Anomaly And Causes Of Soil Radon In Karst Area

The existence of significant prominence of radon concentration has been found in soils derived from the development of poor uranium(U)and radium(Ra)carbonates.It is generally believed that the input of radon from deep exogenous sources along tectonic fissures leads to soil radon anomalies in karst areas.However,the group’s preliminary research found that the weathering of U-poor carbonate rocks by soil formation led to significant enrichment of uranium in the soil.This phenomenon implied that bedrock may also provide a potential source of soil radon anomalies.However,there have been no related studies reported.This paper used the Hunan and Guizhou areas of the southwest karst region as the study area.Soil radon concentration was measured at a total of 18 points on the region,and soil radon from silicate rock development was measured at 6 points as reference.Through the analysis of Ra content in representative soil samples,converted into theoretical soil radon concentration compared with the measured radon concentration,the potential of soil Ra as a source of soil radon was studied and judged.On this basis,an in situ carbonate weathering profile was selected to reveal the release/enrichment behavior of Ra during the weathering of carbonate rocks into soil.The results of the study were as follows.(1)Among the 18 points measured in the karst area,the soil radon concentration in 16 points were higher than the average value of soil radon concentration in China of 7300 Bq/m~3,with the highest value of29500 Bq/m~3 and higher than the soil radon concentration in the non-karst area,and indicated that the soil radon anomaly in the karst area was universal.(2)The theoretical soil radon concentrations converted from soil Ra content were all significantly higher than the measured soil radon concentrations,it showed that soil Ra has the potential to be used as a source of soil radon material.(3)During the weathering process of Ra-poor carbonate rocks into soil,through the rock-soil interface,on the one hand,along with the full dissolution of carbonate,a large amount of Ra was leached out(loss rate of 59%).On the other hand,carbonate dissolution led to a strong reduction in the volume of weathering residues,and therefore to a concentration of Ra in the soil layer,which increased significantly from bedrock to the soil layer at the rock-soil interface(12.9 Bq/kg→152.2Bq/kg).The further changes in Ra were a long and slow process in the evolution of the soil profile.(4)A significant positive correlation was observed between Ra and Mn O in the soil profile(R~2=0.766).And there was no significant correlation with the rest of the main elements and organic matter.Both of them had similar content distribution characteristics in the profile,and indicated that Ra was mainly in the mineral phase composed of Mn O during the weathering of carbonate rocks into soil.(5)During the weathering of carbonate rocks into soils,there was chemical fractionation between uranium-radium,and equilibrium could not be achieved.In the early development of the soil profile,the uranium-radium equilibrium was enriched in radium;while with the upward movement of the profile,the soil uranium-radium equilibrium was enriched in uranium.