US Groundwater Declines More Widespread Than Commonly Thought
In addition to confirming alarming depletion in well-known hot spots such as the Great Plains and Central California, the study identifies a number of other regions, including the lower Mississippi, along the Eastern Seaboard and in the Southeast where water tables are falling just as rapidly. Overall, the report concludes, between 1949 and 2009 groundwater levels declined throughout much of the continental U.S., suggesting that the nation’s long-term pattern of groundwater use is broadly unsustainable.
The report was written as part of the Water Center’s new “America’s Water” initiative, a multidisciplinary program designed to bring together industry, government and academic experts to address the country’s looming water challenges.
According to Tess Russo, the study’s lead author, it is impossible to determine the exact volume of depletion for a particular aquifer without knowing its individual storage capacity. Nonetheless, it’s clear, she says, that “we’re using groundwater in an unsustainable way.”
“Management of groundwater needs to be addressed and rethought,” Russo adds. “I think one of the big points in this paper is to show that it’s something that should be addressed across more of the country than most people would think.”
According to Russo, many previous studies of groundwater depletion have relied on groundwater flow models and extrapolation to estimate groundwater declines. By contrast, the new Water Center report directly analyzes county-level historical records of water table levels from 1949 to 2009.
“Groundwater depletion has been very well studied in the Ogallala [the Great Plains] and Central California, and to some extent other areas of the West, and parts of Texas,” says Russo. “But people don’t think of the lower Mississippi or the East Coast or the Southeast—outside of Florida—as being groundwater stressed areas. We tend to think of those areas as having abundant surface water, so we don’t really think that groundwater should be an issue.”
Access to groundwater is critical for regions that do not have access to surface water from lakes or rivers. And even in relatively wet areas, it can provide an important buffer during dry periods. In the United States, 40 percent of the population relies on underground aquifers for its supply of drinking water; groundwater also provides 60 percent of the water used to irrigate crops.
In dry regions where average precipitation is low, natural recharge of deep aquifers can be so slow that groundwater is essentially a non-renewable resource. Previous studies have suggested that in the High Plains Aquifer system water is being extracted at nearly 10 times the rate of recharge, resulting in the largest depletion of groundwater in the country.
As water tables fall, more and more energy is required to pump the water that remains. Eventually, with less water to support underground structures, aquifers can collapse, permanently reducing the amount of water they will ever store. Climate change adds additional risks; as rainfall patterns change across the Untied States, so will the rate at which underground aquifers recharge, potentially putting even more pressure on already-stressed regions.
The study found that for the most part, counties that have increased the amount of water they extract over time are also the most likely to see declines in groundwater levels. But, Russo, says, there were a surprising number of counties that did not follow that trend, and either saw groundwater levels rise in spite of increasing extraction, or saw water tables continue to fall even as extraction slowed.
In the first case, there may be a few areas where water is so abundant that even high levels of extraction do not draw down the supply—though it is difficult to know how long such practices can last, she says.
More concerning are areas that have continued to see water tables fall even as they have slowed the rate of extraction. “I would say for the counties where they’re slowing down their extraction, but still seeing groundwater declines, those are well-suited for more study, to figure out what’s going on, and to figure out how can they actually manage their groundwater, because it’s obviously more complicated than what they’re already doing,” says Russo.
Russo points to Suffolk County, Virginia—an area that receives relatively high annual rainfall— as an example of a place where groundwater levels are declining, in spite of reduced groundwater pumping. Historical data shows water tables falling at around 0.2 meters per year in almost all wells in the county, including those spanning a large depth range. Adjacent counties to the east and northwest are experiencing the same phenomena.
Groundwater declines in places like Suffolk and neighboring counties are particularly concerning, as they could lead to seawater intrusion, permanently contaminating coastal aquifers. Further analysis is needed, Russo says, to determine whether pumping rates are still too high to be sustainable, or whether groundwater declines are due to larger regional processes affecting groundwater availability.
In addition to comparing water extraction with groundwater declines, the study also analyzed the correlation between declining groundwater and short- and long-term climate variations. The authors found no correlation between water levels and short term climate variations, which was not surprising given that the study was limited to deep wells.
They did find a connection, however, between falling water tables and longer-term climate variations, particularly the Pacific Decadal Oscillation (PDO)—a kind of long-lived El Niño pattern of Pacific climate variability that has strong impacts on the North American climate. The researchers found that a “positive” PDO signal corresponds to wetter periods and lower rates of groundwater decline—either because aquifers are getting recharged or because more available surface water decreases the need to extract water from underground.
Ultimately, Russo says, the most important outcome of the study may be to help direct more targeted research toward at-risk regions. “The results of this study could be used to inform future, smaller-scale studies,” she says, that look much more closely at the complicated dynamics of extraction and recharge in a particular area. These kinds of studies are very intensive, she says, requiring extensive measurements and modeling. “You don’t want to do that for the whole US — you can’t, really. So our study would say, if you’re going to do it, you should probably focus on the lower Mississippi, or the coast in South Carolina — that’s sort of the idea.”