Hydrogeology and Simulation of Groundwater Flow in Cedar Valley, Utah County, Utah, USA

Development of new domestic water supplies driven by rapid population growth in and around Cedar Valley, Utah Coun- ty, has prompted the need for a better understanding of the groundwater system in this north-central Utah valley. Water- level, aquifer-test, and water-quality data from this study, along with previously collected data, significantly improve our conceptual model of the Cedar Valley groundwater flow system and provide the framework for a two-layer, three-di- mensional digital numerical model of the groundwater flow system. Cedar Valley is an arid valley typical of the Basin and Range. We delineate four hydrogeologic units in the Cedar Valley study area, which includes Cedar Valley and parts of the sur- rounding mountain ranges and valleys: (1) an upper clay unit that covers the majority of the valley floor and confines the underlying aquifer, (2) a principal basin-fill aquifer, confined by the clay unit, contained generally in the upper 300 to 400 feet (90–120 m) of the interbedded basin-fill sediments un- derlying the clay unit, (3) fractured Paleozoic bedrock units combined into one bedrock aquifer, and (4) a small perched basin-fill aquifer at Cedar Pass. Groundwater quality is generally good under much of the val- ley except in the southeast and in some wells in Mississippi- an-age bedrock. Aquifer lithology influences major ion water chemistry; wells in the principal basin-fill and Oquirrh Group bedrock (primarily sandstone and limestone) aquifers gener- ally have the least dissolved solutes and a calcium-bicarbon- ate type water. Wells near Fairfield have the same recharge source as Fairfield Spring, as shown by their similar water chemistry, despite separation by more than 100 feet (30 m) of clay. From general chemistry, tritium, stable isotope, and carbon-14 water-chemistry data, we conclude that springs and wells on the western margin of the valley contain water that recharged the aquifer in the last 50 years. Wells in the north-central part of the valley contain the oldest water, be- lieved to be a consequence of a long, deep flow path from the surrounding mountain upland. Many wells in the Cedar Pass area, regardless of well depth, contain some fraction of modern water, possibly an indicator that a component of modern recharge is reaching the wells in an otherwise old fractured bedrock flow system. The unique calcium-chloride type water and independent groundwater flow direction in the perched aquifer at Cedar Pass implicates the Traverse Mountains as a recharge source for this small, hydraulically separate aquifer. The largest recharge source for the basin-fill aquifer is direct infiltration of precipitation in the Oquirrh Mountains and West Canyon Wash drainage basin, which supplies approxi- mately 70% of the recharge to the Cedar Valley aquifer sys- tem. Other sources of recharge include precipitation on the East Tintic Mountains, Traverse Mountains, and valley floor; seepage from streams, canals, and the Sinks east of Fairfield; seepage of unused irrigation water; and seepage from septic tanks and wastewater treatment plants. We show that interba- sin flow from Rush Valley, west of Cedar Valley, is negligible. Groundwater discharges from Cedar Valley via interbasin flow to northern Utah and Goshen Valleys, well pumping, spring discharge, evapotranspiration, and base flow to West Canyon Creek. The volume of recharge entering the aquifer at its western margin is the most important control over long-term ground- water levels in the basin-fill aquifer. In turn, that potentiomet- ric head controls Fairfield Spring discharge. Groundwater in Cedar Valley generally flows from west to east through the upper few hundred feet of the basin-fill aquifer. A continuous clay unit, which is as much as 240 feet (73 m) thick, confines the basin-fill aquifer over much of the valley. Based on water-level and aquifer-test results, a north- south trending normal fault on the eastern margin of the val- ley is a conduit for fault-parallel groundwater flow and a bar- rier to groundwater flow across the fault. This fault directs groundwater flow around the Lake Mountains through bed- rock at Cedar Pass to the north and Mosida Hills to the south. A wedge of Tertiary volcanic bedrock acts as a dam between water in the northern part of the basin-fill aquifer and the downgradient bedrock aquifer at Cedar Pass. Environmental isotopes demonstrate the existence of both deep groundwater flow paths, in which water may take thousands of years to travel from the mountain tops to the basin fill, and relatively shallow flow paths, in which mountain recharge likely only takes a few tens of years to reach the basin-fill aquifer. We created a two-layer, three-dimensional numerical ground- water flow model to simulate the groundwater flow system for Cedar Valley, focusing primarily on the unconsolidated basin- fill aquifer (layer 1) and its connection with the underlying, consolidated bedrock aquifer (layer 2), which receives the majority of recharge. A steady state model simulated condi- tions in 1969 and a transient model simulates the years 1970 to 2007. We calibrated the groundwater flow model using ob- served water levels at 24 wells and discharge measured at the one large basin-fill spring, Fairfield Spring. Model calibration was best in the central part of the valley, where water-level data were more closely spaced. The simulated transient groundwater model flow budget showed a 39-year (1969–2007) average recharge to the Cedar Valley groundwater system of 25,600 acre-feet per year (acre- ft/yr) (31.6 hm3/yr) and discharge of 25,200 acre-ft/yr (31.1 hm3/yr). Over the 39 years simulated, our groundwater flow model allocated an average of 360 acre-ft/yr (0.44 hm3/yr) to aquifer storage to balance recharge and discharge, a difference of about 1% in the overall water budget. Most recharge (approximately 24,000 acre-ft/yr [29.6 hm3/ yr]) is in the form of precipitation on the Oquirrh and East Tintic Mountains, where the majority, 19,700 acre-ft/yr (24.3 hm3/yr), becomes bedrock recharge to the basin-fill aquifer on the western side of the valley. The remaining 4300 acre-ft/ yr (5.3 hm3/yr) travels under the basin fill within bedrock to discharge to adjacent valleys or as bedrock well discharge. In addition to precipitation recharge through the mountain block, the basin fill receives approximately 1600 acre-ft/yr (2.0 hm3/ yr) of recharge on the valley floor, which is roughly 7% of its recharge. Our model shows discharge to northern Utah and Goshen Val- leys through Cedar Pass and Mosida Hills, respectively, aver- aged 14,900 acre-ft/yr (18.4 hm3/yr) during the modeled pe- riod. About twice as much interbasin flow leaves Cedar Valley via Cedar Pass (10,200 acre-ft/yr [12.6 hm3/yr]) compared to Mosida Hills (4700 acre-ft/yr [5.8 hm3/yr]). Discharges from the groundwater system other than interbasin flow (14,900 acre-ft/yr [18.4 hm3/yr]) are Fairfield Spring (3700 acre-ft/yr [4.6 hm3/yr]), evapotranspiration (3000 acre-ft/yr [3.7 hm3/ yr]), and well pumping (2420 acre-ft/yr [3.00 hm3/yr] from basin-fill wells and 1180 acre-ft/yr [1.46 hm3/yr] from bed- rock wells). Groundwater flows through the basin-fill aquifer, entering as bedrock recharge from the west and discharging to the bed- rock in the east, but not in equal proportions. On average, about 19,700 acre-ft/yr (24.3 hm3/yr) flows into the basin-fill and 12,000 acre-ft/yr (14.8 hm3/yr) flows out, which leaves 7700 acre-ft/yr (9.5 hm3/yr) of net flow into the basin-fill aqui- fer. The net flow meets the discharge requirements of well withdrawal, evapotranspiration, and Fairfield Spring. We modeled a variety of possible scenarios, including drought and increased pumping, 30 years into the future (from 2008 to 2037). If 2007 pumping (approximately 6500 acre-feet [8.0 hm3/yr]) and average climatic conditions persist, the model predicts most areas of the basin-fill aquifer will experience as much as 15 feet (5 m) of drawdown from 2007 levels. In scenarios that include doubling the 2007 well extraction rates, our model predicts large areas of the valley will experience up to 50 feet (15 m) of drawdown.