Utah Geological Survey
1594 West North Temple
P.O. Box 146100
Salt Lake City, UT 84114-6100
Telephone: (801)537-3391
Fax: (801)537-3400
e-mail: mattbutler@utah.gov, janaewallace@utah.gov, mikelowe@utah.gov
To delineate bedrock versus valley-fill material, we created a digital geologic coverage of rock units and unconsolidated deposits. Then we generated the TDS and the ground-water classification maps. Using GIS contouring methods within the ArcView 3x Spatial Analyst Extension, we created the TDS map using available TDS values (measured in mg/L) from sampled water wells. Total-dissolved-solids concentrations ranged from 184 to 2,190 mg/L. Contours were generated from a point shapefile, using an interval of 500 mg/L. A polygon coverage outlining the TDS class boundaries was then manually created in ArcInfo using the contoured shapefile (converted to a coverage) as a guide; this coverage was subsequently clipped to the bedrock boundary within the valley-fill areas.
A ground-water quality classification map formally identifies the beneficial use of the ground-water resource based primarily on TDS 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. The ground-water quality classification map was compiled from the TDS map, supplemented with a GIS-nitrate concentration coverage using these criteria. We also calculated the land-surface-area percentage of each ground-water class category by building the polygon coverage in ArcInfo and calculating the corresponding areas. On the basis of chemical analyses of water from 94 wells sampled during 1974 to 2000, Cedar Valley ground water classified as follows: class 1A, 80%, primarily in the central and western parts of the valley; class 2, 19%, primarily in the eastern part of the valley; and class 3, 1%, an area of persistent nitrate contamination northwest of Cedar City. Land-use planners can now use these maps as a basis for enacting regulations to protect water resources in this valley.