GROUNDWATER DEPLETION: A CONSTRAINT ON SUSTAINABLE DEVELOPMENT

Development of groundwater resources for agricultural, industrial, and municipal purposes greatly expanded in the last century, and economic gains from groundwater use have been dramatic. In many places, however, groundwater reserves have been depleted to the extent that well yields have decreased, pumping costs have increased, water quality has deteriorated, aquatic ecosystems have been damaged by reduced groundwater discharge, and land has irreversibly subsided. These impacts tend to reduce the efficiency and sustainability of groundwater development. Some causes and effects of groundwater depletion are neither obvious nor easy to assess. Much groundwater pumped from confined aquifers is derived from storage losses in adjacent low-permeability confining layers. Depletion in confining layers is difficult to estimate and rarely monitored, but it can greatly exceed the depletion from the confined aquifer itself and groundwater drainage from confining layers is partly irreversible. For example, in the confined Dakota Aquifer, about 98 percent of the water removed from storage was derived from depletion in adjacent confining units. If cumulative long-term regional and global depletion is large, it will represent a substantial net transfer of water from land to the oceans, thereby contributing to sea-level rise. A U.S. national groundwater depletion census indicates that about 800 km³ of water was depleted from groundwater systems in the U.S. during the 20^th century—equivalent to a sea-level rise of approximately 2.2 mm—and 1,000 km³ through 2008. Global groundwater depletion since 1900 totals about 3,400 km³ through 2000 and 4,500 km³ through 2008. The rate of annual depletion has increased markedly since about 1950, with maximum rates occurring during the most recent period (2000-2008), averaging about 145 km³/yr. The recent acceleration in groundwater depletion is greater outside the U.S. Although groundwater depletion rates will ultimately be self-limiting, data show that we have not yet reached that point either nationally or globally.