The question will be up to the U.S. Supreme Court after a federal appeals court, affirming the decision of a lower court, dismissed the suit in June because it is an interstate issue. Mississippi had said it had no quarrel with the state of Tennessee, but the Fifth U.S. Circuit Court of Appeals ruled that Tennessee is in fact an "indispensable party" to the suit.
Circle of Blue provides some background about what must happen before the Supreme Court can hear the case:
Before the Supreme Court case can begin, a study must be performed to determine how much of the aquifer belongs to each state, since the underground lake system is fluvial and dynamic. The federal common law doctrine "equitable apportionment" determines how resources such as water and wildlife are allotted to neighboring states.
To date, the Supreme Court has never applied this doctrine to an aquifer and its groundwater-although it has ruled for fluvial rivers like the Laramie River in 1922, the Delaware River in 1931, and the North Platte River in 1945, as well as hearing the current case between the Carolinas for the Catawba River.
The case is particularly important for Mississippi because the state draws some 89 percent of its public water from underground sources. Mississippi Atty. Gen. Jim Hood has said in court documents that Memphis is "the largest city in the world
that relies solely on groundwater wells for its water supply." However, statewide, Tennessee sources only 36 percent of its water from the ground, according to Circle News.
Another factor, as this Business TN article explains, is that DeSoto County's growth has prompted fears of a water shortage.
Here's where this suit could get expensive for Memphis and MLGW -- beyond the $1 billion damages that Mississippi claims.
Memphis and MLGW argue that the withdrawals are appropriate and within legal limits. If they lose, the utility will be forced to build a multi-million dollar water treatment plant for subsequent future withdrawals from the Mississippi River. (emphasis added)
Mmmm, fresh, clean Mississippi River water ...