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Update 6 to: A Dispersion Modeling Analysis of Downwash from Mirant’s Potomac River Power Plant Modeling Baseload Units 3,4,5

Docket No. EO-05-01: This report describes dispersion modeling performed for simultaneous operation of three baseload units at Mirant’s Potomac River Generating Station (PRGS). The units (3,4,5) would operate at maximum load (107 MW) for up to 12 hours and minimum load (35 MW) 12 hours or more in a calendar day. This mode of operation is also referred to as Option B in Mirant Potomac River LLC’s December 30, 2005 letter to the U.S. Department of Energy regarding District of Columbia Public Service Commission, Docket No. EO-05-01. The modeling was performed according to the Protocol approved by the Virginia Department of Environmental Quality. The modeling demonstrates that Option B operations will not cause or contribute to exceedances of the National Ambient Air Quality Standards (NAAQS) for the pollutants PM10 and NO2 . The modeling also demonstrates that Option B operations will not cause or contribute to exceedances of the 3 hour or the annual NAAQS for SO2 and predicts only marginal exceedances of the 24 hour SO2 NAAQS. The 24 hour SO2 NAAQS exceedances are predicted only at the top level and roof of the nearby Marina Towers building and only under a limited set of meteorological conditions. In addition, the marginal exceedances of the 24 hour SO2 NAAQS are predicted to be within the required “margin of safety”.