Freshwater is contained mainly in the unconsolidated basin-fill deposits. In a few areas, sufficient water for domestic and stock use can be obtained from the semiconsolidated and consolidated rocks where they are fractured or weathered. The deeper unconsolidated deposits contain the largest amount of water in storage, but the shallower alluvial-fan and stream-valley deposits currently (1995) yield more water. Very permeable deposits of windblown sand are generally above the water table and, therefore, largely unsaturated; they do, however, form important recharge areas.
The Salinas Valley aquifer system is divisible into upper and lower ground-water basins. The upper basin extends from near the headwaters of the Salinas River and its tributaries to San Ardo, where the unconsolidated deposits narrow. The lower basin extends from San Ardo to Monterey Bay.
In the upper ground-water basin, the degree of confinement varies locally and depends on the presence and total thickness of deposits of fine-grained materials (fig. 120). Most ground water in deep deposits is confined, but the shallow ground water is, for the most part, unconfined. The thickest area of the aquifer in the upper basin is in the Estrella Valley where the unconsolidated deposits are as much as 1,750 feet thick. Recharge in the upper basin is from precipitation, infiltration from streams, and irrigation return flow. Ground-water discharge in the upper basin is by loss to streams, withdrawals by wells, and evapotranspiration. Nearly all the tributary flow to the Salinas River is in the upper basin.
Ground water in the lower basin is mostly under water-table conditions except on the northwest side of the valley from near Gonzales to Monterey Bay. In this area, a clay layer near the land surface provides varying degrees of confinement to water in the aquifers below. Infiltration from the Salinas River provides most of the recharge for aquifers in the lower basin. Nearly all the discharge in the lower basin is by withdrawals from wells.