Potassium and its isotope behaviour during chemical weathering in a tropical catchment affected by evaporite dissolution

This work presents the systematic investigation of K contents and stable K isotopic compositions of surface waters, groundwater, wastewaters, suspended particles, bed sediments, and fertilizers in the Upper Mun Rive catchment, northeast Thailand. This area is of particular interest because its abundant potash deposits and intensive agricultural activities and urbanization, therefore this will improve our understanding whether the K isotope is robust enough to resist evaporite dissolution and anthropogenic disturbances on tracing silicate weathering in the highly weathered tropical region. The dissolved loads in surface waters and shallow groundwater display the large variation in 641Kdiss values from -0.54 parts per thousand to +0.09 parts per thousand relative to suspended particles (-0.60 parts per thousand to-0.41 parts per thousand), river bed sediments (-0.54 parts per thousand to-0.47 parts per thousand the upper continental crust (UCC,-0.44 +/- 0.05%), which agrees with the fact that 39K is preferentially retained in weathering products during silicate weathering. The wet and dry seasons difference ranging from-0.05% to +0.10% 641K, which is slightly greater than our long-term analytical uncertainty of +/- 0.06%. There is likely a limited K input from evaporites dissolution due to weak correlation between delta K-41(diss) values and Cl concentrations or Cl/K ratios in the UMR and world rivers, while a major tributary Takhong River is clearly influenced by the contribution of domestic sewage with lower delta K-41 and higher delta N-15 values. Based on a mass balance of K budge, the dissolved K in the UMR is mostly (>90%) derived from silicate weathering in the unpolluted area, this is consistent with previous studies of large river basins, and the evaporite dissolution and other non-silicate sources may not strongly influence dissolved K and delta K-41(diss) values in rivers, even in evaporite-rich catchment. Therefore, K isotopes cannot be used as lithological tracers in catchments, rather the K isotopic fractionation mainly occurs during silicate weathering, such as the formation of secondary minerals which favor light isotope. Hence, the mechanism of K isotopic fractionation linked to secondary minerals K uptake needs further consideration. Overall, these results clearly show that K isotopes could be an ideal indicator to constrain silicate weathering processes and anthropogenic inputs at catchment scale. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

This study systematically investigated the K contents and stable K isotopic compositions of various water and sediment samples in the Upper Mun Rive catchment, northeast Thailand. The results show that K isotopes can serve as an ideal indicator to constrain silicate weathering processes and anthropogenic inputs at the catchment scale.