In the last 10–15 years in some German drinking water reservoirs an increase in the NOM (natural organic matter) content has occurred. The impact of such a change on the drinking water quality was almost unknown. A research project was carried out at several drinking water reservoirs and water works concerning the change in the NOM quality and the nutrient situation. The results indicate that the NOM increase in the reservoir is predominately caused by an increasing input of high and intermediate molecular weight humic substances from the catchment area into the reservoir. The fractions of the polysaccharides and the high molecular weight humic substances are significantly reduced in the treatment process. In contrast, the elimination rate for the substances with an intermediate and low molecular weight was low. Due to the low elimination rate particularly of the intermediate humic fraction, the NOM content of the treated water increased with the change in the raw water. Furthermore, the BDOC of the treated water increased with the NOM content. For the AOC of the treated water no relationship was found with the NOM content. However, the change of this parameter was affected by the snowmelt and the circulation in the reservoir.

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