Intensive research and studies on interaction, transport, deposition, distribution and accumulation of various organic and inorganic pollutants in natural water systems pointed out especially in the last few years the inconvenience of industrial, agrochemical and household waste products.

Industrial waste waters, that contain besides other pollutants also high portion of heavy metals are the worst pollutants of our rivers and brooks. Because of adsorbtion, hydrolysis and coprecipitation only a small portion of free metal ions stays disolved in water. After a while a large quantity of them deposites in sediments. There, microorganisms transform heavy metals (e.g. arsenic, mercury, selenium, cobalt, chromium, copper and zinc) into biologically active or toxic organic and inorganic compounds of these elements which enter the biological cycles.

Because of dynamic circumstances that are present in running waters the concentrations of pollutants rapidly change. Therefore it is very difficult even with continuous analysis to obtain a clear picture of concentrations of pollutants in our rivers. In the case of heavy metals, the problem is highly complex, because matals occur in waters in many different forms and in such low concentrations that it is inevitable to use special analysis and a number of different measuring methods. Since heavy metals accumulate in sediments, these are good indicators of intensity of pollution of the rivers. And because the picture of condition of waters rapidly changes, the analysis of properly sampled sediment, taken on the right place in typical time intervals shows the devastating influence of industrialization as well as imefficiency of purification plants.

Samples were analysed with a method called the Activation Analysis. Radioactive isotopes of a certain element have exactly the same chemical properties as a stable atom. This feature is used with activation analysis. The wanted sample is first irradiated in a nuclear reactor in a flux of thermal neutrons. As the sample is irradiated, it is activated, which means that radioactive isotopes of the element in the sample are generated. For the analysis only short and medium living isotopes are convenient. These are isotopes with shorter half lives, since analysis with these isotopes are more reliable.

After a sample has been irradiated, it is examined in a scintilation counter. The scintilation counters are different: they can be solid materials as well as liquids and gases (e.g. zinc sulphide, naphtalene, antracene, argon, krypton). The two of us used a sodium iodide / germanium detector linked to a 4200 channel analyser when measuring spectra of gamma rays. Each isotope radiates photones with specific energy. In this way we can determine the element that is in the sample taking into account the energy of radiated quanta. By counting the number of radiated photons the quantity of a certain element is determined.