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Geology, hydrogeology, and groundwater quality of Cedar Valley, Iron County, southwestern Utah: Abstract
Cedar Valley is a N-NE-trending topographic depression on the southeastern margin of the Basin and Range Province. From 1980 to 1998, the valley’s population increased by 75% and water use by public-supply systems increased by 110%. Continued rapid growth and development are creating potential water-supply and water-quality problems.
Cedar Valley’s principal aquifer is in Tertiary sedimentary basin-fill deposits, composed of interbedded sand, gravel, silt, and clay deposited in stream, alluvial-fan, and lacustrine environments. Most recharge is from infiltration of Coal Creek, which drains the Markagunt Plateau east of Cedar Valley, into alluvial-fan deposits near Cedar City. The drainage basin is closed to surface outflow except during extreme precipitation events, but minor underflow occurs through topographic gaps along the valley’s NW and S margins. Interpretation of seismic-reflection data collected by Mobil EPS, Inc. reveals that the Tertiary basin fill is up to 3,800 feet thick, contains three unconformity-bounded units, and has a complicated subsurface structure including two major sub-basins and several smaller intrabasin highs and lows.
Transmissivity of the basin-fill aquifer is greatest in alluvial-fan deposits along the SE and SW valley margins, and decreases toward the valley center as sedimentary deposits become progressively finer grained. Cedar Valley formerly contained flowing wells in its center and springs along its eastern margin, but pumping has lowered the Potentiometric surface below the land surface and dried the springs since 1975. Bedrock units are of secondary importance for water supply, but are hydrologically connected to the basin-fill aquifer and may locally accommodate underflow across the basin-bounding fault system into the basin-fill aquifer.
Ground-water quality in the Cedar Valley basin fill is generally good, with total-dissolved-solids concentrations of 150 to 3,750 mg/L, but nitrate concentrations range from 0 to 59 mg/L. Most of the wells yielding high-nitrate ground water are near Enoch; nitrate sources likely include septic-tank systems, fertilizer, and nitrogen-bearing strata in the Cretaceous Straight Cliffs Formation. Evidence for the Straight Cliffs Formation as a possible nitrate source includes: (1) negligible changes in nitrate concentrations both historically and seasonally, despite implementation of a sanitary sewer system in the Enoch area in 1995, (2) high nitrate concentrations in ground water tapped by both deep and shallow wells, (3) high nitrate concentration in water from a well on the Fiddlers Canyon alluvial fan, upgradient from any septic-tank systems, and (4) high nitrate concentrations in some organic layers in the formation.
We applied a mass-balance equation to three areas in Cedar Valley, using site-specific ground-water flow available for mixing and site-specific ground-water-quality data, to estimate recommended septic-system density/lot size. Allowing for degradation of 1 mg/L with respect to nitrate, the mass balance approach yields average minimum lot sizes of 5.6 acres per system near Hamiltons Fort, a primary recharge area for the basin-fill aquifer, and 54 acres/system near Bauers Knoll, a secondary recharge area. The mass-balance approach is not the best land-use management tool for Mid Valley Estates, a possible ground-water discharge area, where the amount of water from septic-tank effluent is three times greater than the ground-water flow available for mixing. A public sewer system is a better alternative for domestic wastewater disposal in most areas in arid Cedar Valley, especially the Mid Valley Estates area.
Copyright © 2009 by AAPG Pacific Section and Utah Geological Society