Colloid-Filtration Theory for Vadose-Zone Systems

[409174]塞尔斯：要预测土壤中吸收污染物的运动，评估病原微生物进入饮用水含水层的风险，以及估计土壤照明和土壤剖面发展的时间尺度，需要掌握控制胶体大小颗粒通过渗透带的运输过程的知识。气-水界面和固-水界面的胶体沉积对气包带胶体的输运速率和孔隙水浓度起着重要的控制作用。尽管胶体侵入带过程很重要，但目前还没有适合于预测非饱和多孔介质中胶体沉积的理论。这项拟议研究的重点是采取步骤，通过定量地定义空气-水和固体-水界面的胶体沉积动力学与土壤-水-胶体系统的物理性质之间的基本关系，来填补这一知识空白。这些关系将通过分析一套孔隙尺度的水流和通过部分饱和空隙空间的平流扩散胶体运输的模拟而得出。可变饱和多孔介质中空气-水结构的现代模型将用于指定孔隙水域的几何形状，其中孔隙尺度的胶体输运模拟将进行。对流动和输运模拟结果的回归分析有望产生幂律方程，该方程将胶体与空气-水和固体-水界面的碰撞率表示为流体流动速度、胶体大小和密度、多孔介质结构和毛细压力（控制含水饱和度）的函数。除了需要考虑气相的存在之外，推导胶体界面碰撞率幂律方程的方法在概念上类似于用于制定饱和水多孔介质的胶体过滤方程的已发表方法。用幂律方程预测胶体沉积动力学将与实验室柱实验中胶体沉积的测量结果进行比较。在这些实验中，胶体沉积速率对胶体直径、粒度、粒度分布、孔隙水流速和毛细管压力变化的敏感性将被检验。测量和预测沉积速率的比较将揭示新理论的不足，并有助于指导其改进。这项研究的发现将推进目前对部分饱和地质物质的质量传递过程的认识，并大大提高我们对影响渗透带内胶体孔隙水浓度的关键传质反应的理解。本研究首次尝试推导和测试非饱和多孔介质的胶体过滤理论，该理论用于对渗透带中的胶体流动性进行定量推断。这样的理论应该对解决国家关注的关键水质问题做出重要贡献，这些问题与微生物病原体和胶体相关污染物通过近地表环境的运动有关。

0409174SaiersKnowledge of processes that govern the transport of colloid-sized particles through thevadose zone is required to predict the movement of sorbing contaminants within soils, to assessthe risks associated with the entry of pathogenic microbes into drinking-water aquifers, and toestimate the time scales for soil illuviation and soil-profile development. Colloid deposition atair-water and solid-water interfaces plays an important role in controlling the transport rates andporewater concentrations of colloids in the vadose zone. Despite the importance of colloids invadose-zone processes, a theory suitable for predicting colloid deposition in unsaturated porousmedia does not exist.The focus of this proposed research is to take steps towards filling this gap in knowledgeby defining, in a quantitative way, the fundamental relationships between colloid-depositionkinetics at air-water and solid-water interfaces and the physical properties of the soil-water-colloidsystem. These relationships will be derived through analysis of a suite of pore-scalesimulations of water flow and advective-diffusive colloid transport through partially saturatedvoid spaces. Modern models for air-water configuration in variably saturated porous media willbe used to specify the geometry of the porewater domains for which the pore-scale simulationsof colloid transport will be conducted. Regression analysis of the results of the flow-and-transportsimulations is expected to yield power-law equations that express the rate of colloidcollisions with air-water and solid-water interfaces as functions of fluid-flow velocity, colloidsize and density, porous-medium texture, and capillary pressure (which controls watersaturation). Except for the need to account for the presence of the air phase, the proposedapproach for deriving the power-law equations for colloid-interface collision rates isconceptually similar to published approaches used to formulate the colloid-filtration equationsfor water-saturated porous media.Predictions of colloid-deposition kinetics made with the power-law equations will becompared with measurements of colloid deposition made in laboratory column experiments. Inthese experiments, the sensitivity of colloid-deposition rates to variations in colloid diameter,grain-size, grain-size distribution, porewater velocity, and capillary pressure will be examined.Comparison of measured and predicted deposition rates will reveal deficiencies in the newtheory and help direct its refinement.Findings from this study will advance current knowledge of mass-transport processes inpartially saturated geologic materials and substantially improve our understanding of key mass-transferreactions that affect porewater concentrations of colloids within the vadose zone. Thisproposed research represents a first attempt at deriving and testing a colloid-filtration theory forunsaturated porous media that is needed to make quantitative inferences regarding colloidmobility in the vadose zone. Such a theory should make an important contribution towardsaddressing critical water-quality issues of national concern that are related to the movement ofmicrobial pathogens and colloid-associated contaminants through near-surface environments.