Since the early 1970s, water-cooled power plants have installed fish protection technologies largely on a case-by-case basis with limited guidance from the U.S. Environmental Protection Agency (EPA). In 2004, the EPA published rules (the Rule) for implementing Section 316(b) of the Clean Water Act (CWA) at existing plants. The Rule uses national performance standards (a reduction in impingement mortality of 80-95% and, in some cases, a reduction in entrainment of 60-90%, compared to a baseline) as a metric for measuring the effectiveness of compliance options. The Rule is technology-based. All of the compliance options require an examination of cooling water intake system (CWIS) design and operational measures to demonstrate that the protection of aquatic organisms will meet the performance standards. Most of the compliance alternatives require a detailed evaluation of alternative technological and/or operational measures to determine the feasibility, effectiveness, and cost of compliance. Until recently, the mechanisms of fish impingement, collection, spray washing and return to their source water body have not been examined individually in detail. Rather, the entire process has been combined and reported as "fish survival rate." Extensive laboratory and field studies conducted over the past five years have provided great insight into the causal mechanisms of injury and stress to fish from each part of the process. Visual observations during these studies have caused us to realize that many of the assumptions that we, as researchers, have made about what is happening were inaccurate. These assumptions relate to what "impingement" actually is, how velocity and impingement duration affect survival, how important spray wash pressure is to injury, and how injurious collection/return systems are to fish. The results of these combined studies allow us to define conditions needed to achieve survival levels consistent with the §316(b) impingement mortality performance standard.
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