A study on the suitability and quantitative potential of aquifer storage and recovery and brackish water extraction in Dutch coastal areas.

Freshwater demand will increase in the coming years due to climate change and socio-economic developments. One approach to help combat this is through aquifer storage and recovery (ASR), whereby excess fresh water is stored in the subsurface and recovered later as required. Furthermore, brackish groundwater extraction (BWE) can also be used to combat salinization and produce fresh groundwater. COASTAR is a Dutch research consortium that focuses on the use of ASR and BWE to secure sustainable access to freshwater in coastal areas. Previous research within this program produced geohydrological suitability maps at a national level through the assessment of several geohydrological factors using existing data. In this study, we test previous results using numerical validation by ‘blindly’ placing ASR and BWE well systems into the centre of 10 x 10km sub models, using 3D variable-density groundwater flow and coupled salt transport modelling. In total, 12 scenarios were simulated for approximately 170 locations in Dutch coastal areas, resulting in over 2000 model simulations. The scenarios were implemented in two different aquifers (shallow or deep) with extraction rates of 1200, 6.000, and 12.000m³/d. The resulting suitability of BWE and ASR systems at a given location was decided by how the system performs and affects the surrounding environment. A sensitivity analysis provided insights into the main geohydrological parameters and threshold values ​​applicable to ASR and BWE. Overall, results were like the pre-existing geohydrological suitability maps but offered further quantitative insights. On an international level, this knowledge can help to better understand suitability in other areas with similar subsurface characteristics. Additionally, a quick-scan analysis was performed to quantify the total potential extractable volumes for ASR and BWE. The results of this are based on maximum possible extraction/infiltration rates for each model area, by estimating the summed environmental effects of multiple wells. The method used an extrapolation approach based on the numerical model results. For this approach two factors were considered: 1) the effect of multiple extractions on the environment (changes in phreatic groundwater head), and 2) the optimal number of wells given the width of the freshwater ‘bubble’ for ASR scenarios. The quick-scan analysis showed that ASR and BWE systems have the potential to fulfill the increase in freshwater demand in the Netherlands in 2050.