What environmental problems could interstate compacts address, and what solutions could they achieve?

Do you mean interstate contracts?

An example would be the Colorado River.  The water from the river is shared by many states including Colorado, Arizona and California.  Historically, California has taken most of the water, because its population needed it decades ago.  Recently, however, the population of Arizona and Colorado has boomed and they are asking for more water from the river.  Water rights and contracts determine how much each state and city can take from the river.  The supply of water has not increased, but the number of people and farmers that rely upon it has.  Therefore everyone gets less and has to be rationed so that everyone gets some.