Cyclic Processes Within Surface-exposed Fractures Affecting Evaporation and Salinization Mechanisms

影响蒸发和盐化机制的地表暴露裂缝内的循环过程

• 批准号: 0510825
• 负责人: Maria Dragila
• 金额: $ 34.97万
• 依托单位: Oregon State University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-08-01 至 2010-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0510825&HistoricalAwards=false
• 关键词: Cyclic; Processes; Within; Surface; exposed

0510825DragilaThis project will investigate chemico-physical processes in near-surface fractures that may be responsible for enhanced basin-scale evaporation and accelerated transport of solutes (salinity) to groundwater. In arid and semiarid regions where groundwater is a scarce commodity, groundwater salinization has become an ever-increasing concern. This same mechanism is also of interest because of the waste facilities typically located in these type environments. Salt crust formation is the result of a complex process caused by the interaction of various important mechanisms, such as gradients in permeability, osmotic potential and thermal energy. These processes can act in concert to either reduce or enhance salt crust formation, including: (1) accelerated evaporation during nighttime venting; (2) osmotic gradients that result in vapor pumping and saline back-diffusion; (3) salt precipitation that can clog internal pores and/or cause surface sealing of the rock; (4) salt crust morphology that can impact fracture aperture; and (5) haloclasic erosion that can change the surface texture and evaporation rate. Even though these mechanisms havebeen observed either in the field or during preliminary work, the long-term effect has not been evaluated.The working model consists of a vadose zone with exposed surface fractures. When these fractures are air filled, evaporation from the fracture walls triggers capillary forces that draw pore-water and solutes from the matrix where further evaporation occurs, thus resulting in enhanced salt accumulation on the fracture walls. Daytime vapor pressure gradients drive a relatively slow diffusional process that vents moist fracture air. During nighttime, however, unstable air-density gradients permit denser surface air to enter the fracture resulting in mass convection and venting of moist fracture air, thus enhancing evaporation of the fracture surface and enhancing concomitant salt crust formation. Theoretical analysis indicates that nighttime convective venting may increase evaporative potential by a factor of 80. The concept of a near-surface fracture serving as a reactive convection cell and the concept of nighttime convection has only recently been investigated. This project will focus on the impact that this convective venting has on salt crust formation under various natural conditions. The proposed research will use a Climate Control Chamber (OSU) and x-ray CT-scanning (PSU) to investigate pore-scale processes of evaporation and salt deposition within rock cores for a range of permeabilities. Intermediatescale experiments will be conducted in a large Climate Controlled Room (BGU) to explore the aforementioned mechanisms under controlled conditions for a natural fractured rock. Field-scale venting and evaporation dynamic will be investigated in a real fracture (instrumented to a depth of ~1.4 m.) located at a well-characterized fracturedchalksite. Numerical models will be used to quantify basin scale impact of this process on contaminant bypassing, solute loading to aquifer and basin scale evaporation. Most importantly the project will determine the limiting parameters for this process, i.e., under which geologic and climatic conditions will this process be most important.Intellectual Merit:The role of open (air-filled) cavities in vadose zone hydrodynamics has been largely ignored, except as rapid conduits of fluids. There is substantial field evidence that salty crust is indeed precipitated on the surfaces of fractures that cross the upper vadose zone during the long dry season emphasizing the need to investigate this process. We are not aware of any treatments of this topic outside of those recently submitted for publication by thePI's. The project design is such that it will not only develop fundamental understanding of the salt crusting process, but also establish the parameters important for basin scale hydrology. Applicability within arid land hydrology understanding the potential for diverted contamination seepage from waste facilities.Broader impacts:This project is the result of international collaboration between Oregon State University, Pennsylvania State University and Ben Gurion University of the Negev, Israel. Two students involved in the project will work in an international interdisciplinary setting developing collaborative skills essential for their success as future scientists. Because science education is a national priority, the project will also participate in the NSF G-K12 Program with the dual goal of disseminating research results to educators and providing pedagogical training to graduate students readilytangible and inspiring toward the pursuit of further research. The project will develop a workshop unit for in-service K-12 teachers, pilot a thematic unit at a participating primary school, develop a "teaching kit" that can be on loan to interested educators, and a web site for those who wish to replicate the teaching kit and unit.

该项目将研究近地表裂缝中的化学-物理过程，这些过程可能是导致盆地尺度蒸发增强和溶质（盐度）向地下水加速运移的原因。在干旱和半干旱地区，地下水是一种稀缺的商品，地下水盐渍化已成为一个日益严重的问题。同样的机制也令人感兴趣，因为废物设施通常位于这些类型的环境中。盐壳的形成是渗透率梯度、渗透势梯度和热能梯度等多种重要机制相互作用的复杂过程。这些过程可以协同作用，减少或加强盐壳的形成，包括：(1)夜间排气时加速蒸发；(2)导致蒸汽泵送和盐水反扩散的渗透梯度；(3)可能堵塞内部孔隙和/或造成岩石表面密封的盐沉淀；(4)影响裂缝孔径的盐壳形态；(5)盐碎屑侵蚀可以改变表面结构和蒸发速率。尽管在实地或初步工作中已观察到这些机制，但尚未对其长期影响进行评估。工作模型由一个面露裂缝的气包带组成。当这些裂缝充满空气时，裂缝壁上的蒸发会触发毛细力，将孔隙水和溶质从基质中吸走，进一步蒸发，从而导致裂缝壁上的盐积聚加剧。白天的蒸汽压力梯度驱动一个相对缓慢的扩散过程，使潮湿的断裂空气排出。然而，在夜间，不稳定的空气密度梯度允许更密集的表面空气进入裂缝，导致大量对流和潮湿裂缝空气的排出，从而加强了裂缝表面的蒸发，促进了伴随的盐壳形成。理论分析表明，夜间对流排气可使蒸发势增加80倍。近地表裂缝作为反应性对流单元的概念和夜间对流的概念直到最近才被研究。本项目将重点研究在各种自然条件下，这种对流喷口对盐壳形成的影响。拟议的研究将使用气候控制室（OSU）和x射线ct扫描（PSU）来研究岩心内蒸发和盐沉积的孔隙尺度过程，以获得一系列渗透率。中尺度实验将在大型气候控制室（BGU）进行，以探索在受控条件下天然破裂岩石的上述机制。现场尺度的喷发和蒸发动力学将在一个真实的裂缝中进行研究（测量深度约1.4 m），该裂缝位于一个特征良好的裂缝状白垩岩上。数值模型将用于量化这一过程在流域尺度上对污染物绕过、溶质加载到含水层和流域尺度蒸发的影响。最重要的是，该项目将确定这一过程的限制参数，即在何种地质和气候条件下，这一过程将是最重要的。知识价值：除了作为流体的快速管道外，开放（充气）腔在气包带流体动力学中的作用在很大程度上被忽视了。有大量的现场证据表明，在漫长的旱季期间，咸味地壳确实在穿越上部水汽带的裂缝表面沉淀，这强调了研究这一过程的必要性。除了pi最近提交出版的那些，我们不知道关于这个话题的任何处理。该项目的设计是这样的，它不仅将发展对盐结壳过程的基本理解，而且还将建立流域尺度水文的重要参数。在干旱区水文学中的适用性，了解从废物设施转移污染渗漏的潜力。更广泛的影响：该项目是俄勒冈州立大学、宾夕法尼亚州立大学和以色列内盖夫本古里安大学国际合作的成果。参与该项目的两名学生将在国际跨学科环境中工作，培养他们成为未来科学家所必需的合作技能。由于科学教育是国家的重点，该项目也将参与NSF G-K12项目，其双重目标是向教育工作者传播研究成果，并为研究生提供切实可行的教学培训，激励他们继续进行研究。该项目将为在职的K-12教师开发一个讲习班单元，在一所参与的小学试点一个专题单元，开发一个“教学包”，供感兴趣的教育工作者借阅，并为希望复制该教学包和单元的人提供一个网站。