GROUND-WATER MODEL BASED SEPTIC-TANK DENSITY RECOMMENDATIONS USING A MASS-BALANCE APPROACH TO PROTECT GROUND-WATER QUALITY, CEDAR VALLEY, IRON COUNTY, UTAH

Cedar Valley, Iron County, is a semi-rural area in southwestern Utah experiencing an increase in residential development. Whereas much of the development is on community sewer systems, many subdivisions use septic tank soil-absorption systems for wastewater disposal. Many septic-tank systems are on basin-fill deposits that are the principal aquifer for the area. The purpose of our study is to apply a ground-water flow model using a mass‑balance approach to determine the potential impact of projected increased numbers of septic‑tank systems on water quality in the basin-fill aquifer and thereby recommend appropriate septic-system density requirements to limit water-quality degradation.

Ground water in the Cedar Valley basin-fill aquifer is generally a high-quality resource that warrants protection, but has already been impacted in some areas with nitrate contamination. Based on chemical analyses of water from 119 wells sampled during 1974-2000, total-dissolved-solids concentrations range from 184 to 2190 mg/L and average 584 mg/L, while nitrate-as-nitrogen concentrations range from 0.06 to 57.4 mg/L and average 7.6 mg/L.

Nitrogen in the form of nitrate is one of the principal indicators of pollution from septic tank soil‑absorption systems. To provide recommended septic-system densities, we used a mass-balance approach in which the nitrogen mass from projected additional septic tanks is added to the current (background) nitrogen mass and then diluted with ground‑water flow available for mixing plus the water added by septic‑tank systems themselves. Ground water available for mixing was calculated using a regional, three-dimensional, steady-state, ground-water flow model. Results indicate two categories of recommended maximum septic-system densities are appropriate for development using septic tank soil-absorption systems for wastewater disposal: 5 and 15 acres per system. These recommended maximum septic-system densities are based on hydrogeologic parameters incorporated in the ground-water flow simulation, and the modeled area was divided into three ground-water flow domains based on flow-volume similarities; a fourth domain in northern Cedar Valley lacked sufficient data for modeling and was assigned a density of 15 acres per system to be protective of ground-water quality.