Research On Groundwater Chemical Evolution And Water-rock Interaction In Beishan Preselected Area, Gansu

The suitability of hydrogeochemical condition is one of the key issues for site characterization and safety case.Gansu Beishan pre-selected area is the most potential one for geological disposal of high-level radioactive waste?HLW?in China.It is located in the extremely arid Gobi area in the northwest China.In this dissertation,being focused on the hydrogeochemistry of groundwater system in Beishan preselected area,the researches are carried out as the hydrochemical characteristics of groundwater at different scales?Beishan preselected area,sub-areas,candidate site?,water-rock interaction at different depths and the mechanism of groundwater formation by using the comprehensively means and approaches,such as the field investigation in-situ,multivariate statistical analysis,tests&experiment and inversion simulation.The important progress and results have been made as follows:?1?By using multivariate statistical analysis,the hydrochemical characteristics of groundwater have been gotten to know and relevant data have been obtained in the Beishan preselected area and main sub-areas.These results and data have been in the support of hydrochemical conditions for the evaluation of site suitability for the geological repository of HLW.1)The water geochemistry type of groundwater from west part to the eastern basin mainly includes Cl·SO₄-Naand SO₄·Cl-Na;The average p H is between 7.5 and7.9,which is weakly alkaline;The average TDS is 1491.7,3937.7 and 11848.7 mg/L,which increases gradually from west to east.2)The water chemistry type of groundwater in Xinchang sub-area is Cl·SO₄-Na and SO₄·Cl-Na,the TDS of near-surface groungwater is between 1111.5?3750.9mg/L,the shallow groungwater is between 1317.6?1688.2 mg/L and the deep groungwater is between 1750.1?3455.9 mg/L which increase with depth;The water chemistry type of near-surface groundwater in Suanjingzi sub-area is Cl·SO₄-Na·Ca and the TDS is between 690.7?3315.9 mg/L;The water chemistry type of near-surface groundwater in Shazaoyuan sub-area is Cl·SO₄-Na and the TDS is between 2076.6?10285.9 mg/L.?2?Through near-surface water-rock interaction experiments and inverse geochemical modeling studies,the chemical characteristics of water and ion resource from precipitation to unsaturated zone are quantitatively studied.1)Beishan groundwater originates from local precipitation.After vertical and lateral leaching,concentration and cation exchange,high-salinity water is formed,of which concentration is the main force and contribution rate is 61.05%,the contribution rate of leaching is 21.39%,and cation exchange also exists.2)The main ions of groundwater come from the dissolution of halite,sulfates,and carbonates,where Na⁺mainly comes from halite,sodium-containing silicates,and cation exchange;Cl^-mainly comes from the dissolution of halite;Ca²⁺,Mg²⁺,SO₄^2-and HCO₃^-mainly come from the dissolution of calcite,dolomite and gypsum.3)From precipitation to shallow groundwater and deep groundwater in Beishan preselected area,the dissolved minerals are mainly gypsum,dolomite,halite,K-feldspar and precipitated minerals is illite.?3?The effect of high-temperature of deep groundwater chemistry is tested by high-temperature water-rock interaction experiment.It has been shown from the research results that:1)There is obvious change of major components in aqueous solution after high-temperature water-rock interaction.And the cations such as Na⁺and K⁺increase,and the ions of Mg²⁺,Ca²⁺and HCO₃^-decrease gradually.2)As the temperature of the reaction system rises from 60?to 90?,thewill have obvious change,which indicates that the temperature is lower than 60?,the temperature has little or no effect on hydrochemical characteristics of groundwater,but when the temperature exceeds 60?,the temperature has a significant effect.3)As the temperature of the system increases,precipitation and dissolution of minerals occurre,such as precipitation of calcite and dolomite,meanwhile dissolution of part of albite,K-feldspar and anorthite which produce clay minerals such as kaolinite and montmorillonite.This effect is positive significant for the long-term safety of the geological disposal system of HLW.These clay minerals play a role in filling up the near-field fissure network of the repository and reducing groundwater mobility.At the same time,these clay minerals are conducive to the adsorption of radionuclides,which can retard the release of radionuclides from the engineering barrier system.