In a landmark ruling signaling a new lens with which to view the treatment of interstate water allocation, the U.S. Supreme Court issued a decision on November 22 in Mississippi v. Tennessee, et al., 595 U.S. ___ (Case No. 21o143) rejecting Mississippi’s claim to sole ownership of waters of the Middle Claiborne Aquifer within that state’s borders. Instead, the Court held that the traditional remedy of equitable apportionment used to allocate surface waters of streams and rivers should apply. The case represents the first time that the Court has held that equitable apportionment applies to groundwater, which may open the door to new disputes among border states who claim equal rights to underground water resources.

The Middle Claiborne Aquifer underlies eight states: Alabama, Arkansas, Illinois, Kentucky, Louisiana, Mississippi, Missouri, and Tennessee. Since its Tennessee discovery in 1886, the aquifer acts as a source of clean, inexpensive water supply for the city of Memphis. Tennessee drilled vertical wells into the aquifer to tap the water source and no dispute existed in the litigation that all wells were exclusively located within that state. Despite the geographic location of the wells however, interstate impacts did occur in the form of a cone of depression that reduced groundwater storage and pressure in northern Mississippi. Mississippi contended that these impacts interfered with its exclusive sovereignty of groundwater located within its borders.

Rejecting Mississippi’s position, the Court likened the allocation of an aquifer to surface water finding it to have “multistate character,” cross-border impacts, and characteristics akin to its prior cases of using equitable apportionment as a remedy. Citing Hinderlider v. La Plata River & Cherry Creek Ditch Co., 304 U.S. 92, 102 (1938), the Court noted its consistent refusal to hold that a state may exercise exclusive ownership or control of interstate waters flowing within its boundaries. Finally, the Court reasoned that even where surface waters had been ephemeral, or the resource had been fish not water, it had still utilized equitable apportionment as the proper remedy to resolve conflicting disputes to a transboundary resource among competing states.

Finally, the Court sustained Tennessee’s exception to the special master’s ruling permitting Mississippi to amend its complaint to add a claim for equitable apportionment, finding that Mississippi had not indicated any intent to seek such an amendment.