THE HYDROGEOLOGY OF MOAB-SPANISH VALLEY, SOUTHEASTERN UTAH, WITH EMPHASIS ON MAPS FOR WATER-RESOURCE MANAGEMENT AND LAND-USE PLANNING

Moab-Spanish Valley is experiencing an increase in development, much of which uses septic-tank systems. To provide water-resource management tools for the two main aquifers in the area (the Glen Canyon and valley-fill aquifers) we (1) characterize the relationship of geology to ground-water conditions, (2) document the ground water quality (east of the valley only for the Glen Canyon aquifer), and (3) use a mass balance approach to determine the potential impact of projected increased numbers of septic tank systems on water quality in the valley-fill aquifer and thereby recommend appropriate septic-system densities.

Most public water supply is from the Glen Canyon aquifer. The Glen Canyon aquifer generally yields ground water with total-dissolved-solids (TDS) concentrations less than 500 mg/L: 83 percent of the samples collected from 24 water wells completed in the Glen Canyon aquifer had TDS concentrations less than 250 mg/L.

The valley-fill aquifer is up to 400 feet thick and is mainly used for domestic and agricultural purposes. Based on ground-water-quality data from 72 wells completed in the valley-fill aquifer, TDS concentrations range from 140 to 1818 mg/L, and average 690 mg/L. Most of these wells yielded ground water with TDS concentrations greater than 500 mg/L, but ground water with TDS concentrations less than 500 mg/L occurs along the northeastern margin of the valley due to recharge from the Glen Canyon aquifer.

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 nitrogen mass and then diluted with ground water flow available for mixing plus the water added by the septic tank systems themselves. Ground water available for mixing was calculated using a regional, digital, three-dimensional ground-water flow model. Our modeling indicates that two categories of recommended maximum septic-system densities are appropriate for development using septic-tank systems for wastewater disposal: 10 and 20 acres per system.