GROUND-WATER RECHARGE-AREA AND WATER-QUALITY-CLASSIFICATION MAPPING FOR CEDAR VALLEY, SOUTHWESTERN UTAH-- TOOLS FOR LAND-USE PLANNING

Cedar Valley, in southwestern Utah, is experiencing an increase in residential development on unconsolidated deposits of the basin-fill aquifer, the valley’s primary source of drinking water. In response to local governments’ desire to protect the high quality of this resource, we developed two land-use planning tools: ground-water recharge-area and water-quality-classification maps.

Ground-water recharge-area maps identify the relative vulnerability of ground-water resources to surface sources of pollution by delineating, using water-well logs, the presence or absence of clay/silt confining layers overlying the aquifer. Although artesian conditions were present in much of Cedar Valley in the early 1900s, the vertical ground-water gradient in the basin-fill aquifer is currently downward. Secondary recharge areas (42% of basin-floor area; primarily in central parts of valley) contain confining layers thicker than 20 feet, providing some protection to ground-water resources. Primary recharge areas (58%; primarily along valley margins) do not contain thick clay layers and are most vulnerable to land-surface activities.

Ground-water-quality classification maps formally identify the beneficial use of the ground-water resource based primarily on total-dissolved-solids concentrations as follows: class 1A, less than 500 mg/L; class 2, 500 to less than 3,000 mg/L; class 3, 3,000 to less than 10,000 mg/L; and class 4, 10,000 mg/L and greater. Areas where individual constituents exceed drinking-water standards are class 3 water. Cedar Valley ground water is class 1A (80%; primarily in central and western parts of valley), class 2 (19%; primarily in eastern part of valley), and class 3 (1%; an area of persistent nitrate contamination north of Cedar City) based on chemical analyses of water from 97 wells sampled during 1974-2000. Total-dissolved-solids concentrations range from 184 to 2,190 mg/L. Land-use planners may now use these maps as a basis for enacting regulations to protect water resources in this valley.