Reactive convection cells: A mechanism for transport and precipitation of salt in fractures under evaporative conditions

包气带裂隙在沙漠地区的地下水补给中起着重要作用。该项目将评估这一假设，即从上部包气带裂隙表面蒸发触发基质内的毛细管力，将孔隙水和溶质从基质驱动到裂隙表面。蒸发会在裂缝壁上形成盐壳。随后的降雨活动使水穿过裂缝，溶解盐壳，并将其输送到地下水位。此外，在沙漠条件下，夜间的寒冷温度可能会导致裂隙空洞内形成热对流单元，从而极大地提高蒸发速率。该项目将：1.在裂隙岩石场研究现场测试夜间加速对流的概念。2.在实验室研究两种岩石渗透率的盐壳形成速率，以确定：(A)蒸发速率的时间变化率和强度；(B)盐壳形成的速率和程度；以及(C)夜间(对流驱动)和白天(扩散驱动)蒸发速率的比率。开发一个现场尺度的数值模式来分析这种机制在一系列气候、裂缝和基质条件下的意义。这些参数将包括裂缝开度、岩石渗透率和热转化程度。这项研究将有助于更好地了解盐类聚集和运移的机制。它将为在全球范围内探索这一机制的重要性提供宝贵的工具，因为它与地下水资源的盐分负荷有关，并绕过了已经受到污染的地点中的有毒溶质。

Vadose zone fractures are known to play a significant role in groundwater recharge in desert areas. This project will evaluate the hypothesis that evaporation from fracture surfaces in the upper vadose zone triggers capillary forces within the matrix that drive pore-water and solutes from the matrix toward the fracture faces. Evaporation results in salt crust formation on the fracture walls. Subsequent rain events move water through the fractures dissolving salt crusts and transporting them to the water table. Furthermore, under desert conditions, nighttime cold temperatures may result in the formation of thermal convection cells within fracture voids, dramatically enhancing evaporation rates. The project will: 1. Test the concept of accelerated night-time convection at a fractured-rock field study site. 2. Investigate salt crust formation rate in the laboratory for two rocks permeabilities to determine: (a) time rate of change and strength of the evaporation rate; (b) rate and extent of the salt crust formation; and (c) ratio of nighttime (convection driven) to daytime (diffusion driven) evaporation rates.3. Develop a field scale numerical model to analyze the significance of this mechanism under a range of climatic, fracture and matrix conditions. Parameters will include fracture aperture, rock permeability and extent of thermal inversion.This study will provide a better understanding of the mechanism for salt accumulation and transport. It will provide a valuable tool for exploring the importance of this mechanism on a global scale as it relates to salt loading of groundwater resources and bypass toxic solutes in already contaminated sites.