On Monday the U.S. Supreme Court dismissed Mississippi’s claim that neighboring Tennessee has siphoned groundwater across the states’ border for over 30 years.

In a unanimous decision, Chief Justice John Roberts explained that because the groundwater flows naturally underneath state boundaries, the Court must follow established precedent around interstate water disputes, deferring to what’s called the “equitable apportionment doctrine.”

The Supreme Court first heard oral arguments for the case in early October; this marked the first dispute it had ever heard over an interstate aquifer.

“It is certainly true that ‘each State has full jurisdiction over the lands within its borders, including the beds of streams and other waters,’” Roberts wrote, citing Kansas v Colorado and Wyoming v Colorado. “But such jurisdiction does not confer unfettered ‘ownership or control’ of flowing interstate waters themselves.

“Thus, we have ‘consistently denied’ the proposition that a State may exercise exclusive
ownership or control of interstate ‘waters flowing within her boundaries.’”

Mississippi argued that pumping in the Memphis area created a ‘cone of depression,’ forcing water that would be under Mississippi’s territory to slide over under Tennessee. Mississippi asked for over $6oo million in damages.

But, as the Supreme Court reasoned, the groundwater in the aquifer is a continuous body of water, and it naturally flows beneath state boundaries regardless of one state’s pumping.

As Roberts explained, giving one state ownership of a portion of interstate water when it’s in that state’s territory could set a dangerous precedent.

“Mississippi’s ownership approach would allow an upstream State to completely cut off flow to a downstream one, a result contrary to our equitable apportionment jurisprudence,” he wrote.

Equitable apportionment, which the Supreme Court has leaned on in the past to decide interstate water disputes, would decide how to fairly divide the groundwater between the states based on factors such as how many people need the water, what it would be used for, conserving the water, and others.

Although the special master — an expert the Supreme Court appoints for highly technical cases — recommended that Mississippi be able to amend a claim for equitable apportionment, the Court denied the state that option because Mississippi never requested it, and additional states who also share the aquifer might have to be involved.

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