The Bureau of Reclamation, saying its 85-year-old job of irrigating the arid West with taxpayer subsidized water is nearing an end, is proposing a major mission change to emphasize water quality and the environment.

The plan is set forth in documents obtained yesterday by the Associated Press and slated to be unveiled by bureau and Interior Department officials at a news conference this morning.

The plan, which includes shifting the agency's headquarters from Washington to Denver, outlines an agenda to move the bureau away from building huge dams and aqueducts to deliver low-cost water that is frequently used to grow surplus crops. Instead, the plan approved by Interior Secretary Donald Hodel calls for a priorities switch by 1998 to emphasize management of dwindling groundwater supplies, cleanup of toxic substances and enhancing water quality.

The blueprint recognizes a basic fact of life facing the agency: it is running out of rivers to dam.

The plan also acknowledges that "major agricultural water and power projects are becoming increasingly difficult to justify from an economic, budgetary and environmental perspective."

"The Bureau of Reclamation must change from an agency based on federally supported construction to one based on resource management," it said. "Projects in the construction program will be of a smaller scale, primarily nonfederally funded and generally will be directed to making the best use of existing facilities."

The plan said the bureau would explore handing over responsibility for some of its facilities to local users and beneficiaries.