Evaluation of U.S. Geological Survey Monitoring-well network and potential effects of changes in water use, Newlands Project, Churchill County, Nevada

Abstract

Domestic wells tapping shallow ground water are an important source of potable water for rural residents of Lahontan Valley. For this reason, the public has expressed concern over the acquisition of water rights directed by Public Law 101-618. The acquisition has resulted in removal of land from irrigation, which could cause shallow domestic wells to go dry and adversely affect shallow ground-water quality. Periodic water-level measurements and water-quality sampling at a monitoring-well network developed by the U.S. Geological Survey (USGS) provided data to evaluate the potential effects of changes in water use. The USGS, in cooperation with Churchill County, analyzed these data and the monitoring-well network to determine if the network provides an adequate means to measure the response of the shallow aquifer to changes in water use, and to determine if measurable changes have taken place. To evaluate the USGS monitoring-well network, wells were characterized by their distance from active canals or ditches, and from currently (2003) or formerly irrigated land. An analysis of historical data showed that about 9,800 acres of land have been removed from irrigation, generally from the late 1990's to 2003. Twenty-five wells in the network are within about 1 mile of fields removed from irrigation. Of the 25 wells, 13 are within 300 feet of canals or ditches where seepage maintains stable water levels. The 13 wells likely are not useful for detecting changes caused by reductions in irrigation. The remaining 12 wells range from about 400 to 3,800 feet from the nearest canal and are useful for detecting continued changes from current reductions in irrigation. The evaluation showed that of the 75 wells in the network, only 8 wells are likely to be useful for detecting the effects of future (after 2003) reductions in irrigation. Water levels at most of the monitoring wells near irrigated land have declined from 1998 to 2003 because of drought conditions and below normal releases from Lahontan Reservoir. This period coincides with the period of irrigation reductions, tending to mask declines directly caused by the reductions. It is likely that seepage from the diffuse network of canals and ditches in Lahontan Valley also masks declines caused by reductions in irrigation. In addition, the limited number of monitoring wells near land removed from irrigation, yet more than 300 feet from an active canal, does not allow a valid statistical correlation between reductions in irrigation and water-level declines. Water-level declines between the last two periods of below normal releases from Lahontan Reservoir, 1992-95 and 2000-2003, ranged from 0.4 to 4.2 feet at 11 monitoring wells near land removed from irrigation. The maximum observed water declines were about 2 to 4 feet in three wells in the southern part of Lahontan Valley. The three wells are near or surrounded by more than 1,000 acres removed from irrigation, are now more than 3,600 feet from continued irrigation, and are within 300 feet of a canal with greatly decreased use. Water levels generally rose in monitoring wells near Stillwater, Nevada, even though large amounts of nearby land were removed from irrigation. This was likely caused by conditions in 2003 that were not as dry as those in the early 1990's and additional seepage from the increased use and stage of canals for delivery of water to wetland areas. Five wells have been sampled since the late 1990's and two wells have been sampled since 2000 to evaluate long-term changes in water quality. Specific conductance of water sampled from these wells was used to evaluate changes in water quality. One well shows a large decline in specific conductance that may be related to changes in water use. In three other wells that showed a decrease in specific conductance it is uncertain if the decrease was related to changes in water use because samples were not collected shortly before and after the time land was removed