to Bechtel Corporation, which began construction on the site in April, 1970. Generating units 1, 2, and 3 were completed in 1974, 1975, and 1976 respectively at a total cost of about $650 million. Between 1977 and 1990, following Clean Air Act amendments for assessing and protecting visibility in national parks and wilderness areas, the EPA, Salt River Project and other entities cooperated in visibility studies to assess possible effects that NGS emissions might have on those areas. The series of studies culminated in the Winter Haze Intensive Tracer Experiment (WHITEX) and the Navajo Generating Station Visibility Study (NGSVS). Those studies indicated that controlling sulfur dioxide (SO2) emissions might improve winter visual range in the Grand Canyon by 2% to 7%, which prompted the EPA to propose a rule requiring NGS to reduce SO2 emissions by 70%. NGS and environmental groups, however, negotiated an approach that would achieve a greater degree of improvement at lower cost. They agreed to, and recommended a requirement of a 90% reduction in SO2 emissions based on an annual average, with installation to be completed in 1999. The EPA accepted the recommendation and implemented those conditions in its final rule. The technology selected for flue gas desulfurization (FGD) were wet SO2 scrubbers with forced oxidation. Stone & Webster, the architect-engineering firm for the scrubber project, began construction in 1994 and completed the work on Units 3, 2, and 1 in 1997, 1998, and 1999 respectively. The cost of the scrubber project was about $420 million. During spring overhauls in 2003 through 2005, the electrostatic precipitators were completely gutted and rebuilt for reliability and optimum performance. The original collecting plates and weighted wire electrodes were replaced with improved collecting plates and rigid discharge electrodes. Control equipment was replaced with upgraded automatic voltage controls and rappers. In 2007, an analysis of nitrogen oxide (NOx) emissions and control options was conducted for SRP in support of Clean Air Act regional haze reduction efforts, although there were no (NOx) reduction requirements on NGS at the time. The analysis concluded that Low NOx burners with separated overfire air (SOFA) would provide the Best Available Retrofit Technology (BART) alternative in accordance with EPA BART guidelines. Consequently, the plant voluntarily installed low NOx-SOFA burners on units 3, 2, and 1 during eight-week outages in February through March 2009, 2010, and 2011 respectively. The Los Angeles Department of Water and Power withdrew from