Author(s): Vivien Fustos; Peter Saly; Zoltan Szaloky; Balazs Toth; Zoltan Vital; Andras Specziar; Gabor Fleit; Sandor Baranya; Janos Jozsa; Tibor Erős

Linked Author(s): Vivien Fustos, Gábor Fleit, Sándor Baranya, János Józsa

Keywords: Ecohydraulics; Cooling water; Field survey; Fish assemblages

Abstract: Aquatic ecosystems are affected by anthropogenic influence in many aspects. One among those is thermal pollution, which may be caused by discharging cooling water of nuclear power plants back to the river downstream of the intake. Research targeting this effect tends to focus more on coastal areas and small rivers, while less is known about the impact on large rivers. The Paks Nuclear Power Plant is situated on the right bank of River Danube in Hungary and operates by once-through wet cooling relying on the river. After usage, the heated water is returned directly into the river with a discharge of 100 m³/s at most. As the mean Danube discharge here is 2300 m³/s, it creates a locally variable aquatic environment, regarding temperature and flow velocities as well. We aimed to investigate the abiotic and biotic (on fish assemblages) effects of the heat tail through seasonally repeated (spring, summer, autumn) field surveys. 8 study sites were appointed on an approximately 10 km long river reach, 3 located upstream and 5 downstream to the output. Hydromorphologic survey included high resolution acoustic water bathymetry, velocity and thermometry measurements, along with substrate sampling. Fish were collected by offshore bentic trawling and near-shore electric fishing. A clear effect was observed on the water temperatures downstream, causing a 3-4°C temperature differential on the right side of the nearest site. This difference decreased slowly and remained at least 1-2°C on the investigated reach, without complete mixing. Another, but much less apparent impact was on the velocity field, mainly at lower flow regimes, when the returned discharge is proportionally higher. Other hydromorphologic variables, such as water depth and substrate did not seem to be influenced by the returned cooling water. Nearly 16400 individuals of 36 fish species were sampled altogether during the survey campaign. The highest offshore fish abundance was observed near the confluence of a side branch and the main channel, downstream from the output, regardless of season. Overall, the assemblage structure showed great variability within the reach, in which the heat tail did not seem to be determinant.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221471

Year: 2022