On June 1st, 2006, the U.S. Environmental Protection Agency (EPA) issued a directive called the Administrative Compliance Order (ACO) that allows Mirant, during non-emergencies with both Pepco 230-kilovolt lines in operation, to operate the 482-megawatt Potomac River Generating Station at near normal levels, as long as the plant meets the National Ambient Air Quality Standards (NAAQS) in the greater Washington, D.C. and Northern Virginia area.

On the following day, the U.S. Department of Energy (DOE) issued a directive requiring that Mirant comply with the ACO and increase generation to ensure an adequate level of electric reliability in the Central Washington, D.C. area.

In simple terms that means during non-emergency times, as long as the plant meets the NAAQS, the Potomac River facility can operate as many units as needed to ensure an adequate level of electric reliability for the region.

Which emissions does the ACO address?

The ACO specifically addresses sulfur dioxide (SO2), particulate matter 10 (PM10), and nitrogen oxides (NOx), the three emissions that predictive modeling identified as potentially exceeding the NAAQS. SO2 is the focus of the order, as the predicted levels of SO2 were the highest in the modeling. Also, the EPA determined that the measures taken to reduce SO2 will also reduce emissions of PM10.

What is different about how the plant is operating now?

Under the ACO, Mirant is undertaking a Model Evaluation Study (MES) for a 12-month period using the EPA-approved AERMOD computer model. The MES allows Mirant to operate using day-ahead forecasted weather data instead of historical worst-case five year data. As part of the MES, Mirant has installed six ambient air monitors in the vicinity of the plant. The monitor locations were dictated by the EPA, with input from the Virginia Department of Environmental Quality (DEQ). The air monitors will trigger an alarm at the Potomac River Control Room if the actual average measured concentration of SO2 at any monitor is equal to or greater than 80 percent of the NAAQS SO2 limits. Operations at the plant will be adjusted accordingly to mitigate any potential NAAQS exceedances.

The results of this study will be used to determine the best performing model for predicting ambient air quality impacts from plant operations. Mirant is reporting monitoring results monthly to the EPA and the DEQ, and is working closely with the agencies to resolve any issues that may arise during the course of this study, including differences between the actual data versus predicted and modeled data. In the event of predicted or modeled NAAQS exceedances, Mirant will comply fully with the ACO.

How frequently does Mirant monitor emissions?

Mirant continuously monitors emissions whenever the Potomac River Generating Station is operating. There are six real-time monitoring stations collecting data at various locations surrounding the plant.

Will Mirant continue to use trona?

Mirant will continue to use trona, as necessary, as a means to manage SO2 emissions. Trona has been used for over 20 years, and it is a proven and successful emissions reduction technology, especially for SO2. Read more about trona»

In addition, the EPA has determined that reducing SO2 emissions also decreases PM emissions (See Section II.18 of the June 1, 2006 Administrative Compliance Order). This is because particulate matter can form in the atmosphere when emitted gases such as SO2 condense. So if SO2 emissions are reduced, so is particulate matter.

PM 2.5 Now Being Measured at Marina Towers

Following through on our agreement to measure levels of fine particulate matter, Potomac River Generating Station has installed a PM2.5 monitor on the roof of the southeast wing of Marina Towers. The first sample will be collected on November 10, 2006.

The monitor, a Thermo Electron Partisol-Plus PM2.5 sequential sampler, is the same model used by Virginia Department of Environmental Quality's (DEQ) ambient monitoring program, and is designed to meet the particulate sampling standards of the EPA. The Partisol-Plus will collect PM2.5 data every third day in accordance with the Federal PM sampling schedule»

Our air quality consultant, ENSR Corporation, will conduct the field sample collection and reporting program. They will issue data on a monthly basis-approximately three weeks after receiving results from the laboratory. Results will be displayed as a concentration in micrograms per cubic meter (µg/m3), and will be compared to the air quality standards for PM2.5 established by the EPA and DEQ.