GROUND-WATER DEPLETION AND OVEREXPLOITATION: A GLOBAL PROBLEM

Development of ground-water supplies for agricultural, industrial, and municipal purposes "exploded" during the 20th century. In many semi-arid to arid regions, ground water has been withdrawn at rates far in excess of recharge, leading to ground-water "mining," major water-level declines, increased pumping costs, and decreased well yields. Overexploitation anywhere is often accompanied by detrimental environmental side effects, such as land subsidence, water-quality degradation, and reduced ground-water discharge to springs, streams, and wetlands. Some causes and effects of ground-water depletion, however, are neither obvious nor easy to assess. For example, the construction of drainage canals in surficial systems can lead to regional lowering of the water table, and land drainage is frequently ignored as a source of ground-water depletion. Also, the source of much ground water pumped from confined aquifers is derived from leakage from adjacent confining beds, but depletion of low-permeability layers is difficult to estimate, rarely monitored, and usually overlooked. The cumulative long-term depletion of ground water may be so large worldwide as to constitute a measurable contributor to sea-level rise. For example, the net amount of water removed from storage in the High Plains aquifer in the central U.S. is estimated at 270 km³ through 1999, and the depletion is continuing. The total depletion in the Central Valley of California and the alluvial basins of Arizona is about 82 km³ and 114 km³, respectively, although depletion in these basins has essentially ended. If the total volume depleted from just these three aquifer systems were spread over the surface area of the oceans, it would be equivalent to about 1.3 mm of sea-level rise, or nearly 1.0 percent of the sea-level rise observed during the entire 20th century. Although depletion is observed in many areas outside the U.S., such as China, India, and Mexico, the magnitude is poorly documented, particularly in developing countries. Improved efforts to monitor regional water-level declines throughout the world should be implemented; more comprehensive and accurate assessments of rates and effects of ground-water depletion and overexploitation may provide an incentive to improve management of limited ground-water resources.