Geochemistry of U and Th during formation of limestone soils and their effect on environment

Four limestone soil profiles are selected as the subject investigated in karst region, middle Guizhou Province, China. Through carefully sampling and analyzing, this paper discussed geochemical characteristics of U and Th during formation of limestone soils, and their potential risk on superficial environment. It is concluded as follows: (1) the rock-soil interface is an important place for geochemical reaction, where U is both leached plentifully and markedly relatively richened; (2) when pH is neutral, U released from limestone soil profiles is preferentially complexed with CO₃^2- of weathering solution and further leached; (3) Th is much steadier than U in superficial environment. In the rock-soil interface, Th mainly existing in non-silicate phases is released and then remains in situ. During formation of limestone soils, geochemical behaviour of Th is different between limestone soil profiles for the sake of the diversity of micro-environment in them; (4) limestone soils are important reservoirs as well as secondary releasing origin of U and Th in superficial media. Further development of weathering profiles will likely pose potential radioactive risk on ecological environment. During development of limestone soils, released U becomes dominating contributor of high concentration dissolved U in Wujiang River, but contribution of Th is very limited.