Collaborative Research: Unraveling Transport in Porous Media through the Integration of Isotopic Tracers, Geophysical Data, and Numerical Modeling

各种估计表明，与美国地下水污染有关的经济责任高达数千亿美元。这些场址中的每一个都对环境和人类健康构成潜在威胁；近90%的传统法院强制策略未能充分修复受污染的场地。在许多情况下，这些失败可归因于依赖于污染物输送的经典概念模型的补救策略，这些模型既不能充分代表地下非均质性，也不能捕获优先流动和低渗透率溶质圈闭。因此，模型在实际问题中的应用保留了很大的不确定性。虽然存在许多不同的传输模型，但了解对于给定问题哪个是“最正确的”是预测污染物传输、反应和现场清理的基础。该项目将利用污染物输送、地球物理学、元素和同位素地球化学以及混合驱动反应等领域的方法来满足这一需求。该研究最终将大大改进修复设计策略，并改进地下水污染场地对人类健康风险的评估。作为这项研究的一部分，新的高中化学实验室将与丹佛的一所公立高中合作，按照科罗拉多州的高中课程开发，重点是酸碱化学、pH缓冲和金属地球化学。这些实验室将通过国家地球科学教师协会的《地球科学家》杂志在当地发展之外进行传播。污染物迁移预测的可靠性取决于数值/概念模型对现场地质的适宜性。主要问题是：(1)哪些物理可测量参数控制溶质传质和地球化学反应；(2)哪些参数可以先验预测；(3)哪些数学公式能准确描述不同流速和非均质性下的场地特定输运。许多现有输运模型的主要限制是模型中控制溶质输运的拟合参数与物理系统之间联系不紧密。这项工作的目的是探索控制现有的物理传输模式，特别是那些包括化学反应的预测能力差。将结合扩散诱导元素分馏、扩散诱导锂同位素分馏和地球物理表征来探索溶质输运和反应的控制因素。在过去，保守的示踪剂浓度被用来约束顺流-色散输运参数，而同位素示踪剂在扩散过程中会发生分馏，因此可以在扩散主导离子输运的情况下提供关于固定孔隙空间的信息。这个项目的力量将是对溶质输运建模中已知参数的控制增加重要的约束，确定它们对输运过程的控制，并量化多孔介质中活跃的输运长度尺度的分布及其对反应动力学的后续影响。所产生的实验数据将用于验证水文界常用的现有理论（即“局部”与“非局部”运输），并根据新出现的数据探索新的理论，如果需要的话，发展对运输和反应行为的新见解。

Various estimates suggest that economic liabilities associated with contaminated groundwater sites in the U.S. run into the hundreds of billions of dollars. Each of these sites poses a potential threat to the environment and human health; nearly 90% of the conventional court-mandated strategies fail to remediate polluted sites adequately. In many cases, these failures can be attributed to remediation strategies that rely on classical conceptual models of contaminant transport, which neither represent subsurface heterogeneity sufficiently nor capture preferential flow and low permeability solute traps. As a result, the application of models to practical problems retains significant uncertainty. While many different models for transport exist, understanding which is "most correct" for a given problem is fundamental to the prediction of contaminant transport and reaction and site clean-up. This project will utilize methods from the fields of contaminant transport, geophysics, elemental and isotope geochemistry, and mixing-driven reactions to address this need. The research will ultimately enable significantly improved remediation design strategies and improved assessment of human health risk from contaminated groundwater sites. As part of this research, new high school chemistry labs will be developed following the Colorado high school curriculum focusing on acid-base chemistry, pH buffers, and metal geochemistry in collaboration with a Denver public high school. These labs will be disseminated beyond local development via the National Earth Science Teachers Association's "The Earth Scientist" Journal.The reliability of contaminant transport predictions depends on the appropriateness of the numerical/conceptual model to site geology. The primary questions are: (1) which physically measureable parameters control solute mass transfer and geochemical reactions, (2) which parameters can be predicted a priori, and (3) which mathematical formulations accurately describe site specific transport under varying flow rates and heterogeneity. The main limitation of many existing transport models is the poor connection between fitting parameters governing solute transport in models and the physical system. The objective of the work is to explore controls on the poor predictive ability of existing physical transport models, especially those that include chemical reaction. Diffusion-induced elemental fractionation, diffusion-induced lithium isotopic fractionation, and geophysical characterization will be integrated to explore the controls on solute transport and reaction. While conservative tracer concentrations have been used to constrain advective-dispersive transport parameters in the past, isotopic tracers fractionate during diffusion and can therefore provide information on immobile pore space in cases where diffusion dominates ion transport. The power of this project will be to add significant constraints to the controls of known parameters in solute transport modeling, determine their controls on transport processes, and quantify the distribution of transport length scales active in a porous medium and the subsequent impact on reaction kinetics. The experimental data produced will be used to validate existing theories commonly used by the hydrology community (i.e., "local" versus "non-local" transport), and explore new theories as motivated by emerging data, if needed, to develop new insight into transport and reaction behavior.