316(b) Phase II Study

Mid-American Power LLC, a joint venture of Wisconsin Public Service Corp. and Dairyland Power, was planning to replace the 50-megawatt (MW) coal-fired Stoneman Generation Station in Cassville, Wis., with a new 280-MW coal-fired facility. Like the existing facility, the new Cassville Energy Center would use water from the Mississippi River for once-through cooling in the steam condenser.

In February 2002, the U.S. Environmental Protection Agency finalized new regulations implementing Section 316(b) of the Clean Water Act for facilities with new cooling water intake structures. The purpose of Section 316(b) is to minimize the impacts to aquatic biota from entrainment and impingement by cooling water intakes. For new facilities that will not reduce the amount of water withdrawn from a water of the United States to the level obtainable by closed-loop cooling (i.e., cooling towers), the new regulations require the project proponent to conduct a Comprehensive Demonstration Study. The purpose of this study was to show how the proposed facility will reduce its level of impact to aquatic biota to a level equivalent to that which could be obtained by using cooling towers.

• Electrofishing
• Hoopnetting
• Seining
• Ichthyoplankton sampling
• Mussel sampling
• Bathymetric mapping
• Fish spawning habitat mapping

Part of the Comprehensive Demonstration Study is a quantification of the fish and shellfish community that would be vulnerable to entrainment and impingement. Organisms that could be entrained are those small enough to fit through the intake screens, such as the larval stages of fish and mussels. Juvenile and adult fish could become trapped on the screens. Larval fish were sampled using a 0.5-meter diameter, 500-μm mesh plankton net at four stations along a transect across the Mississippi River. At each station, replicate samples were collected at the surface, mid-depth, and near the bottom. A flow meter was mounted in the mouth of the net to provide data for determining the amount of water passing through the net. Samples were collected weekly from late spring through early fall in the morning and again in the evening.

Several methods were used to collect fish. A drag seine was used to collect primarily smaller fish from wadeable areas. A boat-mounted electrofishing unit was used to collect larger fish from along the shore, including locations that could not be seined. Hoopnets were set in offshore areas too deep to be seined or electrofished. The seining and electrofishing were done at night when fish tend to congregate along the shore. The fish collected were identified to species, measured for length and mass, and returned to the river. Adult and juvenile fish were collected monthly from May through September.

Mussels were collected by scuba diver. Quadrates, 50 by 50 centimeters, were excavated at 3-meter intervals along 21 51-meter long transects upstream and downstream of the intake location. The collected mussels were identified to species, measured for length, and returned to the river. The collected data were used to make a map of mussel distribution in the vicinity of the new cooling water intake.

In addition to the biological sampling, a bathymetric map was made of the study area using a boat-mounted, strip-chart recording sonar and GPS. A map of the river bottom substrate was also prepared from data collected using a steel pole to “feel” the bottom and GPS to record the locations of the sampling points.