Sean Lyness, Faculty Fellow, New England Law | Boston
You won’t find the words “public trust doctrine” in the Supreme Court’s November 2021 opinion Mississippi v. Tennessee. But that doesn’t mean the doctrine isn’t there, hiding beneath the surface (figuratively and literally; this case deals with groundwater). And while the case doesn’t change the public trust doctrine’s scope, it does underscore some important facets of the doctrine. After explaining the circumstances of the litigation and the Supreme Court’s opinion, I analyze what this case reveals about the public trust doctrine and how it demonstrates both the promise and limitations of the doctrine.
The Middle Claiborne Aquifer underlies eight states, Mississippi and Tennessee among them. Like all groundwater, the Middle Claiborne Aquifer is susceptible to cones of depression—natural phenomena whereby well-pumped groundwater creates low pressure sites at the wells that draw groundwater from elsewhere. Noting that the City of Memphis had more than 160 wells pumping 120 million gallons of groundwater each day, thereby creating cones of depression that took its groundwater, Mississippi sued to reclaim the allegedly stolen groundwater.
Interestingly, Mississippi filed the original lawsuit (and subsequent invocation of the Supreme Court’s original jurisdiction) based, in part, on its obligations as a public trust doctrine trustee. The public trust doctrine traditionally posits the state as the title-holding trustee of certain natural resources, charged with preventing their impairment. The question, of course, is which natural resources are included in the doctrine. According to Mississippi, groundwater was codified as a public trust asset by state statute in 1985, and that status required Mississippi to sue Tennessee for impairing the public trust.
Not all states have made such claims to groundwater under the public trust doctrine, however. California, for example, has long been associated with an expansive public trust doctrine, but even there a state appeals court recently found that groundwater was part of the public trust only where groundwater extraction harmed a navigable surface water. Additionally, the Minnesota Supreme Court recently declined to extend the public trust doctrine to groundwater. Groundwater as a public trust asset, then, is not well-established.
But Mississippi made its public trust ownership of groundwater a centerpiece of its argument. In its Supreme Court brief excepting to the Special Master’s report, Mississippi asserted that “all groundwater in Mississippi is held by Mississippi in public trust for the use and benefit of its citizens, and it is Mississippi’s duty under the Constitution to protect, preserve, and control its taking for the benefit of its citizens.” Mississippi’s Sur-Reply noted that “Mississippi seeks in this proceeding to discharge its duties as a trustee under the public trust doctrine[.]”
For Tennessee, this focus on the public trust doctrine was beside the point; the main issue in the case was whether the Middle Claiborne Aquifer was subject to the doctrine of equitable apportionment, a judicial means to allocate a shared water resource. Mississippi’s insistent position throughout the litigation was that groundwater was not amenable to equitable apportionment. This hardline position is perhaps more understandable when viewed through Mississippi’s public trust premise; sovereign ownership of a public trust asset does not seem readily susceptible to shared ownership. For Tennessee, however, this case concerned interstate groundwater, and nothing in the public trust doctrine contradicted the longstanding history of using equitable apportionment to deal with interstate natural resource disputes.
The Supreme Court agreed with Tennessee. In a terse, unanimous opinion authored by Chief Justice Roberts, the Court held that the Middle Claiborne Aquifer, as an interstate resource, was governed by the equitable apportionment doctrine. The Court rejected Mississippi’s claim that its sovereign ownership of groundwater conferred unfettered control, noting that permitting such an approach “would allow an upstream State to completely cut off flow to a downstream one[.]” That was that, case dismissed.
To be clear, the Court’s succinct refutation of Mississippi’s sovereign ownership claim did not address the public trust doctrine. But the opinion nonetheless confirms three key points about the doctrine.
First, the Court’s opinion left undisturbed Mississippi’s recognition of groundwater as a public trust asset. In fact, nothing in the oral argument or opinion casts doubt on Mississippi’s sovereign ownership of groundwater within the confines of the state. This is significant given groundwater’s somewhat uncertain status as a potential public trust resource. The Court seemed to condone a state affirmatively asserting control over groundwater as a public trust asset, at least so long as it is within the state’s boundaries.
Second, the Court did not deny the premise of Mississippi’s lawsuit: that Mississippi had obligations as a trustee of public trust resources to preserve those resources. To the contrary, the Court’s repeated emphasis that Tennessee had not physically intruded on Mississippi implies that if Tennessee had done so, Mississippi would have been entitled to act. That tacit assumption strengthens the public trust doctrine’s use as an affirmative doctrine.
Third, and somewhat contradictorily, the case reinforces the limitations of the public trust doctrine. After all, the thrust of Mississippi’s case was that Tennessee was impairing groundwater in Mississippi by pumping in Tennessee. But, to the Court, the groundwater was an interstate natural resource. Any such impairment thus needed to be resolved through equitable apportionment, not the public trust doctrine. However, equitable apportionment requires at the outset a showing of substantial injury by clear and convincing evidence. This is a difficult showing in a surface water case, let alone a groundwater one. For interstate natural resources, then, the public trust doctrine offers little recourse.
As groundwater becomes increasingly scarce and contested, expect more states to follow Mississippi’s tack and make express—either through strategic litigation or state legislative codification—the inclusion of groundwater within their public trust doctrine. At the very least, explicit recognition of groundwater as within the ambit of the doctrine strengthens groundwater protection within a state’s boundaries. And it can hedge against physical intrusions. These are no small feats. But, according to a unanimous Supreme Court, the public trust doctrine is not a panacea for interstate groundwater disputes.
None of these three points changes the scope of the public trust doctrine, at least not in any significant way. But they do underscore the ways that the public trust doctrine can be used—and limited—in furtherance of environmental goals. It only remains, then, for advocates to use these contours to their advantage.
 Mississippi v. Tennessee, 595 U.S. ___ (2021).
 See id. (slip op. at 2).
 See id.
 See id. at 2–3.
 See Envtl. Law Found. v. State Water Res. Control Bd., 237 Cal. Rptr. 3d 393, 397 (Cal. App. 2018).
 See White Bear Lake Restoration Ass’n ex rel. State v. Minn. Dept. of Nat. Res., 946 N.W.2d 373 (Minn. 2020).
 Mississippi v. Tennessee, Exceptions to the Report of the Special Master by Pl. State of Mississippi and Br. in Supp. of Exceptions at 31 (available at https://www.supremecourt.gov/DocketPDF/22/22O143/169578/20210222152919890_MS%20Exceptions%20Brief.pdf) (emphasis in original).
 Mississippi v. Tennessee, Sur-Reply of the State of Mississippi in Supp. of its Exceptions to Report of the Special Master at 6 (available at https://www.supremecourt.gov/DocketPDF/22/22O143/181060/20210607121641394_MS%20v%20TN%20Sur-reply.pdf).
 Mississippi v. Tennessee, 595 U.S. ___ (2021) (slip op. at 10).
 See id. at 11–12.