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

已知包气带裂缝在沙漠地区地下水补给中起着重要作用。 该项目将评估这样一种假设，即上部包气带中裂缝表面的蒸发会触发基质内的毛细作用力，从而将孔隙水和溶质从基质中推向裂缝面。 蒸发作用导致裂缝壁上形成盐壳。 随后的降雨事件使水通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，夜间寒冷的温度可能会导致在裂缝空隙内形成热对流单元，大大提高蒸发率。 该项目将：1。测试加速夜间对流的概念，在一个碎石场研究地点。2.在实验室中调查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）与白天（扩散驱动）蒸发速率的比率。开发一个现场规模的数值模型，以分析在一系列气候、断裂和基质条件下这种机制的重要性。 这些参数包括裂缝开度、岩石渗透率和热反转程度。 它将提供一个宝贵的工具，探讨这一机制在全球范围内的重要性，因为它涉及到地下水资源的盐负荷和绕过有毒溶质在已经污染的网站。

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.