The effect of natural water temperature variation on the monitoring and regulation of thermal discharge impacts : the role of predictive natural temperature models

Pollution control policies have been an outgrowth of increased awareness that measures must be taken to handle the increasing amounts of wastes and by-products of human activity. A particular problem in the policies is how to address wastes that have large natural variations due to natural sources and changing environmental conditions. This is especially true for the control of thermal discharges from steam-electric generating facilities into large bodies of water also influenced by solar heating and inflows of water from natural sources.
The basis for most pollution control policies in the United States is the set of regulations specifying ambient and effluent standards. Technology-based effluent standards have been increasingly used to provide a conservative basis for environmental protection. Ambient standards, based on impacts on humans or other life forms, however provide a viable regulatory approach for those effluents with costly treatment, particularly where large natural variability indicates the environment has a significant capacity to assimilate additional inputs. A major problem with ambient temperature standards indicated by two case studies of large thermal discharges, is the variability in induced and natural conditions which affect facility siting, design, and operation, and verification of compliance.
The Browns Ferry Nuclear Plant is an example of a large thermal discharge into a varying river environment. The final set of ambient temperature limiting standards for the site which have values near naturally occurring conditions, required the owners of the plant to redesign the heat dissipation system. The final design included the use of supplemental cooling (open, helper, or closed mode) to provide flexible plant operation under varying river flow conditions. Problems with real-time monitoring for compliance with the standards led to a study of various methods of verification. Simulation of plant operation found that adjusting the standards higher than naturally occurring values had larger effects than various monitoring strategies utilizing spatial and temporal averaging. A one-dimensional natural change in temperature model used in conjunction with real-time monitoring reduced power losses due to natural variation by about one half, but could not account for all the short-term variations in natural temperatures caused by topographic and river flow changes and density effects.
The Millstone Nuclear Power Station, located in a coastal environment is an example of a thermal discharge into an area with relatively constant long-term mixing conditions. Concerns over natural temperature variation were present throughout the site's history, although this has not affected plant operation since the ambient standards, based on biological evidence, were set to include full open-cycle operation. A natural temperature model, based on finite element circulation and dispersion models was developed as one means of addressing the natural variability issue. The model produced reasonable resolution of the horizontal temperature distribution and relative changes over a tidal cycle. The model had some limitations in those areas where solar heating significantly affects the vertical temperature distribution. If properly combined with baseline temperature monitoring, the natural temperature model provides an assessment tool for characterizing the physical environment around a thermal discharge. It also has potential in verification of compliance by combining with thermal plume monitoring and modeling efforts to define the ambient baseline conditions and the effects of natural conditions on the extent of a thermal plume.
It is recommended that ambient standards continue to be used in the control of thermal discharges to take into account the natural assimilative capacity of large bodies of water. Real-time monitoring of compliance with maximum rise temperature standards should not be used in areas of high natural variability. Natural temperature models which cannot adequately predict highly variable situations should not be used to correct real-time monitoring efforts. Therefore, flexible effluent standards which adapt to large changing conditions should be used based on modeled plant effects and potential biological impacts. Natural temperature modeling (including extensive monitoring of baseline temperature conditions) in both preoperational and operational studies should be used to provide a balance of the understanding between physical and biological characteristics in complex environments.