The problem of diffuse pollution of a major river running through predominantly rural area: the case of potentially detrimental levels of tungsten, silver, and manganese in the Mura River

Our aim was to investigate the diffuse pollution impact on large lowland river, namely the Mura River (Croatia), at nine sites in contrasting periods of high/low water discharges, and to characterize potential pollution sources. Physico-chemical parameters and nutrients were analysed in the water, metals/metalloids (44 elements, including rare earths) in water/sediment, and specific microbial genes in sediments. At several sites, the dissolved Ag (0.200–0.220 μg L −1 ), Mn (133 μg L −1 ), and W (0.800 μg L −1 ) surpassed suggested protective concentrations for aquatic biota (MAC Ag = 0.022 μg L −1 , in new EU WFD proposal; PNEC Mn = 75 μg L −1 , established for freshwaters; MAC W = 0.800 μg L −1 , established in Russia for aquatic systems intended for fishing), whereas high nitrogen (up to 5.8 mg L −1 ) and phosphorus (up to 1.7 mg L −1 ) in water were found across all sites. Sediment enrichment factors were especially high for Mn (up to 3.5), Mo (up to 4.3) and W (up to 4.1). Increased microbial Pb-resistance (high pbrT gene copies, ∼10 −2 gene copies/ rrn copies) was found in the vicinity of the abandoned coal mines, and elevated intI1 gene copies (∼10 −2 gene copies/ rrn copies) throughout the studied river section. The agricultural runoff and the abandoned coal mine drainage were indicated as major contamination sources. Despite the high dilution capacity of the Mura River, the pollution level was detected that can be hazardous for some aquatic organisms. To preserve the rich biodiversity of this UNESCO Biosphere Reserve, further assessment of pollution consequences is highly recommended. • Water/sediment contamination from diffuse sources confirmed in large lowland river. • High total nitrogen and phosphorus levels pointed to agricultural water pollution. • Coal mine drainage confirmed as probable source of high Mo and W in water/sediment. • Dissolved Ag, Mn, W levels, potentially toxic to biota, detected in the river-water. • High microbial int I1 and pbr T gene abundances indicated anthropogenic impact.