PRELIMINARY SYNTHESIS OF FIVE USGS REGIONAL GROUNDWATER AVAILABILITY STUDIES

In 2004, the U.S. Geological Survey (USGS) initiated studies of groundwater availability in regional aquifer systems. Studies of five aquifer systems have been completed or are near completion: Central Valley, California; Atlantic Coastal Plain, North and South Carolina; Denver Basin, Colorado; Mississippi Embayment primarily in Arkansas, Louisiana, Mississippi, and Missouri; and Lake Michigan Basin, Michigan, Wisconsin, Illinois, and Indiana. Study areas range from 6,700 to 78,000 square miles. Similar studies are planned to examine more than 30 regional aquifer systems, and other priority aquifers in under-represented terrains that are important sources of groundwater in the United States.

Groundwater flow models were developed for each study to quantify aquifer-system response to current and future human and environmental stresses. The climatic settings and the responses of each aquifer system to development are quite different. Development for irrigation in the Central Valley aquifer system increased the water flux through the system by a factor of six. In the Lake Michigan Basin or Carolina Coastal Plain, however, withdrawals have changed the flow system in subregional and local areas, but overall fluxes through the system are dominated by climate drivers rather than irrigation. Deep pumping in the Lake Michigan Basin has caused drawdowns of 800-1000 feet in the Cambrian-Ordovician aquifer system, but the water level in this principal aquifer stabilizes if pumping is held constant. Water levels recovered when stress was reduced in the 1990s. In contrast, water levels in the Denver Basin are declining such that aquifers are being converted from confined to unconfined conditions, and simulations indicate that water levels in the basin will not stabilize if withdrawals are held constant.

Challenges for synthesis of regional groundwater availability studies include identifying the physical features that limit groundwater availability; examining the controls and drivers for groundwater availability at varying scales; assessing sources of water that supply withdrawals from regional systems; integrating water-quality issues; and, finally, addressing technical challenges faced when simulating large regional flow systems while also recognizing local availability issues.