In a unanimous decision this week, the United Stated Supreme Court denied Mississippi’s claim that the State of Tennessee was stealing Mississippi’s groundwater. The decision represents a first step in settling a years-long dispute related to the Middle Claiborne Aquifer, a reservoir spanning thousands of square miles beneath eight states, including Alabama, Arkansas, Illinois, Kentucky, Louisiana, Mississippi, Missouri, and Tennessee. In the suit, Mississippi complained that Tennessee, through the City of Memphis, pumped so much water from the Middle Claiborne Aquifer that it created a cone of depression extending across state lines, into Mississippi’s territory, causing “billions of gallons of groundwater” to cross the border from Mississippi to Tennessee. This water, Mississippi contended, would have otherwise existed under Mississippi for thousands of years. Mississippi sought over $600M in damages for this “tortious taking” of Mississippi property.

In a case of first impression, the Supreme Court determined that the Middle Claiborne Aquifer is subject to the doctrine of equitable apportionment, which aims to fairly allocate shared water resources between two or more states. The doctrine has traditionally been applied to other forms of interstate water resources such as rivers and streams, but has never before been applied to underground water resources. Noting that the aquifer is a “single hydrological unit,” albeit flowing slower than a traditional river, the Supreme Court determined that equitable apportionment should still apply.

The Court did not determine how the Middle Claiborne Aquifer would be equitably apportioned among the states, only that it would be subject to equitable apportionment. This leaves the door open for future litigation over apportionment of the aquifer, which the Court noted would require the participation of all states under which the aquifer flows.

If Mississippi does seek equitable apportionment, the state will face a heavy burden. As the Supreme Court determined last term in Florida v. Georgia, the state seeking apportionment of water resources must demonstrate that the other state’s water use is causing the complaining state “significant injury.” As the Florida case showed, proving that level of injury will be difficult.

Read the Supreme Court’s opinion here.