SGER: Documenting the Transient Nature of Natural Free Convection in Groundwater

2008年极端降水事件后阿拉伯联合酋长国(阿联酋)萨布哈的地球物理成像显示，第一个令人信服的证据表明，天然地下水环境中发生了自由对流流动过程。成像的模式与降水引起的盐岩结壳重溶和预先存在的对流模式所导致的指进一致；这些手指的分布与实验室实验和数值模拟中观察到的相似。自然自由对流对于热量和溶质在各种水文地质环境中的传输具有重要意义，人们对它的研究已经超过一个世纪，但几乎完全是在理论、数值模型和实验室实验中进行的。目前，人们对对流不稳定的发展和衰变知之甚少，因为大多数研究都假定为定态条件。为了解决关于对流的起源和持续性及其相关的空间和时间尺度的关键问题，解决不同相互作用方式的瞬变性质至关重要。我们的阿联酋实地基地是解决这些知识差距的理想地点。降水记录表明，这个地点的指法可能每十年才开始一次，因为在干旱的年份，盐岩结壳会形成。我们的试验数据显示了监测密度指进的动态性质的独特机会。我们建议将时间推移的地球物理、水文和地球化学测量与初步的水文模拟相结合。探索性研究将为这一重要领域提供变革性的进展。研究结果将通过研究论文和会议报告进行传播，并将形成包括全面建模工作在内的继续研究计划的基础。

Geophysical imaging of a sabkha in the United Arab Emirates (UAE) following an extreme precipitation event in 2008 shows the first compelling evidence of free convection flow processes in a natural groundwater setting. The imaged pattern is consistent with fingering caused by precipitation-induced redissolution of a halite crust and a pre-existing convection pattern; the distribution of these fingers is similar to those observed in lab experiments and numerical simulations. Natural free convection, which is important for heat and solute transport across a range of hydrogeologic settings, has been studied for over a century but almost entirely in theory, numerical models, and laboratory experiments. Currently, very little is known about the growth and decay of convective instabilities because most research assumes steady state conditions. In order to resolve key questions about origins and persistence of convection, as well as their associated spatial and temporal scales, it is vital to resolve the transient nature of different interacting regimes.Our UAE field site is an ideal location to address some of these knowledge gaps. Precipitation records suggest that fingering at this site may only be initiated once every decade with a halite crust building up during dryer years. Our pilot data demonstrate the unique opportunity to monitor the dynamic nature of density fingering. We propose to integrate time-lapse geophysical, hydrological, and geochemical measurements with preliminary hydrological modeling. The exploratory research will provide a transformative advance for this important field. The results will be disseminated through research papers and conference presentations and will form the foundation for a continued research program including a full-scale modeling effort.