Coupled Silicate Reaction Kinetics in an Aquifer

本提案寻求更新美国国家科学基金会资助的地下水含水层中硅酸盐反应动力学的野外、微观和建模综合研究和教育项目，该项目位于亚利桑那州黑梅萨的纳瓦霍砂岩含水层。现代水文地质学和地球化学的一个基本问题是，从流域和土壤剖面得到的硅酸盐溶解率与实验室测量得到的硅酸盐溶解率之间存在数量级差异。这种巨大的差异表明我们对自然界中控制硅酸盐溶解动力学的基本物理和化学过程缺乏基本的了解。提出的研究将检验两个假设来解释这种差异：(1)长石在自然系统和大多数实验室实验中溶解的饱和状态不同，这在很大程度上是由原位和实验室速率之间的差异造成的。虽然大多数实验室实验都试图测量远离平衡的单一同溶反应的速率，但实地研究在一个复杂的反应网络网络中，在一个非常接近长石平衡的条件下测量原位速率。在这些反应中有粘土的沉淀，它从地下水中除去溶质，从而促进长石的持续溶解。然而，与普遍认为粘土与地下水处于平衡状态的假设相反，我们认为粘土的沉淀比长石的溶解慢得多，因此地下水的化学性质被限制在接近与长石的平衡状态。随着时间的推移，随着长石溶解和粘土沉淀的速率接近恒定，地下水化学达到稳定状态；(2)淋溶层在风化长石表面形成，这是自然界中硅酸盐溶解发生的重要组成部分，必须在恰当描述反应动力学的速率定律中加以考虑。该研究将通过分析含水层中溶解的Al3+浓度，评估饱和度指数及其沿流动路径的变化，以及数值地球化学模型来验证第一个假设，以阐明与含水层中长石溶解相关的复杂反应网络。我们的目标是建立一个理论框架来解释实验室实验和现场数据。为了验证第二个假设，我们将使用原子尺度场发射枪透射电子显微镜来表征长石-粘土界面的微观结构和详细化学。总体目标是促进我们对实验室和现场速率之间差异的两个关键可能性的理解：减弱的热力学驱动和风化长石表面的特征。更广泛的影响pi将把研究活动整合到本科和研究生水平的地质学课程中。夏季指导计划将资助本科生，特别是来自代表性不足的群体的学生，在PI的实验室进行研究。将与国家能源技术实验室和洛斯阿拉莫斯国家实验室建立伙伴关系，以促进研究成果在碳封存和环境恢复计划中的传播和应用。地下水是饮用水的主要来源，对地球上的生命至关重要。地下水还占人类可利用淡水的98%。因此，含水层的反应速率对水资源和水质管理、废物处理和减缓全球变暖战略至关重要。因此，该项目的研究结果将有助于我们对环境的理解，并有助于为环境政策和战略建立科学基础。

0509755ZhuThis proposal seeks renewal of a NSF-sponsored integrated research and education program infield, microscopic, and modeling studies of silicate reaction kinetics in a groundwater aquifer theNavajo sandstone aquifer at Black Mesa, Arizona. One of the fundamental problems in modernhydrogeology and geochemistry is the orders of magnitude discrepancy between silicate dissolutionrates derived from watersheds and soil profiles versus those derived from laboratory measurements.This large discrepancy indicates our lack of basic understanding of the fundamental physical andchemical processes controlling silicate dissolution kinetics in nature.The proposed study will test two hypotheses to explain this discrepancy: (1) A large part of thediscrepancy between in situ and laboratory rates results from different saturation states under whichfeldspars dissolve in natural systems and in most laboratory experiments. While most laboratoryexperiments attempt to measure the rate of a single congruent dissolution reaction far from equilibrium,field studies measure in situ rates amid a complex web of reaction networks and at a condition veryclose to equilibrium with respect to feldspars. Among these reactions is the precipitation of clay, whichremoves solutes from groundwater and hence promotes continued feldspar dissolution. However,contrary to the prevailing assumption that clays are at equilibrium with groundwater, we believe thatclay precipitation is much slower than feldspar dissolution, and hence groundwater chemistry isconstrained to be close to equilibrium with feldspars. Over time, a steady state of groundwaterchemistry is reached with near constant rates of feldspar dissolution and clay precipitation; and (2) Aleached layer forms on weathered feldspar surfaces, which is an important part of how silicatedissolution occurs in nature and must be considered in rate laws that properly describe reactionkinetics.The proposed study will test the first hypothesis by analyzing dissolved Al3+ concentrations inthe aquifer, evaluating saturation indices and their variations along a flow path, and numericalgeochemical modeling to elucidate the complex network of reactions associated with feldspardissolution in aquifers. We aim to establish a theoretical framework for interpreting laboratoryexperiments and field data. To test the second hypothesis, we will characterize microstructures anddetailed chemistry at the feldspar-clay interfaces using an atomic scale Field Emission GunTransmission Electron Microscope. The overall objectives are to advance our understanding of the twokey possibilities for discrepancies between laboratory and field rates: the diminishing thermodynamicdrive and the characteristics of weathered feldspar surfaces.Broader ImpactsThe PIs will integrate the research activities into undergraduate and graduate-level geologycourses. A summer mentorship program will sponsor undergraduate students, particularly fromunderrepresented groups, to conduct research in the PI's laboratories. Partnerships with the NationalEnergy Technology Laboratory and Los Alamos National Laboratory will be developed to facilitatedissemination and applications of the research results to the carbon sequestration and environmentalrestoration programs.Groundwater is a key source of drinking water and is essential to life on Earth. Groundwateralso represents 98% of fresh water readily available to humans. Therefore, reaction rates in aquifers arecritical to water resource and water quality management, waste disposal, and global warmingmitigation strategies. The findings from this project will therefore contribute to our understanding ofthe environment, and help to build a scientific basis for environmental policies and strategies.