Dissolution of gypsum beds in the deep unconsolidated sediments and in consolidated marine deposits on the east side of the valley causes large concentrations of sulfate in ground and surface waters along the Salinas River and San Lorenzo Creek (fig. 122). Dissolved-solids concentrations in ground water in this area are as much as 3,000 milligrams per liter. Because the ground-water system receives recharge from surface water in the lower basin, ground-water quality in areas without gypsum beds can be affected by infiltration from streams that drain areas with such beds. Water in streams on the southwest side of the valley is less mineralized and partly dilutes the highly mineralized water during the wet season.
Aquifers near the coast are subject to seawater contamination when ground-water withdrawals in the area exceed natural recharge. Large withdrawals for agricultural and municipal supplies have lowered the potentiometric surface east of the city of Salinas until it is considerably below sea level (fig. 121). As a result, the natural freshwater gradient has been reversed from seaward to landward, which allows saltwater to enter the aquifer system where it crops out on the sea floor. Saltwater intrusion was already a concern when monitoring began in 1943, and, as of 1995, the area affected has increased greatly in size. The contamination has resulted in the abandonment of some wells.
Another area of concern in the lower basin is on the east side of the Salinas River between Soledad and Salinas (fig. 122). Organic pollutants and excessive nitrate concentrations that result from industrial and agricultural activity are possible threats to ground-water quality in this area.