Evaluation of ground-water quality in the Santa Maria Valley, California

The quality and quantity of recharge to the Santa Maria Valley groundwater basin from natural sources, point sources, and agriculture were evaluated. The results are expressed in terms of a hydrologic budget, a solute balance, and maps showing the distribution of selected chemical constituents in ground water. Point sources of waste discharge that could cause ground-water degradation were investigated including a sugar-beet refinery, oil refineries and oil-field wastewater, stockyards, golf courses, poultry farms, solid-waste landfills, and municipal and industrial wastewater-treatment facilities.

Measurements of specific conductance of ground water and water levels in nearly 400 wells and the analyses of about 100 water samples for most major anions and cations plus detergents, dissolved organic carbon, and trace metals permitted preparation of maps showing the potentiometric surface and the distribution of dissolved solids, chloride, sulfate, calcium plus magnesium, and nitrogen in ground water.

Pumpage of ground water for agricultural, municipal, and industrial uses has exceeded recharge by about 10,000 acre-feet per year (12.3 cubic hectometers per year). Most of the ground water pumped in the Santa Maria Valley is for agriculture. The result of pumping in excess of recharge is a declining potentiometric surface, an accumulation of solutes, and an increase in nitrogen in ground water. At present (1976), the area of confined water is most severely affected by this degradation. Nitrogen concentrations in ground water have reached as much as 50 milligrams per liter in isolated areas, with concentrations in excess of 10 milligrams per liter occurring through most of the area of confined water. Continued pumping in excess of recharge may also lower the potentiometric surface sufficiently to permit intrusion of seawater into the fresh-water zones.

Discharge of wastewater from municipal and industrial wastewater-treatment facilities is also contributing solutes to the ground-water system. In comparison to the influences of irrigation return water on ground-water quality, the contribution from point sources is less than 10 percent of the total solutes. In some areas of the valley, the concentration of dissolved solids and some chemical constituents in water discharging from municipal and industrial wastewater-treatment facilities is lower than in the receiving ground water.