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Ground Water - In Virgin Valley ground water is pumped for irrigation, livestock and domestic use. Ground water samples from wells reflect the presence of geologic formations containing soluble and moderately soluble minerals, such as halite (sodium chloride) and gypsum (calcium sulfate). Sodium and calcium are the principle positive ions found in the ground water while sulfate and bicarbonate are the predominant negative ions. Water Use - About seven percent of the Virgin River's flow is consumed by irrigated agriculture in the reach of the river from Littlefield to Riverside, Nevada. About five percent of the water is consumed by phreato- phytes and evaporation. About 88 percent of the water flows through the Virgin Valley program area. Depletions by phreatophyes and evaporation are almost completely replaced by inflow from small intermittent streams. The reuse of water in onfarm systems is minor. The effect of water reuse on irrigation efficiency is small. The principal reuse is of return flow from the Mesquite and Bunkerville areas which is rediverted for irrigation at the Riverside area. There are about 7,400 acres of phreatophytes in the Virgin River floodplain in Nevada, consisting mostly of saltcedar, some salt- grass, saltbush, cottonwood, mesquite, willow and tules. Depth to water ranges from 0 to 20 feet; but is mostly from one to five feet and varying seasonally. Areal plant density is generally from 25 to 100 percent. An estimated 10,000 acre-feet of water is used in the program area primarily by 2,750 acres which are dominated by trees, shrubs and persistent emergents. An additional evaporative loss of 2,000 acre-feet occurs on open water (channel) wetland areas. Water Budget - Water supply and depletion estimates for Virgin Valley are summarized in Table 4, Water and Salt Budgets. Surface inflow and out- flow have been measured at stream gages. The annual crop consumptive use was determined from onfarm irrigation analyses. Surface and subsurface in- flow between stream gages was estimated from a monthly analysis of concurrent gage data. This estimate compares reasonably with the value used by the State of Nevada, Water Resources-Reconnaissance Series, Report No. 51, (see Bibliography). Water Quality - Virgin River surface water salinity varies considerably with the flow volume. During low flows in the summer and early fall months the water is of poor quality due to high total dissolved solids (TDS). For example, concentrations of salt at Littlefield gage ranged from 2,610 mg/l on October 20, 1978 (mean daily discharge - 83 cfs) to 460 mg/l (mean daily discharge - 1,570 cfs) on May 20, 1979. Concentrations of salt at the River- side gage ranged from 3,770 mg/1 on October 10, 1978 (mean daily discharge _ 10 cfs) to 580 mg/l (mean daily discharge - 1,310 cfs) on May 21, 1979. Typical chemical chemical compositions of surface water are shown in Table 5. The variation in quality and flow makes it possible to apply better quality spring time flows to soils to leach salt. ~ The average concentration of minerals in ground water at the proposed USBR barrier site near Lake Mead is about 4,000 mg/l based on laboratory analysis of 56 samples. Table 6 contains the results of eight analyses that are representative of ground water at the barrier site. Table 7 gives typical compositions of local well water samples. Water quality measuring station locations are shown on Figure 6. OUG~67 10