Paldor A, Bertoni C, Penny B, Michael H A
Department of Earth, Environmental, and Planetary Sciences, University of Tennessee, Knoxville, TN, USA.
Department of Earth Sciences, Oxford University, Oxford, UK.
Sci Total Environ. 2024 Nov 20;952:175834. doi: 10.1016/j.scitotenv.2024.175834. Epub 2024 Aug 26.
Offshore Freshened Groundwater (OFG) reservoirs are gaining attention, as evidence suggests they are more prevalent worldwide than previously thought. OFG systems are generally classified as either passive, a relic of ancient, lower sea levels, or as active, with an onshore-offshore hydrogeologic connection and associated discharge offshore. Previous studies on the mechanisms of OFG were conducted in various hydrogeologic settings, but the role of faults remains understudied. Based on geologic data, we apply hydrogeologic modeling of a faulted submarine confined aquifer in the Levant basin (eastern Mediterranean), to study the impact of faults on OFG. We find that faults that are close to the coastline and within the brackish zone that would have developed without a fault control the offshore salinities regardless of initial conditions. The influence of distal faults, in contrast, depends on antecedent conditions. When initial salinities are such that the distal fault lies in the fresh part of the aquifer, the saline wedge migrates landward toward the fault with sea-level rise, and the fault dictates the steady-state salinity distribution. If the fault is initially within the saline part of the aquifer, freshwater never reaches the fault, likely due to the density-driven flow barrier that the underlying saline wedge generates. These findings suggest a new mode of OFG in which the same geologic system can be either active or passive depending on the hydrologic history. This should be considered in future studies of OFG systems, the functioning of which has implications for marine ecosystems, seafloor geomorphology, and coastal water resources.
近海淡化地下水（OFG）储层正受到越来越多的关注，因为有证据表明它们在全球范围内比以前认为的更为普遍。OFG系统通常分为被动型，即古代较低海平面的遗迹，或主动型，具有陆上-近海水文地质联系及相关的近海排放。以往关于OFG机制的研究是在各种水文地质环境中进行的，但断层的作用仍未得到充分研究。基于地质数据，我们对黎凡特盆地（东地中海）一个断层控制的海底承压含水层进行水文地质建模，以研究断层对OFG的影响。我们发现，靠近海岸线且位于若无断层控制则会形成的咸淡混合带内的断层，无论初始条件如何，都控制着近海盐度。相比之下，远处断层的影响则取决于前期条件。当初始盐度使得远处断层位于含水层的淡水部分时，随着海平面上升，盐水楔向陆地方向迁移至断层，断层决定了稳态盐度分布。如果断层最初位于含水层的咸水部分，淡水可能永远无法到达断层，这可能是由于下方盐水楔产生的密度驱动流屏障所致。这些发现表明了一种新的OFG模式，即同一地质系统根据水文历史既可以是主动型也可以是被动型。在未来关于OFG系统的研究中应考虑这一点，其功能对海洋生态系统、海底地貌和沿海水资源都有影响。
