Science-based land-use planning tools to help protect ground-water quality, Cedar Valley, Iron County, Utah, USA

Cedar Valley, Iron County, is a semi-rural area in southwestern Utah that is experiencing an increase in residential development. Whereas much of the development is on community sewer systems, many subdivisions use septic tank soil-absorption systems for wastewater disposal. Many of these septic-tank systems are on basin-fill deposits that are the principal aquifer for the area. The purpose of our study is to provide tools for water-resource management and land-use planning. In this study we (1) map ground-water recharge and discharge areas for the basinfill aquifer to indicate where ground-water resources are most vulnerable to surface sources of pollution, (2) classify the ground water quality in the basin-fill aquifer to formally identify and document the beneficial use of ground water resources, and (3) apply a ground-water flow model using a mass balance approach to determine the potential impact of projected increased numbers of septic tank systems on water quality in the basin-fill aquifer and thereby recommend appropriate septic-system density requirements to limit water-quality degradation. The Cedar Valley basin-fill aquifer consists primarily of Tertiary to Quaternary alluvial sediments composed of discontinuous, lenticular, commonly elongated, poorly to well-sorted bodies of sand, clay, gravel, and boulders, interbedded with lava flows and some lacustrine, eolian, and volcaniclastic deposits. The basin fill may be as much as 3900 feet (1190 m) thick in the eastern part of Cedar Valley. Ground water in the Cedar Valley basin-fill aquifer occurs under confined, unconfined, and perched conditions. Ground-water recharge areas include the fractured bedrock surrounding the valley, and basin fill along the valley margins, which typically consists of coarse, granular, permeable sediments deposited primarily in alluvial fans, where ground water is generally under unconfined conditions. The basin-fill aquifer is generally under leaky confined conditions in the central, lower elevation areas of the valley where water-yielding coarser grained deposits are overlain by or contain intervening beds of low-permeability silt and clay. Upward ground-water gradients in the central, lower elevation areas of Cedar Valley were once sufficient to supply flowing wells that covered an approximate area of 50 square miles (130 km2 ) in central Cedar Valley in 1939, but due to ground-water pumpage, no flowing wells have existed in Cedar Valley since 1975. This central part of the valley, which contains fine-grained confining layers greater than 20 feet (6 m) thick and where ground water now generally has a downward vertical head gradient, is mapped as a secondary recharge area. Although some wells in the confined part of the basin-fill aquifer have an upward vertical head gradient, they are sporadically distributed and are not mapped as a discrete groundwater discharge area. 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 3000 mg/L; Class III (Limited Use), 3000 to less than 10,000 mg/L; and Class IV (Saline), 10,000 mg/L and greater. Cedar Valley ground water is Class IA (80% of basin; primarily in central and western parts of valley), Class II (19%; primarily in eastern part of valley), and Class III (1%; an area of persistent nitrate contamination northwest of Cedar City) based on chemical analyses of water from 119 wells sampled during 1974–2000. Total-dissolved-solids concentrations range from 184 to 2190 mg/L. Nitrate-as-nitrogen concentrations in the basin-fill aquifer range from 0.06 to 57.4 mg/L, and average 7.59 mg/L. Nitrogen in the form of nitrate is one of the principal indicators of pollution from septic tank soil absorption systems. 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, three dimensional, steady-state, ground-water flow model. Our ground-water flow indicates that two categories of recommended maximum septic-system densities are appropriate for development using septic tank soil-absorption systems for wastewater disposal: 5 and 15 acres per system (2 and 6 hm2 /system). These recommended maximum septic-system densities are based on hydrogeologic parameters incorporated in the ground-water flow simulation and the modeled area was divided into three groundwater flow domains based on flow-volume similarities; a SCIENCE-BASED LAND-USE PLANNING TOOLS TO HELP PROTECT GROUND-WATER QUALITY, CEDAR VALLEY, IRON COUNTY, UTAH 2 Utah Geological Survey fourth domain was assigned to northern Cedar Valley due to insufficient data.