EFFECTIVE GROUND-WATER AVAILABILITY: A CASE STUDY IN THE CENTRAL VALLEY AQUIFER OF CALIFORNIA

The availability of ground water is becoming an important issue in many parts of the world as population growth and development increase demand. Ground-water quality issues can reduce the availability of ground water by decreasing the volume of water suitable for a particular use. The amount of effectively available ground water in an aquifer is the amount of ground water affected by contamination subtracted from total estimates of available ground water, adjusted for treatment feasibility and cost.

The concept of effective ground-water availability is applied to the Central Valley aquifer of California. Large ground-water withdrawals and surface-water imports for irrigation have changed the ground-water budget of this aquifer. More than 80 percent of recharge to the California Central Valley aquifer is from irrigation return flow. This increased cycling of water, along with conjunctive use and land-use practices, have contributed to altering the quality of this aquifer.

Data from the U.S. Geological Survey's National Water-Quality Assessment Program (NAWQA) indicate that about one-third of the ground water in the Central Valley aquifer may be contaminated at levels of human-health concern. Concentrations of various trace elements and volatile organic compounds were greater than health-based screening levels (HBSLs). Concentrations of arsenic and boron accounted for about one-half of all concentrations greater than HBSLs.

The implication of these results is that a substantial volume of ground water in the Central Valley aquifer of California may be effectively unavailable for human consumption, if treatment is not considered. However, many assumptions are inherent in these analyses. If some of the assumptions are not true, this implication may not be applicable. Nonetheless, the potential magnitude of contamination in the Central Valley aquifer indicates that water quality is an important consideration in determining the availability of ground water in heavily-used aquifers.