Water quality of the Danube in Hungary and its major determining factors

Three District Water Authorities cover the whole Hungarian stretch of the Danube. Since the formation of the monitoring network fifteen years ago, the water quality has been observed at fifteen sampling points. Throughout this time, sampling has been performed under different hydrometeorological conditions. Based on these investigations, it was found that the quality of the river was influenced equally by the local and the hydrometeorological conditions. Major pollution sources are: the sewage from Bratislava, Györ and Budapest; the paper pulp, chemical, and sugar beet factories in the Slovakian catchment; the Hungarian chemical, petro-chemical, and food industries; and non-point source pollution from agriculture. The effects of these sources depend on the degree of wastewater treatment, and on the mixing rate. The waste loads provide a continuous source of nutrients, giving rise to bacterial proliferation. The organic nitrate and phosphorus loads are increasing, which is compensated for by biodegradation. In the winter, when the water temperature falls below 10°C and solar radiation is low, saprobic conditions characterize the water quality. In the summer, when solar radiation and temperature increase, trophic conditions determine the water quality. Thus, in winter the ammonia content increases, but in the spring, nitrification starts to improve and, especially in the lower reaches, algal overproduction can be detected. This situation changes during flood periods, when the concentration of polluting material is decreased by dilution, and at the same time, the high level of suspended solids inhibits the growth of organisms needing solar radiation. The most unfavourable water quality conditions occur in the winter low-flow period, when problems may occur in drinking water supply if the water is chlorinated.