The Supreme Court never before addressed whether equitable apportionment applies to interstate aquifers. Mississippi v. Tennessee et al., 595 U.S. ___ (Nov.22, 2021).

Equitable apportionment is a doctrine under which the Supreme Court allocates rights to a disputed interstate water resource after one state sues another under their original jurisdiction. It is highly significant whether one state can sue another and get such relief in a water dispute.

The federal District Court had held that interstate aquifers are comparable to interstate rivers because an aquifer “flows, if slowly” and as a result should be subject to equitable apportionment. Hood ex rel. Miss., 750 F. 3d, at 630. Since the Supreme Court has exclusive jurisdiction over suits between states, however, the case was dismissed and that was affirmed in the Fifth Circuit Court of Appeals. Mississippi then petitioned for a writ of certiorari from that decision, which was denied in 2010. 55 U.S. 904.

Four years later, in 2014, Mississippi sought leave from the Supreme Court to file a bill of complaint against Tennessee, Memphis, and Memphis Light, Gas, and Water Division (MLGW). That is the basis of this recent opinion.
Mississippi contended that MLGW’s pumping has wrongfully taken water from the area under Mississippi that “would have never under normal, natural circumstances been drawn into Tennessee.” Mississippi v. Tennessee et al., 595 U.S. ____, 5 (2021). Mississippi further alleged that the taking is “evidenced by a substantial drop in pressure and corresponding draw-down of stored groundwater. . . .” Id. Mississippi claimed an absolute ownership right to all water beneath its surface, even after that water has crossed its borders. Id.

The Supreme Court, in justification for applying equitable apportionment to these circumstances, made several technical observations: that “The Middle Claiborne Aquifer’s ‘multistate character’ seems beyond dispute,” that “the Middle Claiborne Aquifer contains water that flows naturally between the States,” and that “Tennessee’s pumping has contributed to a cone of depression that extends miles into northern Mississippi. . . Mississippi itself contends that this cone of depression has reduced groundwater storage and pressure in northern Mississippi. . .Tennessee’s pumping is ‘siphoning’ tens of millions of groundwater each day from Mississippi’s portion of the aquifer. . . .” Id. at 8, 9.

The Court held regarding this groundwater that “the speed of the flow, at least in the context of this case, does not place the aquifer beyond equitable apportionment.” Id. at 8. And that “such interstate effects are a hallmark of our equitable apportionment cases.” Id. at 9.

The Supreme Court then ruled that Mississippi does not have sovereign ownership of all groundwater beneath its surface. The Court said it has “’consistently denied’ the proposition that a state may exercise exclusive ownership or control of interstate ‘waters flowing within her boundaries.’” Id. (citing Hinderlider v. La Plata River & Cherry Creek Ditch Co., 304 U.S. 92, 102 (1938).

The Court stated that although past cases dealt with streams and rivers, the Court “see[s] no basis for a different result in the context of the Claiborne Aquifer.” Id.

Once in a while, the Supreme Court issues a seminal ruling based on science explicated in the decision, in the past about Lake Tahoe watershed management, Florida ocean geologic processes, and here slow-moving groundwater aquifers. The Court informed us that the latter falls within their state-state exclusive jurisdiction and triggers access to an equitable remedy fashioned by the Court.

Consequently, this case is of significant interest to practitioners of environmental, interstate, and water law. The Supreme Court doors are open for the judicial remedy of equitable apportionment of states’ rights to interstate aquifers.

Tyler Doan attends Vermont Law School and during 2021 is a Law Clerk at McGregor Legere & Stevens PC