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SEA WATER INTRUSION Overdrafling aquifers also can cause water quality problems - the most widespread being sea water intrusion into aquifers underlying coastal plains. As pumps draw up fresh water from the aquifer, the sea water is pushed inland into the aquifer by the force of the ocean. Sea water intrusion began to occur in the 19208 in southern California, where well water was the only available water supply for many communities. Sea water was drawn into aquifers in Ventura, Los Angeles and Orange counties. For decades. water managers in those areas have taken steps to push the salty water back toward the Pacific. and to restore and expand the ability of coastal aquifers to meet water needs. In the 1950s and 1960s. water managers in south- ern California began injecting into the ground sur- face water imported from the Colorado River and northern California. The injected water creates a hydraulic barrier between the fresh water aquifer and the intruding sea water. As water demands increased, managers in Orange County took the next step of using highly treated municipal wastewater to maintain the barrier. Individual water agencies have tailored their strate- gies to best use their resources. In Los Angeles County, the West and Central Basin water districts are planning to use sophisticated treatment technolo- gies to clean up brackish water - sea water diluted by fresh water - that has been trapped inland of the hydraulic barrier. Since 1925, officials in Ventura County have man- aged the Santa Clara River drainage to conserve water and reduce saltwater intrusion caused by excessive groundwater pumping. More recent efforts have included the damming of Piru Creek to capture runoff and then slowly release water down the stream channel so the water can percolate into the ground before it reaches the sea. Over 2.7 million acre.feet had been replenished by the mld-1990s. The Salinas and Pajaro river valleys of central California's coast have struggled with a serious sea water intrusion problem. Fifty years of overdraft in the Salinas Valley have allowed the sea water to move 5 miles inland - threatening the water supply for a $1.6 billion fresh fruif and vegetable economy and the public water source for communities around Salinas. Decades ago, Nacimiento and San Antonio reservoirs were constructed high in the Salinas river watershed to capture runoff, release it slowly and encourage groundwater recharge. But irrigated agriculture in the upper watershed increased, and recharge at the confined aquifer near the coast is too slow to be effective. One part of the solution is a project that will transport 20.000 acre-feet of highly treated wastewater from the Monterey Peninsula and Salinas to the Castroville Irrigation District, where it will be used to irrigate crops and reduce groundwater use. In one case, over-pumping re- sulted in land subsidence and sea water intrusion - only the sea water was not about to invade the aquifer. but the city streels. Santa Clara County grew up tapping the groundwater under the valley south of San Francisco Bay. By the 1930s. the ground underneath the city of San Jose began to sink - raising the possibility that tidal water could inundate the community. In some cases, salty groundwater can come from ancient sea water that was isolated until pumping began. The northern San Joaquin Willey near Stockton has problems with pockets of sea water that pumps have drawn into the fresh water aquifers. In the northern Sacramento Valley, some pumpers are at risk of tapping into ancient deposits of sea water, which can degrade the quality of the ground- water. The 51.6 hillioJ/fre.\hfruit und I'cgefablc ecol/omy of the S'{/linllS lIIul Pajaro rin'r \,(llIeys is thteaJi'lli'd hy {/ seriolls sell \\'(I/('r int/'lfsiol/ problem. Aho\"e, artichokes 011 {/ Castrorille farm. The first effort to reverse the trend involved construct- ing dams on regional creeks and streams, to capture runoff and release it more slowly so the water could percolate into the ground. As the demand increased. the Santa Clara Valley Water District in the t 960s began to purchase water from San Francisco; when the South Bay Aqueduct came on line in 1965. the district purchased water from the SWP. Subsidence in the area has largely been arrested at 13 feet. During the t987-1993 drought. area water manag- ers also acquired legislative authorization to charge a differential pumping fax so they could develop financial disincentives to the overdraiting of confined aquifers. 13