Eutrophication of gravel pit lakes which are situated in close vicinity to the river Donau: Water and nutrient transport

AbstractFrom the end of the 1970's until the end of the 1990's, the eutrophication process in six gravel pit lakes situated near to the dammed river Donau, was studied. The gravel pits were dredged between 1956 and 1972. Water levels in the river were over 2 m higher than in the lakes and this resulted in considerable hydraulic pressure from the river to the lakes and, therefore, infiltration of nutrient rich water into the lakes. At the end of the 1970's the total phosphorus concentrations showed a trend with lake I having the highest and lake VI the lowest concentration. Lake I was hypertrophic, lake II eutrophic and lakes III to VI were meso- to eutrophic. This trend can be explained by the infiltration pattern: infiltration was greatest at, and upstream of lake I, and lower downstream and in the other lakes as they were shielded by a concrete barrier. In the 1990's the level of eutrophication in lakes I and II decreased because the phosphorus loads in the river decreased. The level of eutrophication in the other lakes stayed constant or slightly increased in this time due to ageing. The water level in the lakes increased in the course of the years, probably due to the lake bottom and sides becoming more tightly sealed from the sedimentation of autochthonous material. Chloride concentrations in the lakes and the surrounding water were high compared to natural conditions. Chloride concentrations in lake VI increased considerably in the course of the three decades. A drainage ditch runs through this lake. The increase of chloride concentration in this lake probably results from the long term application of road salts during snowy or icy periods, which get washed off roads into the surrounding groundwater and surface water