Improving the recovery efficiency of the Aquifer Storage and Recovery system in Hoorn

Abstract

PWN is responsible for supplying sufficient quantities of high-quality water to its customers. The growing population and the effects of climate change, such as heat waves and droughts, are straining the capacity of the water supply network, especially during dry and warm periods when water demand increases. A significant portion of the drinking water is produced in Andijk. From there, the drinking water is transported to Hoorn, where it is further distributed to customers. The connection between Andijk and Hoorn is therefore crucial, and in the event of a pipeline failure due to a calamity or maintenance, capacity issues can arise. To maintain the redundancy of this connection and to smooth out the daily water demand and supply fluctuations, PWN is exploring the implementation of an Aquifer Storage and Recovery (ASR) system in Hoorn.

The ASR system in Hoorn faces strict requirements, which address the challenge of maintaining water quality standards and optimising recovery efficiency. These requirements must ensure that the extracted water remains suitable for consumption, with no more than 1% dilution with ambient groundwater. The objective of this study is to identify a method to improve the recovery efficiency of the ASR system in Hoorn. The ASR system operates by injecting drinking water into an aquifer during periods of water availability and recovering it when needed. Compared to installing a new pipeline, the ASR system offers a more cost-effective solution with additional benefits such as space efficiency and temperature stability. However, the ASR system in Hoorn faces challenges related to maintaining water quality standards and optimising recovery efficiency. Processes such as lateral flow, dispersion, and buoyancy affect the system’s performance, and a thorough understanding of these processes is crucial for accurately predicting recovery efficiency. A comprehensive analysis of a pilot ASR system in Hoorn was conducted by PWN to address these challenges. The pilot system consists of a single well where two pumps operate at two different filter depths in an aquifer. In the final layout of the ASR, additional wells are necessary to achieve the desired capacity.

A radially symmetric model was used to simulate groundwater flow, conservative solute transport, and heat transport. Due to the stringent water quality requirements, the radially symmetric model must accurately capture essential processes in an Aquifer Storage and Recovery (ASR) system, such as flow, dispersion, retardation, and buoyancy. The performance of a radial symmetric model in SEAWAT and MODFLOW 6 was assessed based on analytical methods and 3D models. Through this analysis, it was decided to utilise a radial symmetric model in SEAWAT due to the presence of numerical dispersion in a model using MODFLOW 6.

After this analysis, the model’s performance was tested against various measurements, including hydraulic head, temperature, and electrical conductivity. It is evident that the model effectively captures both solute transport and heat transport. Discrepancies between measurements and the model can be attributed to assumptions made during the study and uncertainties in the measured values. However, the presence of clay layers between the deep and shallow filters in the pumping well significantly contributes to local differences between the model and the measurements. The main reason for this difference is that these layers are not homogeneous throughout the depths, allowing water to flow between them. This heterogeneity cannot be simulated with a radially symmetric model. However, despite this heterogeneity, these clay layers consistently result in low recovery efficiency in the current system.

The objective of this study was to identify a method to improve the recovery efficiency of the ASR system in Hoorn. The current system has a recovery efficiency of about 30%. This can improved by implementing a check valve in the shallow filter of the pump well, with 60% to 65% of the filter dedicated to recovery to achieve a recovery efficiency of 80%. During the testing of this system, an injection period was followed by a recovery period with specific pumping rates. It took three cycles to achieve the desired recovery efficiency. It is important to note that these cycles did not include storage and rest phases. The system’s recovery efficiency may change when these phases are incorporated. However, an important assumption is that homogeneous layers are present. Heterogeneity of layers can lead to deviations from the modelled recovery efficiency. This research contributes to a better understanding of the pilot ASR system in Hoorn and provides insights into improving its recovery efficiency. With the lessons learned from this study,
PWN can assist in developing the final design for the ASR system. This design will involve multiple wells to meet the required capacity.