Employing a use-attainability analysis to modify NPDES permit provisions

Rocky Flats Environmental Technology Site (Site) is a 6000-acre facility located 16 miles from the center of Denver, Colorado. Rocky Flats is part of the Department of Energy's (DOE) nuclear weapons complex, and was operated by its prime contractor, EG ampersand G Rocky Flats, Inc. (EG ampersand G). The Site is currently undergoing extensive environmental restoration. To serve the Site population of 7,000 workers, Rocky Flats has an activated sludge wastewater treatment plant with tertiary treatment discharging 0.1 to 0.2 MGD. The WWTP discharges into the head waters of Big Dry Creek, which flows eastward to the South Platte River, the major drainage of northeastern Colorado. As a result of intense public scrutiny in 1989, the state of Colorado adopted the most stringent stream standards available based on use classification. These standards include 0.1 milligrams per liter (mg/L) for unionized ammonia, a standard used by the Environmental Protection Agency (EPA) to propose that the WWTP discharge no more than 1.0 mg/L total ammonia to meet the stream standard. This effluent limitation would require that DOE fund the installation of ammonia removal treatment at the WWTP, at high cost and with little improvement to the environment. In 1994, DOE and EG ampersand G undertook a Use Attainability Analysis of Big Dry Creek in accordance with federal guidance. It was expected that the receiving waters would not demonstrate that they were meeting the aquatic life use classification assigned by the state, and that a request for the removal of that classification would be successful. Samples for water chemistry were collected beginning in the spring, and biological observations were made throughout the same period. Rather than impaired segments, the Use Attainability Analysis identified sensitive aquatic organisms, and found excellent water quality downstream from the Rocky Flats boundary. Also, ammonia does not persist in the segment at the current levels of discharge from the WWTP