GROUND-WATER QUALITY CLASSIFICATION AND SEPTIC-TANK DENSITY ANALYSIS IN CASTLE VALLEY, SOUTHEASTERN UTAH

Ground water is the principal source of drinking water in Castle Valley. Ground-water quality classification is a relatively new tool that can be used in Utah to manage potential ground-water contamination sources and protect the quality of ground-water resources. Results of ground-water quality classification for Castle Valley indicate the valley-fill aquifer contains mostly high-quality ground-water resources that warrant protection. Utah’s ground-water quality classes are based mostly on total-dissolved-solids (TDS) concentrations as follows: Class IA (Pristine), less than 500 mg/L; Class II (Drinking Water Quality), 500 to less than 3,000 mg/L; Class III (Limited Use), 3,000 to less than 10,000 mg/L; and Class IV (Saline), 10,000 mg/L and greater. Water samples from 14 ground-water wells representing the aquifer in the area have TDS less than 500 mg/L (Class IA), and 39 wells have TDS ranging from 501 to 2,442 mg/L (Class II), based on chemical analyses of water from 53 wells and one surface-water source sampled between 1991 and 2001 (TDS range of 204 to 2,442 mg/L).

Septic tank soil-absorption systems are used to dispose of domestic wastewater in Castle Valley. Many constituents in septic-tank effluent are not removed from the waste stream by percolation through subsurface soils; dilution is the principal mechanism for lowering concentrations of these constituents. We used nitrate in ground water as an indicator for evaluating the dilution of constituents in wastewater that reaches the aquifer; the evaluation uses a mass-balance approach based on ground water available for mixing with septic-tank effluent in the aquifer. We also delineated four ground-water flow domains based on modeled flow-volume similarities. Background nitrate concentrations for the domains range from 0.18 to 0.48 mg/L. Based on hydrogeologic parameters incorporated in the ground-water flow model, the mass-balance results indicate that two categories of recommended maximum septic-tank system densities are appropriate for new development in Castle Valley: 5 and 15 acres per system.