| Groundwater in fractured rocks is mainly controlled by uneven-distributed fractures or faults and their hydrogeological properties are extremely heterogeneous.Traditional methods of obtaining these hydrogeological parameters are costly and difficult to use.Radium and radon isotopes behave as effective tracers for quantitative research on groundwater migration and have advantages on tracing hydrogeochemical processes in fractured aquifers.The method using radium and radon isotopes overcomes the drawback of traditional methods.However,there are limited applications in estimation of hydrogeological parameters of fractured media.Therefore,taking the fracture groundwater system of the Meilin Reservoir area as the research area in this study,we conducted field investigation and sampling,water chemistry and stable isotope(2H and18O)analysis,radioactive isotopes(223Ra,224Ra,226Ra,228Ra and 222Rn)tests.We aim to find the sources of groundwater recharge in the formation and discuss the main hydrogeochemical processes of groundwater based on the temporal and spatial variations of the water chemistry at different locations.The hydrogeological conditions will be analyzed after estimating the hydrogeological parameters based on the radioisotope contents of the water and rock samples.Main results of the study are as follows:1)Stable isotopic compositions of water samples suggested that springs and runoff were mainly recharged from precipitation.Springs and runoff were oxidative and weakly alkaline with the average p H of 7.61.TDS value of springs and runoff was between 23.67?33.41 mg/L,while the content of dissolved solids was relatively low in rainwater.Piper diagram and Scholler diagram intuitively displayed that springs and runoff were HCO3-Na type.Hydrochemical compositions were mainly controlled by silicate weathering and cation exchange reaction according to correlation and principal component analysis,Gibbs diagram,ion ratio and saturation indices of minerals.2)Activities of four radium isotopes fluctuated in the same trend with TDS and major ions,while in the opposite trend with p H and DO.Varation of radium activities was mainly influenced by hydrochemical conditions and water-rock interation.Varation of radon activity was mainly controlled by hydrodynamic conditions and fracture properties.The activity ratio of 224Ra/228Ra in water samples was significantly greater than 1,indicating short residence time of spring water and runoff.The ratio of 224Ra/222Rn was far smaller than 1,revealing strong adsorption for radium isotopes.The ratio of 228Ra/226Ra generally greater than 1 suggested higher recoil rates of232Th decay series compared to that of 238U decay series.3)The hydrogeological parameters were estimated through radium and radon reactive transport model.For S1,S2,&S3,the average residence time was 5.3 d,9.7 d and 2.6 d,respectively.The average effective aperture was 10.75?m,8.22?m and 1.78?m respectively.The average hydraulic conductivity was 9.92×10-5m/s,5.74×10-5m/s and 0.28×10-5m/s respectively.This study provide not only a new powerful tool for the hydrogeological parameters evaluation in bedrock area,but also guidance for development and sustainable use of fractured groundwater in the study area. |
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