The impacts of dual-porosity aquifers on Aquifer Storage Recovery (ASR) schemes

摘要

Dual porosity formations, such as the Cretaceous Chalk, can be prolific aquifers and have the potential to store large volumes of artificially recharged potable water. The unique characteristic of these aquifers is the large volume of effectively immobile pore water stored in the rock matrix (up to 40percent porosity) and the relatively small volume of mobile water stored in fractures (1 to 2percent porosity). Diffusive mixing of the fracture water and pore water does occur however, so the quality of the native pore water impacts on the quality of the water recovered subsequent to injection and storage. The SWIFT/486 model was used to investigate the relative significance of diffusive and dispersive mixing. The diffusive mixing process was shown to have a significant deleterious effect on the quality of the recovered water after a period of storage. This would not happen if only dispersive processes are significant. A good fit between measured and modelled data for non-reactive solutes gave confidence that the diffusive model adequately describes the significant transport properties of the chalk aquifer. If the native groundwater contains solutes at unacceptable concentrations then large volumes of injected water need to the invested to create a buffer zone within which an operational scheme can be used. Water-water and water-rock geochemical reactions, both detrimental and beneficial, are not considered here and need to be monitored and modelled separately and, if possible, linked to the predictive operational scenarios.