Who can intervene in cases of "original jurisdiction"?

Courts of "original jurisdiction" are those having priority over other tribunals to decide a case; Black's Law Dictionary defines an "intervenor" as "one voluntarily entering a pending lawsuit because of a personal stake in the outcome." That clear cut it's presumably not always...

A Law.com article this morning reviewed one such matter in one of last Tuesday's Supreme Court's hearings, but "during oral arguments in a water dispute between two states that came to the Court under its "original jurisdiction" over conflicts between states, discussion of the issues led the justices into some broader questions about the nature of original jurisdiction cases and the role of special masters appointed by the Court to assist in their adjudication."

The case, South Carolina v. North Carolina, involves a dispute over the apportionment of water from the Catawba River, which flows between the two states, the article says, but the issue now before the justices is whether three non-state parties – the City of Charlotte, an interstate water supply organization, and a hydroelectric power company-- may join as interveners in the case. A special master appointed by the Court to oversee fact-finding in the case recommended that the parties be allowed to intervene. South Carolina, supported by the U.S. Solicitor General's office, is appealling that recommendation, citing the Supreme Court’s standard set forth in New Jersey v. New York in 1953 that "intervention by a non-state entity is proper only when the putative intervenor demonstrates (1) a 'compelling interest in [its] own right,' (2) 'apart from [its] interest in a class with all other citizens and creatures of the state,' (3) 'which interest is not properly represented by the state.'"

ScotusBlog has more.

Special Master Kristin Linsley Myles’ report
Transcript of oral arguments