Chemical and magnetic tracing of coal slag pollutants in karstic river sediments

The Mrežnica River near Duga Resa (Croatia) serves as an ideal ”natural laboratory” for studying downstream transport of material in river systems. This study expands the so-far brief knowledge about magnetic and chemical properties of sediments along the lower course of this river, contaminated by discharge of coal slag and ash from a former textile factory, directly released into the river for 110 years (1884–1994). Magnetic susceptibility of the river sediments is very high close to the pollution source and shows a decreasing trend downstream, still being above the level of unpolluted sediments just before the confluence with the Korana River app. 7 km away from the point source. Thermomagnetic curves reveal magnetite as the dominant magnetic mineral, and low-frequency-dependent susceptibility indicates an anthropogenic origin. Most heavy metals and other elements in the sediments show a decreasing trend in the downstream direction, similar to the magnetic concentration signal. Correlation analysis between magnetic susceptibility and concentrations of 29 elements shows strongest correlations with Co (R = 0.70), Fe (R = 0.92), Sr (R = 0.86), Al (R = 0.74), U (R = 0.78) and B (R = 0.96). Cluster analysis distinguishes three clusters: Cluster 1 is under strong influence of contamination and contains four sampling sites; Cluster 2 has extreme values of Cd, Cr, Mo and Ni and contains seventeen sites; Cluster 3 reflects rather clean sites and contains nine sites. According to existing sediment quality criteria, Ni and Cr show the largest degree of contamination among six evaluated heavy metals. Strong variability in the data and frequent extremes are to a large degree caused by heterogeneity in the spatial distribution of the coal-derived material. To account for this situation and to resolve the variability in more detail, statistical sampling of sediments in river cross sections and estimation of the sediment volume are needed, being a target for further research.