The overuse of groundwater is a concern that looms over discussions of water supply in many regions around the world. Many groundwater aquifers are pumped more quickly than they can be replenished, meaning wells have to be drilled deeper and deeper to reach an ever-diminishing resource.

The Ogallala Aquifer, which extends from Nebraska to Texas, is a prime example. There, groundwater irrigation has made productive agriculture possible in an otherwise dry region. The rate of water use, unfortunately, is not sustainable. In some places, groundwater is pumped over 20 times faster than it is recharged by precipitation, and water levels have steadily dropped. Some refer to this as “groundwater mining” because it took some 30,000 years to fill the aquifer—once it’s pumped dry, it won’t soon refill.

Even in the water-rich Great Lakes region, over-pumping can locally lower water levels in “cones of depression”. While limited in extent, these depressions can reach impressive depths. Groundwater levels beneath Chicago have dropped by up to 900 feet, which significantly increases the energy needed to pump the water up. Milwaukee, which has seen a drop of up to 450 feet, has begun to rely on Lake Michigan for much of its drinking water. Battles have been brewing as other communities turn a thirsty eye towards the lakes, which are tightly controlled by agreements between the US states and Canadian provinces bordering the Great Lakes.

In areas of the American west, groundwater depletion has reached such an extent that the land surface itself is sinking. Groundwater counteracts compaction of the sediment, so pumping out the water can actually lower the elevation of the ground surface. Parts of the San Joaquin Valley in California subsided by 30 feet between 1925 and 1975, as groundwater levels dropped by 400 feet. Las Vegas has dropped by about 6 feet since the 1950s as groundwater pumping has maintained Sin City’s extravagance in the middle of arid Nevada.

A hydrogeologist at the United States Geological Survey has compiled the most rigorous accounting of global groundwater depletion to date, which was published recently in Geophysical Research Letters. Since much of the water pumped out of the ground eventually ends up in surface water bodies, it ultimately makes its way to the ocean where it could potentially contribute to rising sea level. To evaluate this, the researcher used historical water level observations, measurements of aquifer characteristics, pumping data, groundwater flow models, and even some data from the Gravity Recovery and Climate Experiment (GRACE) satellites.

The total amount of groundwater lost between 1900 and 2008 was estimated at 4,500 km3—enough to raise global sea level by about 12.6 mm. That’s a little more than 6 percent of the total sea level rise seen in that time period. It’s interesting to note, however, that the retention of surface water in reservoirs behind dams has had the opposite effect—counteracting sea level rise. A related paper in press (also in Geophysical Research Letters) shows that between 1972 and 2008, when groundwater depletion contributed an average of 0.3 mm/yr to sea level rise, surface water retention decreased sea level rise by 0.4 mm/yr.

Perhaps the most remarkable thing about the groundwater analysis is that 25 percent of the total 1900-2008 depletion occurred between 2000 and 2008. During those final eight years, the volume of groundwater lost is equal to 13 percent of global sea level rise. In the US, this acceleration is largely due to increasing use of the Ogallala Aquifer and aquifers along the Gulf Coast. In fact, over-pumping of the Ogallala is responsible for nearly half of the total US depletion during that period despite conservation efforts and changes in farming practices.

Of course, groundwater depletion cannot continue to accelerate indefinitely. Water use will ultimately be curtailed by decreasing availability and the increasing expense of pumping from greater depths, if nothing else. It’s highly uncertain when that will happen, though. As far as sea level is concerned, surface water retention is likely to level off soon as dam construction becomes less common, even as groundwater depletion rises. The more immediate and pressing concern, however, is likely water supply. The fact that contributions to sea level rise can even enter into the discussion indicates the magnitude of the issue.

Geophysical Research Letters, 2011. DOI: 10.1029/2011GL048604; Geophysical Research Letters, 2011. DOI: 10.1029/2011GL048794 (About DOIs).