The 2021 U.S. Supreme Court ruling in Mississippi v. Tennessee involved the allocation of rights to groundwater in the Middle Claiborne Aquifer, which straddles the boundary between Mississippi and Tennessee. Mississippi argued that the increased pumping of water from the aquifer by Tennessee was a wrongful appropriation of water belonging to Mississippi because it increased the flow of water in the aquifer across the state line from Mississippi to Tennessee.

The question of whether the doctrine of equitable apportionment could apply to an interstate aquifer was a matter of first impression for the Court, which held that the doctrine did apply to the Middle Claiborne Aquifer. Important to the Court’s analysis were that (1) the Middle Claiborne Aquifer is a transboundary resource, (2) the aquifer “contains water that flows naturally between [Mississippi and Tennessee],” and (3) the impact of Tennessee’s pumping on the portion of the aquifer that underlies Mississippi was reached “through the agency of natural laws.”