Dissolved iron and isotopic geochemical characteristics in a typical tropical river across the floodplain: The potential environmental implication

Iron (Fe) is an essential element for bio-physiological functioning terrestrial organisms, in particular of aquatic organisms. It is therefore crucial to understand the aquatic iron cycle and geochemical characteristics, which is also significant to obtain the key information on earth-surface evolution. The stable iron isotopic composition (delta Fe-56) of the dissolved fraction is determined in the Mun River (main tributary of Mekong River), northeast Thailand to distinguish the human and nature influenced riverine iron geochemical behavior. The results show that dissolved Fe concentration ranges from 8.04 to 135.27 mu g/L, and the delta Fe-56 ranges from 1.34% to 0.48%, with an average of 0.23%, 0.14% and -0.15% in the upper, middle and lower reaches, respectively. The delta Fe-56 values of river water are close to that of the bulk continental crust and other tropical rivers. The correlations between delta Fe-56 and Fe, Al, and physicochemical parameters show mixing processes of different Fe end-members, including the rock weathering end-member (low Fe/Al ratio and high delta Fe-56), the urban activities end-member (high Fe/Al ratio and moderate delta Fe-56), and a third end-member with probable sources from the Chi River and reservoir. For the most river water samples, the primary contribution is attributed to rock weathering, and the second is urban activities (only a few samples are from the upper and middle reaches). Thus, Fe isotopes could be employed as a proxy to identify and quantify the natural and anthropogenic contributions, respectively. These findings also provide data support for the scientific management of water resources in the Mun River catchment and other large tropical rivers.

Automated summary

Iron (Fe) is an essential element for terrestrial and aquatic organisms, and understanding the aquatic iron cycle and geochemical characteristics is important for earth-surface evolution. The stable iron isotopic composition in the Mun River shows a primary contribution from rock weathering and a secondary contribution from urban activities. The correlations between delta Fe-56 and Fe, Al, and physicochemical parameters can be used to distinguish different iron sources in river water.