The Doctrine of Equitable Apportionment Applies to Underground Interstate Aquifers In Mississippi v. Tennessee, No. 143, Orig. (Nov. 22, 2021), the state of Mississippi sued the state of Tennessee for damages and declaratory and injunctive relief, alleging that Tennessee’s mechanical pumping of groundwater from the Middle Claiborne Aquifer (the “Aquifer”) had taken hundreds of billions of gallons of water that were once located beneath Mississippi that would never, under normal, natural circumstances, have been drawn into Tennessee. The Aquifer underlies portions of eight states and Mississippi argued that the City of Memphis, through its public utility, altered the historic flow of groundwater within the Aquifer. The Doctrine of Equitable Apportionment The United States Supreme Court explained that under the doctrine of “equitable apportionment,” the Court allocates rights to a disputed interstate water resource after one State sues another under the Court’s original jurisdiction. Equitable apportionment supports the principle that states have an equal right to make reasonable use of a shared water resource and aims to produce a fair allocation of the shared water resource. Equitable apportionment is the exclusive judicial remedy for interstate water disputes unless a statute, compact, or prior apportionment controls. Equitable Apportionment Applies to Underground Interstate Aquifers The Court noted that it had never considered whether equitable apportionment applied to interstate aquifers but determined that equitable apportionment of the Aquifer would be sufficiently similar to past applications of equitable apportionment and thus should apply. The Court found that the Aquifer contains water that flows naturally between Mississippi and Tennessee and that Tennessee’s mechanical pumping contributed to a cone of depression that extended miles into northern Mississippi and had affected the portion of the Aquifer that underlies Mississippi. The Court noted that an equitable apportionment case would require it to consider a broader range of evidence and may require the joinder of additional parties as other states rely on the Aquifer for groundwater. Disposition The Court dismissed the case, noting that Mississippi did not seek leave to amend or put forth a proposed complaint seeking equitable apportionment.