KEYNOTE SPEAKER: CHANGES IN GROUNDWATER BUDGETS FROM PREDEVELOPMENT CONDITIONS AND THE EFFECTS OF DEVELOPMENT ON GROUNDWATER QUALITY IN SELECTED SOUTHWEST BASIN-FILL AQUIFERS

The hydrogeologic settings of many basin-fill aquifers in the arid to semiarid southwestern United States have changed from natural, predevelopment conditions mostly as a result of human-induced changes in water use and to the landscape. In combination, artificial recharge and withdrawals from wells have enhanced the downward movement of water, increasing the susceptibility of the basin-fill aquifers to water-quality changes. Part of the U.S. Geological Survey’s National Water-Quality Assessment (NAWQA) Program, the Southwest Principal Aquifers (SWPA) study includes the California Coastal Basin aquifers, the Central Valley aquifer system, the Basin and Range basin-fill aquifers, and the Rio Grande aquifer system.

Before human development of the landscape and water resources began in many Southwest basins in the 1800s and early 1900s, recharge to basin-fill aquifers was focused mainly at the basin margins and along stream reaches where water seeped into the ground. With land and water development, artificial recharge, mostly from the infiltration of excess irrigation water, has become a major source of groundwater recharge in developed basins. In the West Salt River Valley, Arizona and Central Valley, California the amount of groundwater recharge is now more than six times the estimated amount of recharge prior to development.

Despite the large amounts of artificial recharge introduced to many of the basins, pumping has caused an even larger change in discharge. In 11 of the 15 SWPA case-study basins, groundwater pumping from wells accounts for more than half the discharge from the basin-fill aquifers. Water-level declines, changes in vertical head gradients and geochemical conditions, and preferential pathways for vertical movement of water in wells are some of the factors associated with development that affect groundwater quality in the studied basins. Elevated concentrations of dissolved solids, arsenic, uranium, and (or) nitrate, and the detection of volatile organic compounds and pesticides in groundwater have been documented in areas of the case-study basins.