On November 22, 2021, the U.S. Supreme Court decided Mississippi v. Tennessee, holding that water in an underground aquifer that flows across State lines is subject to equitable apportionment between the States, in similar fashion to interstate streams and rivers.

The Middle Claiborne Aquifer “underlies portions of eight States in the Mississippi River Basin,” including Mississippi and Tennessee. Water from the aquifer flows very slowly, underground, in a direction going from Mississippi into Tennessee. “[A]lthough the transboundary flow … may be a mere one or two inches per day,” or “30 to 60 feet per year,” the huge size of the aquifer means that “that amounts to over 35 million gallons of water per day, and over ten billion gallons per year.”

The City of Memphis’s public utility “pumps approximately 120 million gallons of groundwater from the Middle Claiborne Aquifer each day.” Mississippi alleged that this created an area of low pressure in the aquifer that greatly accelerated the subterranean flow of water across the State line. “Mississippi claims an absolute ownership right to all groundwater beneath its surface,” and so it sued Tennessee and Memphis under the Supreme Court’s original jurisdiction, seeking $615 million for a tortious taking of its groundwater.

The Supreme Court dismissed the claim in a unanimous opinion. The Court observed that, under the Court’s long-settled precedents, interstate disputes over surface water rights are governed by the doctrine of “equitable apportionment,” which “aims to produce a fair allocation of a shared water resource” considering “not only the physical properties and flow of a water resource, but also existing uses, the availability of alternatives, practical effects, and the costs and benefits to the States involved,” and guided by the overarching “principle … that States have an equal right to make a reasonable use of a shared water resource.” Although the Court had “never considered whether equitable apportionment applies to interstate aquifers,” it noted the aquifer here, like an interstate stream, “contains water that flows naturally between the States.” Therefore, the Court held that Mississippi did not have absolute ownership of groundwater in the aquifer within its boundaries, but instead that the water is subject to equitable apportionment between the States that share the aquifer. Since Mississippi had not asserted a claim for equitable apportionment, the Court dismissed the case.

Chief Justice Roberts authored the opinion for a unanimous Court.

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