In case you missed it while preparing for your Turkey dinner, on November 22, 2021, the United States Supreme Court decided 9-0 that the Equitable Apportionment Doctrine, which had prior to this decision been held to apply only to surface waters, now also applies to interstate aquifers i.e., underground waters. Mississippi v. Tennessee, et al. 
In 2006, the state of Mississippi filed suit against Tennessee, the City of Memphis, and Memphis Light, Gas, & Water division, alleging that the Tennessee respondents were stealing Mississippi’s groundwater by pumping water from the Middle Claiborne Aquifer—water that Mississippi claims would have remained in Mississippi for centuries. Mississippi further alleged that due to the pumping by the Tennessee respondents, the only way that Mississippi would be able to access its water is by drilling deeper and more costly wells. Mississippi asked the Court to establish a special standard for groundwaters, separate from the Equitable Apportionment Doctrine that has been established among states for surface waters.
The Equitable Apportionment Doctrine is applied by courts to disputed interstate waters by balancing each state’s sovereign interests in a waterway by describing how that water will be shared between those states. The underlying principle of the Equitable Apportionment Doctrine is that states have an equal right to reasonable use of shared water resources. Federal common law has applied equitable apportionment in disputes between states over the division of surface waters of rivers and streams passing between states for over a century. However, equitable apportionment had not been applied to underground waters until this recent Supreme Court decision.
The Court held that the Middle Claiborne Aquifer was a single hydrological unit that was interconnected beneath both Mississippi and Tennessee and that water slowly flowed across the border between the states and was thus an interstate water. Interestingly, the High Court, following the opinion of the appointed Special Master, ruled that there was no basis to apply a different standard between surface and groundwater simply because as Mississippi argued, the water in the aquifer “moved slower” than water in rivers and streams. Specifically, the High Court said “the speed of the flow does not place the aquifer beyond equitable apportionment.” The determination that surface waters and groundwater must be treated similarly is significant because the views of hydrologists and their assessments of potentiometric pressure, flow rate, return flow, connection to surface waters, and barriers to flow, form the basis of many water law disputes. Accordingly, states overlying aquifers like the Ogallala that cut across multiple state borders should note the application of the Equitable Apportionment doctrine to subsurface waters regardless of the rate of flow within the interstate aquifer.
 MS v. TN https://www.supremecourt.gov/opinions/21pdf/143orig_1qm1.pdf
 Special Master’s Report https://www.supremecourt.gov/DocketPDF/22/22O143/159926/20201106120944261_No.%20143%20Orig.%20SM%20Report.pdf
 Mississippi v. Tennessee 595 I.S. ____ (2021). https://www.supremecourt.gov/opinions/21pdf/143orig_1qm1.pdf