| Deep geological disposal of high-level radioactive waste is currently consideredas the most technically feasible method for the final safe isolation of high-levelradioactive waste from human environment. That is, select a natural geological barrierand set multiple engineered barriers in the underground repository in order to achievelong-term or permanent isolation of HLW from human environment. In safetyassessments, groundwater flow is the most important factor in transportingradionuclides released from wastes to the biosphere. Radioactive wastes should beplaced in a site where groundwater flow is very low or stagnant. As the high levelradioactive waste is characterized by highly radioactive, long half-life and high heatrelease, we often focus more on the long time-scale behavior and performance of thesite hydro-geological system in site selection and safety assessment. The chemical andisotopic compositions in groundwater contain a wealth of hydro-geologicalinformation, they can provide the most direct and effective means for the study ofgroundwater recharge sources and residence time, long time scales of regionalgroundwater system characteristics, cyclical pattern and dynamic characteristics of thedeep groundwater.The study aims to reveal the deep groundwater formation mechanism of HLWrepository in Beishan preselected area, and provide a basis for site selecting andevaluation. The thesis is mainly focused on the following aspects:(1) hydrochemistryand isotope distribution characteristics of groundwater;(2) penetration characteristicsof the medium and groundwater recharge;(3) groundwater dating and deepgroundwater formation mechanism research.In geological disposal of high level radioactive waste engineering requirementsand key hydro-geological problem-oriented, the thesis study on the issues about deepgroundwater formation mechanism of Beishan preselected area. In summing up theresults of previous research and progress, and based on regional hydro-geologicalbackground, it mainly through analysis bedrock fractures and permeabilitycharacteristics, groundwater chemistry and isotopic distribution of water, Multilevel groundwater monitoring, application of the chloride ion tracer method in estimation ofgroundwater recharge in unsaturated zone and saturated zone, groundwater datingtechniques to carry out the related research. It focuses on the study of groundwaterrecharge, groundwater cycle and its renewal characteristics, dating groundwater age,and proposed groundwater recharge and flow patterns.This study attains the following achievements:1. This study adopts the international advanced technology of the double-packerhydraulic test system, by pumping test for nearly four months and using the isolationof double packer, water-sampling campaign was conducted in the sections of330m～340m and390m～400m. The real groundwater samples were taken from differentdepth in deep environment of BS15, and the chemistry of groundwater, isotopes andnoble gas were tested.2. The permeability characters under different geomorphic units wereobtained by permeability tests, the permeability coefficient is minimum inlow-lying land, the average permeability coefficient of low-lying land is7.69×10-6m/s, the average permeability coefficient of the valley and gentleslope zone is1.6×10-5m/s and4.0×10-5m/s, the impact structure zone has thestrongest permeability, and the coefficient can reach to10-4m/s. The chloride iontracer method was used to estimate groundwater recharge in unsaturated zone andsaturated zone, The results showed that the recharge rate in valley zone is only1.57mm/a, and the recharge rate in shallow groundwater is0.28mm/a, less than1%ofprecipitation.3. The study focus to illustrate the chemistry and isotopic characteristics ofprecipitation, shallow groundwater and deep groundwater in the region, the shallowgroundwater chemical mainly type is Na-Cl-SO4-and Na-Ca-Cl-SO4, and the deepgroundwater chemical mainly type is Na-Cl-SO4. The deep groundwater flow wasthrough a relatively longer path and longer water-rock interaction, the groundwatercirculation rate was slower. Hydrogen and oxygen isotope data line in the vicinity ofthe meteoric water line, indicating the shallow groundwater originates from the localprecipitation. 4. Through the groundwater chemistry and isotopic analysis, with considerationof hydro-geological conditions, the age of deep groundwater in BS15was analyze byusing a variety of isotopes (2H,18O,3H,14C,81Kr,85Kr) and environmental tracer(4He).According to the result of carbon-14age dating, the deep groundwater age between5.7ka to8.7ka.Krypton-81data of groundwater were first got from checkout in home,and the result of krypton-81age dating were25ka and46ka.The groundwaterresidence time estimated from the4He accumulation rate were3.8ka and5.0ka. Thegroundwater renewal rate for shallow groundwater was0.28mm/a, and the deepgroundwater flow rate was between9.8×10-4m/a and9.5m/a, the deep groundwaterrenewal rate was between0.006%and0.012%.5. Combined with groundwater data of chemistry, isotope, penetration testing andgroundwater monitoring, it proposed formation mechanism of deep groundwater, asthe recharge of deep groundwater is lateral deep ancient groundwater and the youngshallow groundwater, the recharge from fossil groundwater is greater than80%and ithas no deep crust and mantle sources, shallow groundwater recharge is mainly fromprecipitation during the rainy season. It revealed the formation mechanism of the deepgroundwater and provided realistic basis for selecting and evaluating potential sites.The study would deepen the understanding of hydro-geology conditions in thepreselected area; this investigation on krypton-81dating of groundwater was firstcarried out in our country. The integrated quantitative evaluation on recharge andrenewal rate of groundwater was made by isotope analysis and chloride ion tracermethod. We tested three methods for dating the deep groundwater, the mechanism ofgroundwater recharge, groundwater flow pattern and formation mechanism wasproposed. In conclusion, the studies of this dissertation reach the anticipate targetswith some innovation and integrity, and has important significance in promotingChina’s HLW geological disposal. |
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