| Artificial recharge with storm water for saline aquifers is an efficient way forsaline aquifer restoration and treatment. However, once the storm water is introducedinto aquifers, water-rock balance would be dramatically undermined to triggercomplicated hydrogeochemical reactions, which would influence groundwater quality,result in permeability reduction, and eventually reduce the efficiency of the rechargeproject.In our study, based on the monitoring results for groundwater quality, thedistribution characteristics and formation process for groundwater chemical field wereexplored. On this basis, through artificial recharge simulation experiment andhydrogeochemical reaction experiment, groundwater kinetic parameters andwater-rock reaction parameters were measured, components equilibrium distributionof the test water samples and the equilibrium state between water and main mineralswere calculated, different influence factors on mineral equilibrium state wereanalyzed, and main hydrogeochemical interactions (mixing, cation exchange,precipitation-dissolution, and etc.) between recharge storm water and undergroundsaline aquifer were simulated. Meanwhile, a mathematical model was built to predictmulti-component solutes transport and transformation during artificial recharge. Andtemporal and spatial changes of groundwater components and their impact ongroundwater quality during field-scale groundwater recharge were analyzed. Severalconclusions were obtained as follows.(1) The seasonal changes of salinity and main component composition inresearch area were not significant. However, their spatial changes showed clearzoning from north-west to south-east. Hydrochemical type for groundwater wastransferred from HCO3-Ca type and HCO3-Ca·Mg type to Cl·SO4-Na·Mg type andCl-Na type as salinity gradually increased.(2) The cation exchange capacity values obtained respectively by dynamic elution, static extraction, empirical formula were similar to each other. The cationexchange capacity value (2.83meq/100gSoil) obtained with static extraction methodwas used for reactive transport simulation. Based on the results of cation exchangeexperiments, the selective coefficients for each cation was calculated based onGaines-Thomas equation:K Na/K=0.09，K Na/Mg=0.44，K Na/Ca=0.43. With CaCl2as atracer, hydrodynamic parameters for aquifer media were determined. Dispersioncoefficient, dispersity and effective porosity valued at1.16E-06m2/s,0.058m and0.38respectively.(3) Based on analysis of chemical component speciation and concentration,conventional components Na, K, Mg and Ca were in simple ionic form, S and C werein the form of SO2--4and HCO3respectively, and Si and Al were in the form of H4SiO4and Al(OH)-4respectively. The calculation results of mineral saturation indexesindicated that carbonates in the water were supersaturated and the saturation index forcarbonates were minimized when volume fraction for recharge water was60~70%. Asfor silicates, the saturation index was gradually decreased with the increase ofrecharge water volume fraction. Chalcedony and feldspar were under unsaturated statein solutions. As for Illite and Kaolinite, with increase of recharge water volumefraction, they gradually turned into unsaturated state from saturated state.(4) The results of laboratory simulation experiments suggested that thepermeability of the water-bearing media displayed a decrease trend with time. Byanalyzing component concentrations in leachates, it was found that thehydrogeochemical interactions which occurred during recharge mainly involvedmixing, cation exchange and carbonates precipitation-dissolution.(5) By comparing the numerical simulation results and experiments results forgroundwater artificial recharge, it was discovered that when “cation exchange andcalcite precipitation” was employed as reaction module, the best correlation wasobtained between numerical simulation results and experiment results, which reflectedthe dominant hydrogeochemical reactions occurring during artificial recharge.(6) According to the prediction results for temporal and spatial changes of multi-solute concentration during field-scale artificial recharge, temporally speaking,the ions which were mainly influenced by mixing would achieve stable concentrationsfaster. Nevertheless, the time needed for the ions which were involved inhydrogeochemical reactions was longer. Spatially speaking, the further they werelocated from the recharge well, the more profound ion concentration changes werefound. As the temperature increased, the precipitation of calcite was strengthened,triggering a decrease of aquifer permeability with precipitates within pores. As thepCO2increased, the dissolution of calcite in mixed water was enhanced, renderingincreased Ca2+and HCO-3concentrations to enhance the hardness. The recharge flowmerely impact artificial recharge rate, yet made no difference for concentration ofsolute components in groundwater. |
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