Numerical evaluation of community-scale aquifer storage, transfer and recovery technology: A case study from coastal Bangladesh

Abstract

Aquifer storage, transfer and recovery (ASTR) may be an efficient low cost water supply technology for rural coastal communities that experience seasonal freshwater scarcity. The feasibility of ASTR as a water supply alternative is being evaluated in communities in south-western Bangladesh where the shallow aquifers are naturally brackish and severe seasonal freshwater scarcity is compounded by frequent extreme weather events. A numerical variable-density groundwater model, first evaluated against data from an existing community-scale ASTR system, was applied to identify the influence of hydrogeological as well as design and operational parameters on system performance. For community-scale systems, it is a delicate balance to achieve acceptable water quality at the extraction well whilst maintaining a high recovery efficiency (RE) as dispersive mixing can dominate relative to the small size of the injected freshwater plume. For the existing ASTR system configuration used in Bangladesh where the injection head is controlled and the extraction rate is set based on the community water demand, larger aquifer hydraulic conductivity, aquifer depth and injection head improve the water quality (lower total dissolved solids concentration) in the extracted water because of higher injection rates, but the RE is reduced. To support future ASTR system design in similar coastal settings, an improved system configuration was determined and relevant non-dimensional design criteria were identified. Analyses showed that four injection wells distributed around a central single extraction well leads to high RE provided the distance between the injection wells and extraction well is less than half the theoretical radius of the injected freshwater plume. The theoretical plume radius relative to the aquifer dispersivity is also an important design consideration to ensure adequate system performance. The results presented provide valuable insights into the feasibility and design considerations for community-scale ASTR systems but installation of systems in different hydrogeological contexts (i.e. high regional hydraulic gradient and aquifer TDS concentration) may require re-evaluation of the system design.