The US Supreme Court Monday unanimously sided with Tennessee in a groundwater dispute with Mississippi.

The dispute arose out of a Mississippi suit which alleged that the city of Memphis, Tennessee had wrongfully taken water from the Middle Claiborne Aquifer. The aquifer, which spans eight states in the region, is an underground water source which allows for extraction of water.

The key issue was that water lies beneath the boundaries of multiple states. As a result, water can be pumped from a well in Tennessee but come from another state. Therefore, states may feel aggrieved, as Mississippi did, when another state takes water from within their boundaries.

The court, in a decision written by Chief Justice Roberts, stated that Mississippi is not entitled to exclusive ownership over the water within its boundaries. The state must abide by the doctrine of equitable apportionment. The doctrine, which governs other types of shared water resources, attempts to work out disputes under the assumption that the water is a shared resource.

The case is significant because the court has never explicitly ruled that underground water is subject to the doctrine of equitable apportionment. Interstate groundwater exists throughout the United States and future disputes will now be governed by this precedent. Importantly, however, the dispute is not entirely resolved.

If Mississippi seeks relief in the case, it must now file under equitable apportionment. In doing so, the state must prove that Tennessee’s use of the water is causing Mississippi significant injury. While it is unclear if Mississippi will file a future equitable apportionment suit, the court has laid the groundwork which will govern the issue.