Lake Mead (below right) supplies nearly 90 percent of the water used by Las Vegas Valley Water District customers. Virtually all water in Lake Mead originates as snowmelt in the Rocky Mountains that flows down the Colorado River. The remaining water—about 10 percent—comes from wells that tap a deep groundwater aquifer beneath the Las Vegas Valley. Groundwater is used primarily between May 1 and Oct. 1 annually to meet peak water demand; in addition, several groundwater wells operate Oct. 1–March 31 to reduce trihalomethane levels in key areas and ensure compliance with state and federal water-quality requirements. If you live or work within several miles of the Water District’s offices at Charleston and Valley View boulevards, or in the northwest valley, you may receive a blend of treated Lake Mead water and groundwater.