Design and Optimisation of an Aquifer Storage and Recovery System - Options for long term storage under natural hydraulic gradients

Sammanfattning: Aquifer storage and recovery (ASR) is a concept where water is stored in an aquifer during times when it is available, thus creating artificial groundwater, which can later be recovered by pumping when it is needed (Pyne, 1995). One ASR cycle consists of a injection, a standby and a recovery phase. The purpose of this work is to investigate the feasibility of large scale ASR systems intended for long term water storage purposes under a natural hydraulic gradient. A framework for the design and optimisation of such systems is presented, in which the individual ASR phases are considered separately. For each phase, the design objective is identified, the most important optimisation parameters highlighted and a methodology established to evaluate the performance based on the desired result. The design approach is demonstrated on a synthetic example by means of numerical groundwater modelling with the software FEFLOW. The results show that implementation of suitable operating schemes which prevent a migration of the injected freshwater with the natural groundwater flow are essential to increase the ASR systems performance and make it feasible for long storage durations. For such applications, optimisation of the standby phase is decisive. In the design process, the aquifer hydraulics determine the exact layout of a well-field. Once this is established, the optimisation process should be directed at (i) removing the native groundwater from the well-field domain during the injection phase, (ii) mitigating the drift of the injected water with minimal pumping efforts in the standby phase and (iii) maximising the quantity of extracted water in the recovery phase. A basic layout of the well-field can be obtained through hydraulic testing at the scale of a cluster of wells, while the operating schemes for the different ASR phases should be established at the scale of a full sized well-field. Different schemes were derived for the individual ASR phases based on the respective design objectives, optimisation parameters and evaluation methods.