13 former gravel pits covered roughly 300 acres in Fremont, California, and now form part of a groundwater recharge system that is operated by the Alameda County Water District (ACWD), which directs water from Alameda Creek into the pits so it can seep underground. The district was working to protect the Niles Cone groundwater basin from saltwater intrusion in the early 1970s, which led to the establishment of the project. The problem was that seawater from San Francisco Bay had entered parts of the freshwater aquifer, which threatened a supply that was used by communities in southern Alameda County, and the district needed to replenish groundwater and limit the movement of saline water into freshwater zones.
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