Abstract
Aquifer storage and recovery (ASR) refers to injecting freshwater into an aquifer and later withdrawing it. In brackish-to-saline aquifers, density-driven convection and fresh-saline water mixing lead to a reduced recovery efficiency (RE, i.e., the volumetric ratio between recovered potable water and injected freshwater) of ASR. For a layered aquifer, previous studies assume a constant hydraulic conductivity ratio between neighboring layers. In order to reflect the realistic formation of layered aquifers, we systematically investigate 120 layered heterogeneous scenarios with different layer arrangements on multiple-cycle ASR using numerical simulations. Results show that the convection (as is reflected by the tilt of the fresh-saline interface) and mixing phenomena of the ASR system vary significantly among scenarios with different layer arrangements. In particular, the lower permeable layer underlying the higher permeable layer restricts the free convection and leads to the spreading of salinity at the bottom of the higher permeable layer and early salt breakthrough to the well. Correspondingly, the RE values are different among the heterogeneous scenarios, with a maximum absolute RE difference of 22% for the first cycle and 9% for the tenth cycle. Even though the difference in RE decreases with more ASR cycles, it is still non-negligible and needs to be considered after ten ASR cycles. The method to homogenize the layered heterogeneity by simply taking the arithmetic and geometric means of the hydraulic conductivities among different layers as the horizontal and vertical hydraulic conductivities is shown to overestimate the RE for multiple-cycle ASR. The outcomes of this research illustrate the importance of considering the geometric arrangement of layers in assessing the feasibility of multiple-cycle ASR operations in brackish-to-saline layered aquifers.
Highlights
Aquifer storage and recovery (ASR) refers to injecting freshwater into an aquifer for temporary water storage when freshwater is in surplus, and later withdrawal at shortfalls of freshwater supply using the same or, less commonly, a nearby well [1,2,3,4]
The aquifer in the ‘homogeneous case (Hom)’ case can be considered as five layers of identical hydraulic conductivity
This indicates relative low density effect during injection, and it is consistent to the small value of mixed convection ratio (M = 8.772 × 10−3)
Summary
Aquifer storage and recovery (ASR) refers to injecting freshwater into an aquifer for temporary water storage when freshwater is in surplus, and later withdrawal at shortfalls of freshwater supply using the same or, less commonly, a nearby well [1,2,3,4]. Ward et al [27] conducted the mixed convection analysis of ASR In their models, the pumping-induced forced convection and density-driven free convection simultaneously control the flow during injection and recovery, whereas flow during storage is only controlled by the free convection. A higher M value indicates a stronger intensity of density-driven convection and it leads to an earlier saline water breakthrough at the bottom of the ASR well during recovery phases, thereby reducing the RE [27]. Since natural aquifers are heterogeneous and a different K value results to a different density effect (refer to Equation (2)), Ward et al [28] investigated a layered aquifer incorporating successive horizontal, isotropic layers with alternating low and high hydraulic conductivities. The outcomes gained from this research are expected to improve the planning and feasibility assessment of ASR in heterogeneous layered brackish-to-saline aquifers
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