It is a statement of fact from which the rest of the argument can proceed. It doesn't really function as a premise as it's not part of the argument, but it's the basic information that is needed for the rest of the argument to be necessary. If the government had not spent heavily on toxic chemical cleanup, then the rest of the discussion is pointless. That said, it's not going to fit within a diagram of premises because it doesn't direct the reader toward a logical end. It's simply the starting point.
