Las Vegas Wash water quality monitoring program: 1996 Report of findings

Las Vegas Wash, a natural wash east of the city of Las Vegas, Nevada, carries stormwater, groundwater drainage, and sewage effluent from three sewage treatment plants to Lake Mead. The Wash provides nearly the only surface water outlet for the entire 2,193 mi2 of Las Vegas Valley. A drainage area of 1,586 mi2 contributes directly to the Wash through surface flow which is channeled to Las Vegas Bay of Lake Mead, while drainage of the remaining 607 mi2 is presumably subsurface and may drain toward Las Vegas Wash. In the 1930\u27s and 1940\u27s, sewage treatment plants were built for the Las Vegas community which ultimately discharged into Las Vegas Wash. In 1942, water was also imported from Lake Mead to process magnesium, and it too was discharged into the Wash. This increasing water availability resulted in the development of a wetland area which extended nearly the entire length of the Wash and became important habitat for waterfowl and other wildlife. Prior to 1928, approximately 1 ft3/s was reported to be the normal summer flow in the Wash. Since then, the Las Vegas metropolitan area has grown considerably, and the average annual discharge now approaches 200 f3/s. Mean monthly discharges ranged from 171 to 214 f3/s entering Lake Mead during Water Year 1996. Over time, this increasing discharge has resulted in additional upstream erosion, downcutting, and channelization in the Wash which has resulted in a falling water table adjacent to the Wash and draining of much of the previously inundated flood plain. Las Vegas Wash now essentially flows in a totally confined channel. The groundwater entering Las Vegas Wash is quite saline and contributes some salt loading to Lake Mead and the Colorado River. In 1974, as part of the Colorado River Basin Salinity Control Act, Title II (Public Law 93-320), a bypass pipeline was authorized for construction to eliminate the interception of saline groundwater in the Wash. An environmental impact statement was filed in 1977 (FES 77-15, Final Environmental Statement: Colorado River Water Quality Improvement Program). The bypass pipeline was not constructed. At present, the Las Vegas Wash unit is not included in an overall Salinity Control Plan for the Colorado River. The present monitoring program in Las Vegas Wash was begun in April 1989 to identify present levels and trends associated with this salinity and the increasing flows, as well as to document the general water quality and the levels of nitrogen and phosphorus compounds entering Lake Mead as they may relate to present or future water quality conditions or standards. Applied Sciences Referral Memorandums No. 90-2-6, 91-2-6,92-2-3,93-2-3 and 94-2-5 and Technical Memorandums No. 8220-95-3 and 8220-96-13 report the results of seven years of recent monitoring. This report presents the results of the 1996 monitoring and offers suggestions for future direction of study