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MERCURY CONTENTS IN WATER, SEDIMENT AND TUFA IN KARSTIC KRKA RIVER, CROATIA

Željko Kwokal

Abstract

The Krka River is a pristine karstic stream and unique model system for the understanding and interpretation of environmental records in tufa precipitating rivers worldwide. The river is characterized by 7 main and numerous small tufa barriers grown in a constant and dynamic process depending upon many physicochemical factors. An evaluation of the quality status of the pristine karstic, tufa depositing aquatic environment of the Krka River National Park based on the analysis of mercury contents was performed for the first time. The aim of the study was to compare total mercury concentrations obtained in surface sediments with those in the recent tufa. Moreover, mercury concentration in tufa and sediments was compared with those in ambient water. Research was undertaken during three years, and analyses of mercury in water, sediment and tufa were conducted by cold vapor atomic absorption spectrometry (CVAAS). The concentrations of mercury in water were very low revealing a pristine aquatic environment. During the three year research period, mercury concentrations in water varied from 0.5 ngL-1 to 17 ngL-1. In the surface sediments concentrations varied from 0.01 to 0.13 mgkg-1 dry weight. Mercury concentrations in the sediment column vary from 0.07 in the deepest layer to 0.13 mgkg-1 dry weight in the surface layer, indicating a continually increasing anthropogenic pressure during the last fifty years. In recent tufa, mercury concentrations varied from 0.008 to 0.092 mgkg-1 dry weight. Box plot statistical analysis was performed, which found only one anomaly of lower degree (outlier) in sediment of location K-11. Cluster analysis of Q-mode was performed on dataset consisting of sediment and tufa data. Two clusters were obtained: Cluster 1 with mean Hg-concentration of 0.075 mgkg-1 and Cluster 2 with mean Hg concentration of 0.024 mgkg-1. It is obvious that Cluster 1 is under some anthropogenic influence, while Cluster 2 is rather clean. Majority of samples of Cluster 1 are located in area around Brljan lakes and is assumed that this Hg-contamination originates from industry and waste water of the Knin town. It seems that self-purification mechanism exists in Krka River, as elevated Hg-concentrations are not present downstream Ro?ki slap waterfall and all samples in the downstream part of the river belong to anthropogenically clean Cluster 2.

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