Association of dissolved organic matter characteristics and trace metals in mountainous streams with sabo dams

Metals undergo various processes transforming from one phase to another during transportation from head streams to oceans. The major chemical processes in the streams and rivers appear to be strongly dependent on dissolved organic matter (DOM). The mutualistic interactions between DOM and metals are important in determining the fate of many elements. We collected surface water samples from four stations along two mountainous streams in 2015 and analysed DOM quality and quantity and metal concentrations with the aim of understanding how changes in DOM characteristics are associated with dissolved metals along the stream with sabo dams. Both DOM and metal concentration were very low at the upstream site and did not show any significant correlation. However, sites within the dam vicinity (St.2 and St.3) had significantly high concentrations of manganese (Mn) and iron (Fe) which strongly correlated with molecular weight and aromaticity (> 0.6, p ≤ 0.01). Fe, Mn, copper (Cu) and aluminium (Al) significantly varied (p ≤ 0.05) downstream with highest average concentration for Al (17.7 ± 6.0 μg/L) and Cu (0.63 ± 0.3 μg/L). Cu had significant correlation with fluorescence components (fulvic-like, humic-like and protein like) and fluorescence index, while Fe significantly correlated to coloured DOM properties for all the sites. In the midstream section, strong correlations were exhibited by Fe, Cu, Zn and Mn whereas all metals did not show any significant correlation in the upstream site. Additionally, Cu, Mn and lead (Pb) showed strong correlation to DOM properties at the downstream site. The Fe to DOC and Mn to DOC ratios gave significant correlations with aromaticity, molecular weight and fulvic-like component at the midstream sections whereas Cu to DOC ratio lacked significant correlations to DOM at all the stations. Sabo dams enhanced Fe and Mn dissolution, while human activities in the downstream impacted Cu, Al, DOC, Mn and Fe stream concentrations. On the other hand, Fe and Mn were closely associated to DOM properties in regions of changing decreasing redox potential. The significant correlations presented by Mn, Fe and Cu with fluorescence index stresses the role of DOM heterogeneity in natural environments.