodeled area, we calculated the amount of ground-water flow available for mixing based on the volume of basin-fill deposits and specific yield of sediments of the type present in these basin-fill deposits. Using an allowable degradation of ground water of 1 mg/L with respect to nitrate (the amount of water-quality degradation deemed acceptable by local government officials), we derived septic-tank density recommendations for each domain on the basis of flow volumes, background nitrate concentration (from 0.09 to 6.58 mg/L), and number of existing septic-tank systems. The results of our modeling indicate that three categories of recommended maximum septic-system densities are appropriate for development using septic tank soil-absorption systems for wastewater disposal: 3, 5, and 10 acres per septic system. This study provides land-use planners with a tool to use in approving development in a manner that will protect ground-water quality.