The decision could complicate states’ ability to pursue groundwater natural resource damages actions.
On November 22, 2021, the US Supreme Court held that equitable apportionment applies to a dispute between states about their respective interests in groundwater that flows through multiple states in Mississippi v. Tennessee. The decision may have implications for natural resource damages (NRD) claims. Natural resource trustees often assert claims to pursue damages to groundwater. In assessing these claims, courts often must determine (1) whether the trustee has a trusteeship interest in the groundwater resource at issue, and (2) if so, the extent of the trustee’s interest in that groundwater resource relative to the interests of other trustees in the same groundwater.
In this original jurisdiction action, Mississippi alleged that Tennessee wrongfully appropriated groundwater belonging to Mississippi through Tennessee’s pumping of groundwater from the Middle Claiborne Aquifer, which lies underneath eight states, including Mississippi and Tennessee. The Special Master appointed by the Supreme Court recommended dismissal of Mississippi’s complaint, finding that equitable apportionment between Mississippi and Tennessee was the appropriate remedy. Equitable apportionment is a judicial remedy to fairly allocate shared water resources based on consideration of all relevant factors, guided by the principle that each state has equal rights to the reasonable use of a shared water resource.
The Supreme Court adopted the Special Master’s recommendation, holding for the first time that equitable apportionment is the proper remedy to determine a state’s interest in an interstate groundwater aquifer. As the Court explained, “When a water resource is shared between several States, each one ‘has an interest which should be respected by the other.’” The Court rejected Mississippi’s argument that it has an exclusive sovereign right to the water beneath its surface, even after that water had crossed its borders, reasoning that the Court has “‘consistently denied’ the proposition that a State may exercise exclusive ownership or control of interstate ‘waters flowing within her boundaries.’”
In an equitable apportionment proceeding, the Supreme Court explained, a “broader range of evidence” must be considered, including the physical properties and flow of the water resource, its existing uses, available alternatives, practical effects, and costs and benefits to all States affected by that water resource. These considerations may also require the joinder of additional states that rely on the groundwater resource.
The Supreme Court’s holding that equitable apportionment is required to determine states’ respective interests in interstate groundwater may suggest that an equitable approach, taking into account all relevant factors and the interests of all affected natural resource trustees, is required to resolve relative NRD trusteeships when there are multiple trustees for the same groundwater resource.
 Mississippi v. Tennessee, 595 U.S. ___ (2021).
 Id. at *9-10 (quoting Wyoming v. Colorado, 259 U.S. 419, 466 (1922)).
 Id. at *9 (quoting Hinderlider v. La Plata River and Cherry Creek Ditch Co., 304 U.S. 92, 102 (1938)).
 Id. at *11.