Intermittently working hydropower plants create a periodic change of the water regime of the water body concerned, leading to a very short term change from high water discharge to very low water regime.
Laboratory tests have proved lindan and different PCB to be hormone-active. Ecosystems polluted with such substances may interfere with the reproduction and development of the fauna. In the North Sea, a shift in the sex-ration in favour of female individuals has been observed for distinct fish species.
Hexachlorobenzene (HCB) arose as by-product, above all, when producing pentachlorophenol (PCP) and was used as softening agent and fungicide. Even though, in the Rhine bordering states, the application of this persistent organic pollutant has been forbidden for a long time and though it is no longer being produced, this substance is still detected in the Rhine sediments and in eel, above all on the Upper Rhine.
Polychlorinated biphenyls (PCB) were formerly used as softening agents in plastic, in transformers and were a compound of hydraulic fluids. They are persistent and accumulate in the food chain as well as in sediments. Some eel from the Rhine with a high fat content are still too contaminated. Apart from the substances mentioned, further synthetic organic substances, e.g. micropollutants may pose a problem. Thus, the surveillance of Rhine water quality remains a permanent task for Rhine water protection.