The control of particulate matter and hydrogeochemistry on uranium–thorium partition coefficients in two representative lakes, Qaidam Basin, China

Radionuclides (including those of uranium and thorium) are commonly used to quantify complex geochemical processes in aqueous environments. The estimation of mass transport fluxes for uranium (U) and thorium (Th) isotopes is plagued, however, by uncertainties predominantly driven by the inability to determine solute–particulate partition coefficients (K_d). Such uncertainties are especially pronounced for lakes in rarely studied, environmentally sensitive areas. This study examines the factors controlling the K_d values of U and Th isotopes in both freshwater (Keluke Lake) and saline water (Tuosu Lake) lakes within the Qaidam Basin, China, a basin sensitive to the impacts of climatic change. The analysis found a negative correlation of both K_d(Th) and K_d(U) with the concentration of suspended particulate matter (SPM), suggesting that particle concentrations affect K_d values in the sandy Keluke Lake. In contrast, there was no significant correlation between K_d values and SPM in the muddy (fine-grained) Tuosu Lake. Compositionally, three types of particles were found in both lakes and were derived from the same sediment sources. The K_d values were primarily controlled by lake evaporation and/or secondary water–rock interactions that were influenced by hydrogeochemical ecosystem differences between the lakes. In addition to differences in particle composition and concentrations, redox potential, ionic strength, organic matter concentration and pH of the lake water were likely to influence the K_d(Th) and K_d(U) values. These results are not only of significance to quantitative studies of U and Th isotopes in water (e.g., reducing transport flux uncertainties and improving the precision of U–Th isotope dating), but also provide an important reference for the application of radionuclides in other climate sensitive regions of the world.