The Water Replenishment District of Southern California (WRD) is the largest groundwater agency in the State of California, managing and protecting local groundwater resources for four million residents. WRD's service area covers a 420-square-mile region of southern Los Angeles County, the most populated county in the United States. The 43 cities in the service area, including a portion of the City of Los Angeles, uses about 250,000 acre-feet (82 billion gallons) of groundwater annually which accounts for approximately half of the region's water supply. WRD ensures that a reliable supply of high quality groundwater is available through the use of recycled water and stormwater capture. WRD is responsible for monitoring and testing groundwater throughout the region using effective management principles.