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

Ground water is the main source of drinking water in Moab-Spanish Valley. Ground-water quality classification is a tool that can be used in Utah to manage potential ground-water contamination sources and protect the quality of ground-water resources. 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. Results of ground-water quality classification for Moab-Spanish Valley indicate the valley-fill aquifer contains mostly high-quality ground-water resources that warrant protection. Ground-water samples from five wells representing the aquifer in the area have TDS less than 500 mg/L (Class IA; 11% of wells) and 44 wells have TDS ranging from 564 to 1,820 mg/L (Class II; 89% of wells), based on chemical analyses of water from 49 wells and one surface-water source sampled between 1968 and 2002 (TDS range of 140 to 1,820 mg/L; NO3 of 0.6 to 7.2 mg/L).

Septic tank soil-absorption systems are one type of wastewater disposal in Moab-Spanish Valley, and their use will likely increase with population growth. Dilution is the principal mechanism for lowering concentrations of many constituents present in wastewater effluent. We used nitrate in ground water as an indicator for evaluating the dilution of constituents in wastewater that potentially 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 recommended maximum septic-system densities based on: hydrogeologic parameters incorporated into a ground-water flow simulation and the resultant delineation of three geographic areas on the basis of flow-volume similarities, background nitrate concentration per domain (from 0.78 to 3.5 mg/L), and number of septic systems. The mass-balance results indicate three unique acre-per-system categories of recommended maximum septic-system densities that are appropriate for new development in Moab-Spanish Valley for each domain. This study provides land-use planners with a tool to use in approving development in a manner that will be protective of ground-water quality.