A FRAMEWORK FOR ASSESSING GROUNDWATER SUSTAINABILITY

Sustainable groundwater use can be maintained indefinitely without causing unacceptable consequences. While we can quantitatively estimate the hydraulic effects of pumping on an aquifer system and its boundaries, determination of what consequences are ‘unacceptable’ requires some partly subjective assessments based on socioeconomic, political, environmental/ecological, and legal considerations. Judgement and tradeoffs are key players, and these are calls that can differ among various interested parties. Total groundwater withdrawals in the U.S. increased consistently from 1945-75, but withdrawals more or less stabilized after 1975 at about 110 km³/yr. This stabilization does not mean that the amount of groundwater use is generally sustainable because storage depletion and water-level declines have continued to grow—almost linearly—for decades after the pumpage levelled off. Continued depletion and water-level declines are not sustainable indefinitely. C.V. Theis’ 1940 paper offers a quantitative and objective framework for assessment and decision making. Such an assessment can be made with results from a well-calibrated, high-resolution, mathematical model of an aquifer system that has reasonable boundary conditions to represent adjacent surface water bodies and evapotranspiration. Theis’ principles state that pumpage must be balanced by some combination of storage depletion, increase in recharge, and decrease in discharge. The latter two factors can be lumped together as “capture.” The average long-term natural recharge rate does not enter into this assessment. Analysis of long-term records in the U.S. indicates that on average about 85% of groundwater withdrawals are balanced by capture, while only about 15% are balanced by storage depletion in aquifers. But the variation between aquifer systems is large, and detailed hydrogeological analyses of individual groundwater systems are required to assess sustainability at local to regional scales. A growing population drives increased demands for food and water supply; meanwhile, climate and hydrologic systems are changing. We will need (1) improved water management, (2) improved regulations, (3) technology advances, and (4) adaptation to change. If these efforts fail to achieve sustainable use, natural processes will ultimately constrain withdrawals—forcing involuntary adaptation.