It also means coal-fired generation at the station will be ceased by the end of 2044.

The Department of the Interior, the Central Arizona Water Conservation District, Navajo Nation, Gila River Indian Community, Salt River Project, Environmental Defense Fund and Western Resource Advocates have signed the agreement.

The agreement, containing a proposed “reasonable progress alternative to best available retrofit technology”, has been submitted to the Environmental Protection Agency for consideration in developing the final BART rule for the NGS.

The NGS is the largest coal-fired power plant in the west.

It is also one of the largest single sources of nitrogen oxide air pollution in the US, contributing to ozone and fine particle pollution in the region – which includes the Grand Canyon and 10 other national parks and wilderness areas.

The NGS is also significant because it provides more than 90% of the power for the Central Arizona Project, which is the state’s primary water delivery system.

The station plays a critical role in numerous tribal economies too.

In February the EPA issued a proposed BART rule that would require the NGS to install selective catalytic reduction technology on each of the station’s three units between 2021 and 2023 to reduce nitrogen oxide emissions.

It would likely have cost NGS’ owners $US1.1 billion.

However, in light of the NGS’ important role in the regional economy, the EPA invited the submission of alternative proposals that would achieve the same or greater benefits.

A technical working group consisting of NGS owners, the DoI, affected tribes and other interested parties came together to develop an alternative proposal.

Under the agreement sent to the EPA, emissions of nitrogen oxide and carbon dioxide will be significantly reduced while also maintaining essential operations at the plant for the foreseeable future.

Consistent with President Barack Obama’s commitment to clean energy and reducing carbon pollution, the DoI will voluntarily reduce CO2 emissions by 11.3 million tonnes or 3% annually, no later than December 31, 2035.

It will be accomplished through measures taken at the plant or through qualifying low-emitting clean energy projects.

Interior will also boost the development of clean energy at a pace and scope to achieve 80% of clean energy by 2035 for the US share in the NGS.

To do this it will secure more than 26.9 million megawatt hours of clean energy projects.

The agreement calls for the DoI to help carry out the national renewable energy laboratory phase 2 study to identify options for replacing the federal share of energy from the NGS with low-emitting energy resources and guarantees that the environmental review for the plant will consider clean energy generation options.

It also contains a commitment by NGS’ owners to cease their operation of conventional coal-fired generation at the plant no later than December 22, 2044.

For people living within 100 miles of the NGS or the Kayenta mine that supplies coal to the station, the agreement provides a $5 million local benefit fund for community improvement projects.

It also promotes the development of clean energy by tribal communities, which includes a federal commitment to work with the Gila River Indian Community to develop a 33MW solar energy facility.

The agreement also includes a commitment by the DoI to provide $100 million over 10 years, beginning in 2020, to provide financial assistance to tribes in Arizona that rely on water from the CAP.