Seasonal Variations of Uranium in Karst Waters from Northeastern Sichuan, Central China and Controlling Mechanisms

Abstract—: Uranium (U) (or U/Ca ratio) in speleothems has been suggested to be an indicator of soil humidity and effective rainfall in the Asian summer monsoon region. However, this argument has not yet been confirmed in modern cave monitoring studies. A 3-year (July 2011–August 2014) cave monitoring study was conducted to explore the seasonal variations of U in karstic waters (including cave water and river water) from Northeastern Sichuan, Central China and to gain a better understanding of the processes influencing the U variations in these waters. With the exception of some pool waters, U concentrations and U/Ca ratios of cave and river waters exhibit seasonal variations with relatively higher values occurring in the winter half year and lower values in the summer half year, anti-correlated with the seasonal variations of local rainfall amount. Soil redox conditions, which are controlled by soil moisture and hence effective precipitation, may have a significant influence on the seasonal variations of U in waters. We demonstrate that speleothem U concentrations and/or U/Ca ratios could serve as a potential proxy, at least at the study site, for the regional effective rainfall.