## Facts

- Mississippi sued Tennessee, the City of Memphis, and Memphis Light, Gas & Water Division in an original action concerning groundwater in the Middle Claiborne Aquifer, a multistate aquifer underlying both States.
- Mississippi alleged that intensive pumping near the border in Tennessee created a “cone of depression” that drew groundwater from beneath Mississippi into Tennessee.
- Mississippi characterized the alleged conduct as a tortious taking, trespass, and conversion of Mississippi’s property, asserting exclusive sovereign ownership of all groundwater within its borders.
- Mississippi expressly disclaimed any theory of shared rights in the aquifer and rejected equitable apportionment as a remedial framework.
- Mississippi sought at least $615 million in damages, declaratory relief recognizing exclusive rights to the groundwater it claimed, and an injunction limiting Tennessee’s pumping.
- A Special Master concluded the dispute concerned a transboundary water resource governed by equitable apportionment and recommended dismissal with leave to amend to pursue that remedy.

## Issues

1. Whether groundwater in an interstate aquifer is subject to the federal common-law remedy of equitable apportionment.
2. Whether a State may obtain damages and injunctive relief based on exclusive territorial ownership of groundwater while disclaiming equitable apportionment for a transboundary aquifer dispute.
3. Whether Mississippi should be granted leave to amend to replead an equitable apportionment claim after years of litigating an exclusive-ownership theory.

## Decision

- The Court unanimously held that the waters of the Middle Claiborne Aquifer are subject to equitable apportionment.
- The Court rejected Mississippi’s theory that it had exclusive sovereign ownership of groundwater within its borders as applied to a transboundary aquifer.
- The Court dismissed Mississippi’s complaint because it did not plead the type of injury and claim equitable apportionment is designed to address and instead disclaimed that remedy.
- The Court dismissed without leave to amend, declining to permit Mississippi to recast the case after persistently rejecting equitable apportionment throughout the litigation.

## Legal Principles

- Equitable apportionment is the judicial remedy for interstate disputes over shared water resources, including groundwater that naturally crosses state lines and whose use in one State affects the resource in another.
- In interstate water disputes, States have equal rights to reasonable use of the shared resource; no State may assert absolute, exclusive ownership based solely on territorial location.
- A State may not pursue, in substance, an allocation of a shared interstate water resource while disclaiming equitable apportionment and relabeling the dispute as tort or property claims.
- Leave to amend in an original action may be denied where a plaintiff State strategically and repeatedly refused to pursue the governing equitable apportionment framework and amendment would prolong the proceeding.

## Conclusion

The Court treated the Middle Claiborne Aquifer as a shared interstate water resource subject to equitable apportionment, rejected Mississippi’s exclusive-ownership tort theory, and dismissed the case without allowing amendment because Mississippi chose not to plead equitable apportionment despite clear notice that it was the controlling doctrine.


Mississippi v. Tennessee, 595 U.S. 15 (2021)