Thermally induced biological effects caused by once-through cooling systems: A rationale for evaluation

The thermally induced biological effects of power plants with once-through cooling systems can be evaluated in a logical, scientifically defensible manner. First, we combined a near-field integral model (the Shirazi-Davis model) with a far-field model (the Okubo-Pritchard model) to predict the fields of excess temperature and velocity associated with a hypothetical power plant sited on a river and an estuary, and to establish the time-excess temperature exposure histories resulting from the interaction of an assumed distribution of organisms with these fields. Next, we developed a new thermal response model to assess the thermal effects of these exposures on these organisms.Our thermal response model can be used with data from existing and proposed power plants to estimate what fraction of plankton in waters contiguous to the plant will be exposed to thermal doses greater than those that cause death at any stated level. The model can also be used to aid in the design of once-through cooling systems to keep the mortality rate caused by thermal stresses below any designated threshold. The inputs to the model are the frequency distribution of time-excess temperature histories experienced by particular plankton (the Representative Important Species), thermal resistance curves for those organisms, and the spatial and temporal variations of the natural temperature of the receiving waters.To illustrate, we applied our thermal response model to three important aquatic environments of New York State, Long Island Sound, the lower Hudson River, and Lake Ontario. The Representative Important Species were identified for each of these environments and the thermal resistance data for their early (entrainable) life stages were reviewed. Fromall of the thermal resistance data available for any of the entrainable species composing these three communities, we constructed community thermal resistance curves and showed how they could be combined in graphic form with the distribution of thermal doses computed from the physical models and an assumed distribution of organisms in the receiving waters. For each community it appeared that the most sensitive organisms were the ichthyoplankton and juvenile fish; therefore, protection of these organisms from thermal stress should protect the entrainable populations as a whole.Finally, we made several recommendations for future research so that the full management potential of our approach can be achieved.