River water provides an essential cooling function for most thermoelectric power stations, but what effect does this have on the river and the species that live in it, and how far downstream do these impacts extend? To address these questions, Robert Stewart from the University of New Hampshire, US, and colleagues modelled the impact of thermoelectric power stations on rivers in the Northeastern US, a region with a high density of power plants. They combined a hydrology and water-temperature model with a thermoelectric power and thermal-pollution model to assess how the excess heat was distributed and what kind of impact this might have on local ecosystems.

Of the total heat generated by thermoelectric power stations in the Northeastern US between 2000 and 2010, the scientists found that one third (34.3%) was converted to electricity, while one quarter (25.9%) was dissipated by cooling towers, and a whopping 28.4% was transferred directly into rivers. Their findings are published in Environmental Research Letters (ERL).

The impact on rivers varied greatly and depended upon the location of the power plant along the river, the magnitude of the heat input and the local climate conditions. Power plants that were closer to the river mouth, such as those on the Hudson and Deleware rivers, had less of a thermal impact than plants situated far upstream, such as those on the Connecticut and Susquehanna rivers. "River-mouth locations are optimal in the sense that their heat impacts do not extend over significant river lengths before being diluted by tidal water," explained Stewart. Of course this location may not be such good news for marine species, but this was not something that the study investigated.

Increases to water temperature were greater in winter (because of the larger difference between ambient river-water temperature and effluent temperature), but stretched further down the river in summer, due to reduced efficiencies of the river system to re-equilibrate water temperatures during that period.

Perhaps counter intuitively, Stewart and his colleagues found that the thermal pollution has a greater impact on warm-water fish than cool-water species. "This is because conditions are already marginal for cool and cold-water species in this region's large rivers and these species find refuge in headwaters and low-order streams that do not receive power-plant effluent," he said.

The power-plant technology was also important. "Once-through cooling" (OTC) technologies had the most severe impacts because of the large amounts of waste heat that they discharge to rivers. "Re-circulating cooling towers" (RCC) consume significantly more water but have a lower thermal impact on the river because of the relatively cool temperature of the effluent they release. "The impacts these cooling technologies have on aquatic ecosystems are likely to be far more severe in regions that are less water-rich than our region of study, such as the Mediterranean," Stewart told environmentalresearchweb.

In a companion paper in the ERL Focus on Electricity, Water and Climate Connections, led by Ariel Miara of the City College of New York, the authors explored how increases in energy demand and changes to climate will impact thermoelectricity production and the rivers it relies on. "We found that projected changes to climate result in reduced river-water available for thermoelectricity operations during the summer period, when electricity demand is relatively high," said Stewart.

Based on their findings, Stewart and his colleagues think that it is time to take stock and consider the impact of power production on rivers and their ecosystems. "The full ancillary costs associated with relying on rivers to buffer heat loads from power plants should be considered when deciding how to meet future energy demands," he said.

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