Industrial polluters will be allowed to buy credits from cleaner competitors to help comply with the Clean Water Act, under a plan released yesterday by the Bush administration. The National Water Quality Trading Policy would allow industrial, agricultural, and wastewater-treatment operations that cannot meet clean water regulations to purchase credits from cleaner facilities in the same watershed. In keeping with the administration’s fondness for incentives-based environmental protection over government regulation, the policy would rely on financial incentives to boost water quality. Some environmentalists welcomed the plan, but others worried that it would contribute to a significant decline in the nation’s water quality, especially because it is just one in a series of steps the administration has taken to weaken water protections.