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November 30, 2021  |  Written by Michelle E. Chester

U.S. Supreme Court Broadens Equitable Apportionment Doctrine to Include Groundwater

The United States Supreme Court unanimously decided its first interstate groundwater case, finding that Mississippi must rely on the doctrine of equitable apportionment to invoke the Court’s original jurisdiction for relief related to allegations of excessive pumping of groundwater by Tennessee. See Mississippi v. Tennessee, 595 U.S. ___ (2021). Equitable apportionment is a judicial remedy intended “to produce a fair allocation of a shared water resource between two or more states,” and “[t]raditionally, . . . has been the exclusive judicial remedy for interstate water disputes, unless a statute, compact, or prior apportionment controls.” The Court has previously applied the doctrine of equitable apportionment to interstate rivers and streams, interstate river basins, and even where the pumping of groundwater has affected the flow of interstate surface waters—but never before addressed whether equitable apportionment applies to interstate aquifers.

Mississippi brought an original jurisdiction action in the United States Supreme Court in 2014 against the State of Tennessee, the City of Memphis (Memphis), and the Memphis Light, Gas & Water Division (MLGW) seeking damages and other relief related to the pumping of groundwater from the Middle Claiborne Aquifer by Memphis’ public utility, MLGW. The Middle Claiborne Aquifer underlies Mississippi, Tennessee, and six other states, spanning tens of thousands of square miles. MLGW pumps approximately 120 million gallons of groundwater from the Middle Claiborne Aquifer each day from over 160 groundwater wells in and around Memphis—just north of Mississippi.

Mississippi’s complaint alleged that MLGW’s pumping altered the historic flow of groundwater within the Middle Claiborne Aquifer, “allowing Memphis to take billions of gallons of groundwater that otherwise would have remained under Mississippi for thousands of years.” Mississippi sought $615 million in damages on allegations of a tortious taking of property by Tennessee. Mississippi  asserted that it has an absolute ownership right to all groundwater beneath its surface, including any groundwater that was once in Mississippi but flowed outside of its boundaries. Importantly, Mississippi expressly stated that the doctrine of equitable apportionment did not apply in these circumstances.

Chief Justice Roberts, writing on behalf of the Court, rejected Mississippi’s ownership approach and opted to apply the doctrine of equitable apportionment based on three criteria. First, the Court has applied equitable apportionment where transboundary resources are at issue. The Court noted that  “[t]he Middle Claiborne Aquifer’s ‘multistate character’ seems beyond dispute.” Second , like all of the waters at issue in the Court’s equitable apportionment cases, the Middle Claiborne Aquifer “contains water that flows naturally between the States.” Mississippi contended that the aquifer was distinguishable “because its natural flow is ‘extremely slow,’” but the Court noted that the aquifer’s transboundary flow of one or two inches per day still amounted to over 35 million gallons per day and over ten billion gallons per year and, therefore, “does not place the aquifer beyond equitable apportionment.” Lastly, the Court acknowledged that pumping in Tennessee has affected groundwater storage in Mississippi and “[s]uch interstate effects are a hallmark of [the Court’s] equitable apportionment cases.” “For these reasons,” the Court concluded, “we hold that the waters of the Middle Claiborne Aquifer are subject to the judicial remedy of equitable apportionment.”

Because Mississippi expressly rejected the doctrine of equitable apportionment and never requested leave to amend its complaint to seek equitable apportionment, the Court dismissed Mississippi’s suit and denied leave to amend. The next stage of this litigation is uncertain—if Mississippi wishes to further pursue its claims, it must  request leave of the United States Supreme Court to file a new complaint seeking equitable apportionment under the Court’s original jurisdiction.

The Court’s full opinion can be found here. For additional information on this case or other interstate water cases, please contact Michelle Chester at

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