Several Issues On Numerical Simulation Of Groundwater:From Quantification Of Medium Microstructure To Modeling Of Groundwater Reactive Transport At Basin-scale

Aquifers composed of granular materials are major repositories of groundwater resource in which water can be stored abundantly.With the rapid development of economy,the pressure from water consumption and demand is increasing.As a result,groundwater resource has been exploited seriously and various contaminants from human activities have entered into subsurface environment,which led to serious contamination and the change of recharge,circulation and discharge of groundwater system For example,groundwater overexploitation,highly toxic hydrocarbon compounds,nitrogen fertilization.More seriously,lacking efficient groundwater exploitation management has increased the contradictions of water demand.How to quantify properties of aquifer media?What factors can affect contaminants migration in aquifer?How to identify subsurface contaminant source?What measures can be taken to protect subsurface environment?What changes will appear for the interactions between surface water and groundwater influenced by groundwater overexploitation?What can be done to slow down the depletion of groundwater resource and achieve sustainable use?These all are important issues needed to be solved.This research firstly has studied the microstructure of granular aquifer and developed a new technique-light transmission micro-tomography(LTM)to quantify properties of translucent granular materials,simultaneously,associated representative elementary volume(REV)of granular materials are also estimated using a criterion of relative gradient error(?'g).Afterward,a relative permeability-saturation model(RPM)is proposed for the complexity of dense non-aqueous phase liquids(DNAPLs)migration in porous media.What's more,a reactive transport model of petroleum contaminant typical component is built for a mesoscale site to identify subsurface contaminant source.Moreover,to improve the effect of groundwater optimization model,a new efficient harmonic oscillator genetic algorithm(HOGA)is proposed.Finally,a nitrogen reactive transport model is established to simulate the migration process of nitrogen species in the large-scale Shijiazhuang(SJZ)region of North China Plain(NCP)and corresponding groundwater management optimization model is conducted to improve the sustainability of groundwater utilization.Furthermore,the changes of interactions between surface water and groundwater caused by groundwater overexploitation are studied for complex large-scale basin-the arid mid-eastern Yanqi Basin(MEYB).Summing up all studies in this paper,the main innovative research results are as follow:(1)This research firstly quantifies the properties of granular materials by light.A new LTM technique is established to step forward to accurately quantify micro-structures of translucent granular materials rapidly,handily and economically.Results suggest heterogeneity of granular materials is widely existed.The frequencies of minimum REV sizes of translucent silica are close to Gaussian distribution in 0.0-12.0 mm and the average REV size approximately is 10.0 mm(2)Based on the quantification of heterogeneities of translucent granular aquifer using LTM,a RPM is proposed to simulate complex long-term DNAPL migration in porous media.Results suggest DNAPL migration is not only affected by the heterogeneity of porous media,but also controlled by the relative permeability of DNAPLs over long-term periods.PCE infiltration and spreading behavior is well modeled over long term including calibration,verification and validation periods.(3)Use GA to identify subsurface contaminant source.Based on the spot field investigation,a BTEX reactive migration model is built in mesoscale site and genetic algorithm(GA)is used to identify contaminant source.To improve the effect of groundwater optimization model,a new HOGA is developed to achieve better global searching ability and higher optimization efficiency.(4)Simulation results of groundwater-surface water coupled model in large-scale basin-MEYB suggest that human activities such as groundwater overexploitation and agricultural irrigation greatly affect the runoff of Kaidu River into Bosten Lake.The increasing river seepage induced by groundwater overexploitation plays the most important role in the water level decline of Bosten Lake and water environment deterioration.To make water consumption in agriculture becoming more efficient,adopting water-saving technologies,changing the crop rotation type,adjusting irrigation and surface water diversion schemes also can improve efficiency of water use in MEYB and protect the regional environment.Future work will focus on contaminants reactive in subsurface environment and groundwater-surface water resources management in MEYB.(5)A nitrogen reactive transport model and associated groundwater resource management optimization model are conducted to achieve groundwater sustainable utilization in the large-scale SJZ region of NCP.Simulation results suggest the concentration of ammonia and nitrite is quite low.Simultaneously,the contamination of NO3-N is mainly distributed in the metropolitan SJZ city and its concentrations can be up to 109 mg/L,while its concentration throughout the surrounding rural counties is not so high,less than 10 mg/L.The concentrated industrial and domestic contaminants play a major role in groundwater nitrogen contamination over the whole study area.Compared with the optimization model without water quality constraints,this study could provide a more useful tool for developing cost-effective strategies for groundwater sustainable management,preventing the nitrogen contamination plume from expanding and improve subsurface environment in the NCP.