Vertical Flux in a Two-Layer Aquifer System Absent of Aquitard

Abstract

Heterogeneously layered river-basin or reservoir-bank aquifers are widely distributed, and the two-layer aquifer system is a simple representative of such aquifers. In this research, a conceptual model of vertical groundwater flux induced by a rising prescribed head boundary in a two-layer aquifer system (without an aquitard in between) is developed. A semianalytical solution is derived using Green’s function method (GFM) and compared with finite-element numerical solutions. The result indicates that differences of hydraulic parameters have profound effects on the hydraulic head distribution in both layers and dynamics of vertical mass exchange between the layers. A larger media anisotropy (Am) leads to a smaller buildup contrast between the two layers and a greater peak value of vertical groundwater flux across the interface of two layers. The buildup in the upper layer is insensitive to KLD [which is the permeability ratio of the lower (less permeable) layer to the upper (more permeable) layer], whereas the buildup in the lower layer is sensitive to KLD. A greater permeability contrast between the two layers causes a larger groundwater flux across the interface and a greater peak flux. A larger specific storage contrast leads to smaller buildups, and a smaller specific storage contrast leads to a greater peak vertical groundwater flux. Among many applications, the proposed solutions have profound chemical, biological, and thermal transport implications because tracer elements or heat of different layers can dynamically interact with each other, driven by the vertical groundwater flux of different layers.