Freshwater lenses in otherwise saline environments contain an important source of fresh water for natural vegetation and agricultural crops. Such lenses are regularly found in areas where both upward seeping saline groundwater and downward infiltrating fresh recharge water occur simultaneously during part of the year, resulting in shallow freshwater lenses which are highly susceptible to changes in recharge or seepage. In a series of two papers, we consider the water – and solute transport in a 2D cross-section between two parallel outflow faces. In this second part of the series, we build upon expressions presented in the first part to consider a situation where the density of seepage water exceeds that of recharge water, as typical for many deltaic areas around the world. Analytical expressions and approximations are given to obtain the steady state position of the interface between the two types of water using a sharp interface approximation, with a focus on the position midway between two outflow faces. Results show that the effect of a heterogeneous density distribution is limited when the seepage flux exceeds the density difference induced flux, but increases rapidly for ratios of the seepage flux over the density flux falling below 1. The heterogeneous density distribution then results in a decrease in freshwater lens thickness and, correspondingly, a decrease in fresh water availability. We also consider time-variant, oscillatory boundary conditions, and show that for heterogeneous density distributions the interface approaches its equilibrium position faster than for a corresponding situation with a homogeneous density distribution, indicating a higher vulnerability for changing boundary conditions. We also demonstrate that heterogeneous density distributions have limited effect on the amplitude of the interface oscillations. Analytical results obtained with the simplified model are validated using the numerical code SUTRA, which solves the full model for a numerical grid.

中文翻译：

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由向上和向下渗流产生的土壤水之间的界面和混合区 - 第二部分：非均质总密度。


盐碱环境中的淡水透镜体包含自然植被和农作物的重要淡水来源。这种透镜体经常出现在一年中的部分时间同时出现向上渗出的含盐地下水和向下渗入的淡水补给水的区域，导致浅淡水透镜体极易受到补给或渗流变化的影响。在一系列的两篇论文中，我们考虑了两个平行流出面之间二维横截面中的水和溶质传递。在本系列的第二部分，我们以第一部分介绍的表达式为基础，考虑了渗流水密度超过补给水密度的情况，这在世界上许多三角洲地区都是典型的。给出了解析表达式和近似值，以使用尖锐界面近似获得两种水之间界面的稳态位置，重点是两个流出面之间的中间位置。结果表明，当渗流通量超过密度差诱导通量时，非均质密度分布的影响是有限的，但当渗流通量与密度通量之比低于 1 时，非均质密度分布的影响会迅速增加。然后，异质密度分布导致淡水晶状体厚度减少，相应地，淡水可用性减少。我们还考虑了时变振荡边界条件，并表明对于非均质密度分布，界面比具有均匀密度分布的相应情况更快地接近其平衡位置，这表明变化的边界条件具有更高的脆弱性。 我们还证明，异质密度分布对界面振荡幅度的影响有限。使用简化模型获得的分析结果使用数字代码 SUTRA 进行验证，该代码求解数字网格的完整模型。