Groundwater Resources of the Virgin River Basin in Utah

The Division of Water Resources is conducting a study of further water development in the Virgin River Basin. This report examines the effects of groundwater development as a part of the overall study. The study area includes about 1000 square miles in the Central Virgin River Basin east of the Hurricane Fault. The deeply incised Virgin River has cut a youthful drainage network with deep canyons and steep escarpments and drains out of southwest Utah across Arizona to Lake Mead in Nevada. A basin-wide geologic map emphasizing groundwater features has been prepared from available reports. Outcrops of the principal sedimentary formations are shown as well as alluvial depositis, lawa flows, and other features. Groundwater in usable quantities occurs in both consilidated and unconsolidated rock formations. The pincipal consolidated aquifer is the Navajo Sandstone. Its water quality is usually excellent and large recoverable reserves are present. in recent years many wells in the Navajo Sandstone aquifer have been developed for public water supply. Groundwater resources in unconsolidated alluvial aquifers also are extensive, but in some areas may be of poor quality. Wells in alluvial aquifers have long been a major source of water especially for irrigation. Springs are natural indicators of groundwater availability while wells show where groundwater resources have been artificially developed. Maps show the locations of the 800 springs in the basin and the 700 wells. Other maps show the use (public water supply, irrigation, and other) and the growth of each kind of use. Recorded water level data are reported in two ways. Hydrographs of 29 wells have been plotted to show changes over time and groundwater contour map of parts of the basin has been prepared to show the general pattern of groundwater movement. The annual withdrawal by wells for various uses, the total annual groundwater recharge, and groundwater discharge are summarized. Groundwater quality from typical wells and srpings in the Virgin River Basin is presented by table and map. The opportunities for additional groundwater development occur mainly by the following means: Reduce evapotranspiration of groundwater by lowering water tables, reduce seepage to gaining streams and reduce flows to springs and seeps by lowering water tables (this may require transfer of surface water rights and spring rights to the groundwater), and artificially increase the recharge to the groundwater from melting snow and rain. Principal recommendations are: *Selectively increase monitoring of groundwater levels, withdrawals, and water quality from wells. *Extend groundwater contour mpas to cover more of the basin. *Study future aquifer yields under proposed development levels and aquifer management options. *Continue the cautious development of the Navajo Sandstone aquifer espeically for public water supply uses. *Initiate programs to protect the recharge (outcrop) zones of the Navajo Sandstone aquifer from pollution. *Encourage groundwater use from alluvial aquifers for irrigation particularly where the water quality is marginal for other uses. *Where groundwater is the only economic supply available and amounts are small, careful development for domestic use should continue. *Water development plans should consider both surface and groundwater supplies and pick the best mix