The Supreme Court’s Decision on the Mississippi-Tennessee Aquifer Conflict Will Change U.S. Water Wars

On Monday, the U.S. Supreme Court unanimously rejected Mississippi’s claim that Tennessee was stealing its groundwater in a decision that legal experts say could have major implications for future battles over water amid the worsening climate crisis.

If the Supreme Court had sided with Mississippi, it would have “created chaos in the long-established world of interstate water allocation,” says Christine Ann Klein, a professor at the University of Florida Levin College of Law who specializes in water law. “[The ruling] is a very big deal.”

The term “groundwater” refers to freshwater that’s stored beneath the earth’s surface. The U.S. Environmental Protection Agency has determined that groundwater is responsible for about half of the country’s municipal, household, and agricultural water supply. (The term “surface water,” on the other hand, refers to any body of fresh water that’s above ground.)
[time-brightcove not-tgx=”true”]

Monday’s case stretches back to 2014, when Mississippi sued Tennessee for allegedly stealing its groundwater by allowing a Memphis water utility company to pump from the Middle Claiborne Aquifer, which sits below the Mississippi-Tennessee border. Mississippi claimed that the water was its own since 1817 when it arrived in America. It sought $615million in damages from Tennessee.

However, the Supreme Court disagreed. Instead, the high court ruled that the legal doctrine of “equitable apportionment”—which has long been used to determine what states get control of interstate surface water—also applies to groundwater. Mississippi and Tennessee both have the right to use the Middle Claiborne Aquifer. But if they want an official decree dictating how they have to share it, they’ll need to go through an “equitable apportionment” process, in which they must go before a court and argue their case. The court will divide the water according to its own discretion.

In other words, the high court ruled that “states have to share,” writes Robin Craig, a professor at the University of Southern California Gould School of Law who specializes in water law. “They can’t claim all the water for themselves.”

Monday’s ruling was the first time the Supreme Court has ever weighed in on the issue of interstate groundwater. The unanimous ruling not only ended Mississippi and Tennessee’s long-running dispute, but also provided a framework for other legal battles that may emerge in the coming decades, says Craig. Now, in order to claim legal ownership of both interstate groundwater and surface water, states will need to go through the “equitable apportionment” process to determine who gets what.

As the climate crisis intensifies and droughts worsen, particularly in the American West, groundwater will only become a more precious resource—and interstate groundwater disputes will likely become more common, she argues. Craig is aware that Nevada and Utah have been in close litigation on the matter, while the High Plains Ogallala Aquifer (which runs through the Great Plains) has already been heavily mined. Many aquifers are scattered throughout the United States.

In Klein’s opinion, the case helps bridge the gap between the way litigation has historically treated groundwater and how scientists understand groundwater actually works. “The law often treats groundwater differently than surface water, partly as a relic of slow-developing hydrologic knowledge,” she says. But at this point in 2021, she adds, researchers have found “little justification” for the separate treatment of surface and groundwater.

If Mississippi had been awarded the $615 million it sought, she says, “groundwater wars would have become very expensive, very fast,” says Craig. The Supreme Court’s high standard for showing injury might have encouraged states to seek to divide aquifers instead of immediately going to court to claim damages.