Abstract

Sanpete Valley, Utah, is experiencing increases in residential development on unconsolidated deposits of the valley-fill aquifer, the principal source of drinking water in the valley. In response to state and local governments’ desire to protect the high quality of this drinking water source, we provided three tools for land-use planning: a ground-water quality classification map, a ground-water recharge-area map, and a septic-tank density map.

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. Based on data from 408 wells analyzed for general chemistry (118 for total dissolved solids [TDS] and 290 for converted specific-conductance data), 84.9% of the valley-fill aquifer is Class IA (Pristine), 14.9% is Class II (Drinking Water Quality), and 0.2% is Class III (Limited Use). 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 the vertical ground-water gradient. The mountains surrounding Sanpete Valley and the upper parts of alluvial fans along the valley margins are the primary recharge areas. Most of the discharge areas are in the central part of the valley. Secondary recharge areas are present mostly east of the San Pitch River, between the primary recharge areas and discharge areas.

Septic tank soil-absorption systems are the primary means of wastewater disposal in many areas of Sanpete Valley. Based on ground-water flow, background nitrate levels, and projected nitrate loading from new septic tanks, we performed mass-balance calculations to map recommended septic-system density/lot size. We used nitrate data from 340 wells as a ground-water quality indicator. The background nitrate concentration (nitrate as nitrogen) in the principal valley-fill aquifer is 3.3 mg/L. We used a regional, three-dimensional, steady-state MODFLOW model to estimate ground-water flow available for mixing in the principal valley-fill aquifer. Ground-water flow available for mixing is the major control on projected aquifer nitrate concentration in the mass-balance approach. Areas having lower ground-water flow rates require larger lot sizes. Our ground-water flow analysis using a mass-balance approach indicates that low and high flow rates in different parts of the valley necessitate two categories of recommended maximum septic-system densities for developments that use septic tank soil-absorption systems for wastewater disposal: 5 acres per system and 10 acres per system (2 and 4 hm²/system). We base our recommended maximum septic-system densities on hydrogeologic parameters incorporated in a ground-water flow simulation that geographically divided the study area into three ground-water flow domains on the basis of flow-volume similarities.