GROUND-WATER/SURFACE-WATER RELATIONS AND WATER QUALITY WITHIN A VULCANOKARSTIC TERRAIN, MARKAGUNT PLATEAU, SOUTHWESTERN UTAH

The Markagunt Plateau, in southwestern Utah, is at an altitude of about 2,900 meters (m), largely within Dixie National Forest. The plateau is capped by Tertiary-age volcanic (basalt) rocks that overlie Eocene-age marly limestones of the Claron Formation, which forms the prominent escarpment of Cedar Breaks National Monument. Volcanic pseudokarst (vulcanokarst) characterizes large parts of the Markagunt Plateau, as indicated by extensive sinkhole development in the basalt. Lava tubes occur within the upper part of the basalt and locally have collapsed; however, most of the vulcanokarstic terrain on the plateau likely results from dissolution of the underlying limestone and subsequent collapse of the basalt, producing sinkholes up to 300 m across and 30 m deep. Numerous large springs discharge from the basalt and limestone on the Markagunt Plateau, including Mammoth Spring, one of the largest springs in Utah. Discharge of the spring generally ranges from less than 0.3 m³/s during base flow to more than 3 m³/s at peak flow during the snowmelt runoff period. Mammoth Spring probably discharges from the Claron Formation; however, much of the recharge to the aquifer that supplies the spring likely takes place by both focused and diffuse infiltration through the overlying basalt.

Results of major-ion analyses for samples collected from Mammoth Spring during base-flow conditions when most of the water is presumed to be from aquifer storage, indicate a calcium-bicarbonate type water containing dissolved-solids concentrations of about 110 mg/L. Results of analyses for sulfur-35, tritium, and chlorofluorocarbons in water sampled from the spring indicate that residence times are a mixture of short- (months or less) and long-term (years) components. However, results of a dye-tracer test from 13.7 km southwest of and 450 m higher than the spring indicate a maximum ground-water travel time of 27 days during the snowmelt runoff period. Specific conductance and water temperature also show an inverse relation to discharge during snowmelt runoff and rainfall events. Variations in discharge and turbidity in addition to the presence of total coliform bacteria in the spring water further indicate a significant potential for transport of contaminants from surface sources to the spring in a relatively short timeframe.