Structural features, such as faults and anticlines, alter or restrict ground-water flow at several places in the Los Angeles--Orange County coastal plain basin. The most prominent structural zone is the Newport--Inglewood Uplift (figs. 125 and 126), which trends northwestward, extends virtually the entire length of the basin, and is approximately perpendicular to the direction of natural ground-water flow. The aquifers might be interrupted by faults or thinned near the upwarped structural zone, but, for the most part, such restrictions are not complete barriers to ground-water flow. The sediments that form the mesas along much of the coast have minimal permeability and also impede ground-water flow; however, erosion formed gaps in the mesas, which subsequently filled with alluvial deposits; these gaps allow water to move between the inland aquifers and the sea.
Ground-water flow directions have been altered by withdrawals in the basin. Rapid urban development and the accompanying increase in withdrawals resulted in severe declines in water levels that began in the early 1900's. As a consequence, ground-water gradients near the coast reversed from seaward to landward in some areas in the 1920's, and saltwater intrusion was detected in 1932. Virtually the entire coastline was affected by the 1950's (fig. 128). The hydraulic gradient is now (1995) primarily from recharge areas toward withdrawal centers, rather than toward the ocean (fig. 129). Withdrawals in the deeper aquifers also have created a downward hydraulic gradient over much of the basin. Large expanses of the basin surface have been urbanized, thus decreasing the potential for direct recharge to the aquifer system and increasing the potential for saltwater intrusion.
The critical need for a solution to the seawater encroachment problem brought about coordinated management of water use in the basin beginning in the 1950's. Several methods have been employed to stop the steady progression of seawater into the basin (fig. 130). Ground water, which accounted for 40 to 50 percent of the water used in the basin, was augmented by large amounts of surface water imported from the Colorado River, the Owens Valley, and northern California. In some areas, particularly near the coast, withdrawals have been reduced or wells abandoned (fig. 130A). This has, to some extent, lessened the landward gradient. Artificial recharge (fig. 130B) through ponds or by water spreading, using imported water or reclaimed wastewater, replaces some of the ground water lost from storage and partly compensates for the loss of recharge potential that results from urbanization. Three barriers have been constructed near the coast in areas where seawater was encroaching into the freshwater aquifer system. The barriers consist of a series of either pumping wells that will remove saltwater from the aquifer and form a trough barrier (fig. 130C) or injection wells that pump reclaimed waste water into the permeable sediments, and thus establish a narrow zone in which the freshwater gradient is seaward (fig. 130D). In some places, wells that withdraw saline water from the aquifer system on the seaward side of the injection wells are part of the barrier. To date (1995), the combination of methods has been successful in halting seawater intrusion and also reducing the area of the basin with ground-water levels below sea level (fig. 131).
