LAND-USE PLANNING TOOLS FOR PROTECTING GROUND-WATER QUALITY IN UTAH

Many rural areas in Utah are experiencing increases in residential development on unconsolidated deposits of valley-fill aquifers. In response to local governments' desire to protect the high quality of these drinking water sources, the Utah Geological Survey provides three land-use planning tools: ground-water quality classification maps, ground-water recharge-area maps, and septic-tank density maps.

Ground-water quality classification maps are tools local governments can use for managing potential ground-water contamination sources and protecting ground-water quality. Classifying the water quality of aquifers is a way to formally identify and document the beneficial use of ground-water resources. Utah's ground-water quality classes are based mostly on total-dissolved-solids (TDS) concentrations as follows: Class IA (Pristine), <500 mg/L; Class II (Drinking Water Quality), 500 to <3,000 mg/L; Class III (Limited Use), 3,000 to <10,000 mg/L; and Class IV (Saline), 10,000 mg/L and greater. Water having lower TDS values deserves higher protection levels.

Ground-water recharge-area maps identify the relative vulnerability of ground water to surface sources of pollution by identifying (using well logs) the presence or absence of clay confining layers overlying aquifers, and the direction of vertical ground-water gradient. Discharge areas contain confining layers >20' thick and have an upward gradient. Secondary recharge areas contain confining layers >20' thick, and have a downward component of ground-water movement, providing some protection to ground-water resources. Primary recharge areas do not contain thick clay layers and also have downward gradients; these are most vulnerable to land-surface activities.

Septic tank soil-absorption systems are the primary means of wastewater disposal in many rural Utah valleys. Based on ground-water flow, background nitrate levels, and projected nitrate loading from new septic tanks, we perform mass-balance evaluations at various scales to map recommended septic-system density/lot size. Areas having lower ground-water flow rates require larger lot sizes.

These land-use planning tools provide local governments a basis for enacting defendable land-use regulations in areas where population growth has potential to affect Utah's water quality.