Nearly three decades after PCB's [polychlorinated biphenyls] were discovered in the upper Hudson River, General Electric made a binding agreement yesterday to dredge them from the river in one of the largest and most expensive industrial cleanups in history...But there are no guarantees that the $700 million project will go smoothly, because the consent decree splits the cleanup into two phases. While General Electric has agreed to Phase 1, it will not make a decision about the second phase until the first is completed.