Simulating groundwater flow and contaminant migration in heterogeneous alluvial sediments

塞拉菲尔德是坎布里亚海岸的一个大型核燃料后处理、核废料储存、核退役和前核能发电站。它将于2120年完全退役。英国的许多核设施都位于沿海平原上，并被厚厚的河流-冰川冲积沉积物覆盖。这些沉积物本质上是高度不均匀的，由粘土大小到鹅卵石大小的物质组成。沉积结构的横向变化反映了沉积环境的变化。从历史上看，曾发生过几次未经授权向地下水释放放射性核素和/或含碳氢化合物的流体的事件。这些放射性核素包括在地下水中具有高度流动性的放射性核素，如在地下水中具有高度流动性的TC-99和H-3，以及表现出吸附控制迁移的其他放射性核素，如锶-90、碳-137和铀，以及碳氢化合物及其溶解产物。在塞拉菲尔德，有200多个地下水监测孔，记录了含水层材料和地下水数据。这表明在这一非常复杂的松散含水层结构中存在广泛的放射性和化学污染羽流。该项目将开发新的建模方法，考虑到松散河流含水层的三维复杂性，以及存在的各种沉积物和地下水成分，以更好地了解和预测污染物的迁移。将进行一项数值模拟研究，以确定沉积物非均质性对放射性核素、化学污染物和冰川沉积物中盐分界面行为的影响。在工作的第一阶段，将评估模拟溶质运移的基本方法，如粒子跟踪、平流-弥散和连续时间随机游走(CTRW)，以确定它们是否适合于解决这些问题。一旦确定了适当的基本建模方法，就必须审查和选择可供实施的准则。在第二阶段，冰川沉积物的渗透性和吸附特性(例如分配系数Kd)将从现场岩心样本的预先存在的数据中获得[在目前正在进行的EPSRC-DTP资助的PHD项目中进行的实验室实验中测量]。渗透率和Kd将根据粒度和沉积物组成的岩性数据进行经验估计，并确定空间统计特性。对模拟渗透率/Kd场随机生成的现有方法进行了综述。选定的方法将用于生成代表沉积物的渗透率/Kd场。在工作的第三阶段，将对生成的渗透率/Kd场应用溶质传输模型，以便限制可能来自现场的污染物影响的包络。现场地下水监测数据将约束模拟的水力坡度和边界条件。可研究非均质沉积物渗透性和吸附特性对与污染物影响有关的三种特定溶质迁移情景的影响：一)放射性核素羽流的迁移(即非吸附污染物U和H-3；吸附污染物SR-90、TC-99和Cr-137)；二)溶解相烃污染物的迁移、分配和降解(根据拟议的100年期至站点关闭，这与此类污染物的寿命有关)；三)该沿海站点盐水-淡水界面的行为，这可能会影响溶解相污染物的吸附/迁移和降解。

Sellafield is a large nuclear fuel reprocessing, nuclear waste storage, nuclear decommissioning and former nuclear power generating site on the coast of Cumbria. It is due to be fully decommissioned by 2120. Many of the UK nuclear sites, are located on coastal plains and are underlain by thick fluvial-glacial alluvium deposits. These sediments are highly heterogeneous in nature and consist of layers ranging from clay size right up to cobble sized materials. Lateral variability in sediment texture reflects changing depositional environments. Historically there has been several events where unauthorised releases of radionuclide and/ or hydrocarbon containing fluids to groundwater have occurred. These include radionuclides such as Tc-99 and H-3 that are highly mobile in groundwater, others such as Sr-90, C-137 and U which display sorption controlled transport, and hydrocarbons and their resulting dissolution products. At Sellafield, over 200 groundwater monitoring boreholes exist, where aquifer materials and groundwater data have been recorded. This shows extensive radioactive and chemical contamination plumes exist in this very complex unconsolidated aquifer structure. The project will develop new modelling approaches which considers the three-dimensional complexity of unconsolidated fluvial aquifers, and the variable sediment and groundwater compositions present, to better understand and predict contaminant migration. A numerical modelling study will be undertaken to establish the influence of sediment heterogeneity on the migration of radionuclides, chemical contaminants and saline interface behaviour in glacio-fluvial sediments. In the first phase of the work, fundamental approaches to modelling solute transport such as particle tracing, advection-dispersion, and Continuous Time Random Walk (CTRW) will be assessed for their suitability for tackling these problems. Once appropriate fundamental modelling approaches have been identified, it will be necessary to review and select from the available codes for implementation. In the second phase, permeability and sorption properties (e.g. distribution co-efficient Kd) of the glacio-fluvial sediments will be obtained from pre-existing data on the site core samples [measured in lab experiments conducted in a currently-running EPSRC-DTP funded PhD project]. Permeability and Kd will be estimated empirically from lithology data on grainsize and sediment composition, and spatial statistical properties will be determined. Available approaches for the stochastic generation of simulated permeability/Kd fields will be reviewed. Selected approaches will be used to generate permeability/Kd fields representing the sediments. In the third phase of the work solute transport modelling will be applied to the generated permeability/Kd fields in order to constrain the envelope of possible contaminant impacts from the site. Site groundwater monitoring data will constrain modelled hydraulic gradients and boundary conditions. The influence of heterogeneous sediment permeability and sorption properties on three specific solute transport scenarios relevant to contaminant impacts may be investigated i) migration of radionuclide plumes (i.e. non-sorbing contaminants U and H-3; sorbing contaminants Sr-90, Tc-99 and Cr-137) ii) transport, distribution and degradation of dissolved phase hydrocarbon contaminants (relevant to the longevity of such contaminants in terms of the proposed 100 year period to site closure) and iii) behaviour of the saline-freshwater interface at this coastal site, which may influence both sorption/migration and degradation of dissolved-phase contaminants.