CHANGES IN THE CONCENTRATION OF HEAVY-METALS IN THE ZALA MINOR BALATON-ZALA SYSTEM (WATER, SEDIMENT, AQUATIC LIFE)

The changes in the concentration of seven heavy metals (Hg, Cd, Pb, Cr, Ni, Ou, Zn) in the water, sediment and aquatic organisms were studied in the River Zala, the largest tributary to Lake Balaton, and in the retention pond (Balaton Minor) created on the tributary. The samples have been collected at four stations, in ice-free periods, at monthly intervals for four years. In the water the mean concentrations of the heavy metals studies are one order of magnitude below the levels mentioned as allowable limits in the WHO recommendations on drinking water. In the sediment the concentration measured for the heavy metals remain below the limit acceptable for plants in soils (except for lead in the samples retrieved from the mouth of the River Zala). The concentration of solid lead and chromium compounds has surpassed in all samples that of the dissolved ones. The concentration of lead and chromium, but also that of nickel and zinc decrease thus perceptibly from the upper reaches of the River Zala towards the outflow from Minor Balaton. The analyses on fish and snails demonstrated the retention of Cu and occasionally of Pb and Hg in the pond, but also that the pollution reaching the pond is subject to seasonal and yearly fluctuations.