GROUND-WATER AVAILABILITY AND LINKAGES BETWEEN WATER QUANTITY AND QUALITY IN THE HIGH PLAINS AQUIFER

The High Plains aquifer has been substantially affected by human activities. Withdrawals from the aquifer have far exceeded natural recharge thus causing large declines in local and regional water levels. The High Plains aquifer in Texas has experienced the greatest ground-water depletion with water in storage declining about 27 percent relative to predevelopment volumes. Large-scale pumping has resulted in some areas having insufficient supplies of fresh water to support local populations, agricultural economies, and aquatic ecosystems. Ground-water availability is dependent on water quantity and quality, yet ground-water quantity and quality studies have typically been conducted independently. Experience gained from the U.S. Geological Survey's High Plains Regional Ground-Water Quality Study demonstrates that there are linkages between quantity and quality that can directly affect ground-water availability.

The linkages between ground-water quantity and quality are not immediately apparent and the resultant effects typically are slow in developing. In the High Plains aquifer, hydrologic responses to long-term regional withdrawals, primarily from irrigation, have resulted in fundamental changes to the ground-water flow system. Reductions in hydraulic heads due to long-term ground-water withdrawals have reduced natural discharge to streams, lakes, and evapotranspiration, depleted ground-water storage, increased large-scale vertical mixing of contaminants in the aquifer, reduced or reversed hydraulic gradients between the aquifer and underlying formations with saline waters, and induced infiltration of degraded surface water into the aquifer. Irrigation also has resulted in altered recharge patterns and accelerated transport of agricultural chemicals and natural salts from the land surface to the water table.

The long-term implications of these observed changes in the hydrologic system and their effects on water availability are uncertain but it is clear that current conditions cannot easily be reversed. The lack of sufficient water supplies for specific uses will be compounded if the remaining resource is deemed unsuitable due to deteriorated water quality. The goal for the future will be to balance competing demands and optimize water use in a way that protects both ground-water quantity and quality.