While previous studies conclude that riparian freshwater lenses are the result of buoyancy forces that drive river water circulation in the adjacent aquifer, only highly idealized conceptual models have been assessed. A numerical study is conducted to investigate the influence of evaporation from unsaturated bare soils, including accumulation and precipitation of salt, on riparian freshwater lenses, assuming otherwise similar conditions to those adopted in recent studies. Non-evaporating simulations that account for the unsaturated zone are first compared to previous saturation-only numerical and analytical results, and reasonable matches are obtained. The modeling analysis of evaporative simulations includes three different approaches to the treatment of solute in evaporated water: (a) no evaporative salt accumulation, (b) evaporative salt accumulation, and (c) as for (b) except resistance to evaporation caused by salt accumulation is considered. Significant evaporative fluxes caused gaining river conditions to change to losing conditions, leading to the cessation of buoyancy-driven freshwater circulation and to freshwater bodies that are not lenticular. Rather, evaporation reduces regions where the water table and/or unsaturated zone are fresh relative to the freshwater lens volume. Evaporation rates depend on the hydraulic connection between the floodplain surface and water table, which is limited where the water table is deep, the soil hydraulic conductivity is low, and under higher potential evaporation rates. The results show that floodplain evaporation may control the freshwater-saltwater distribution and lead to hypersaline groundwater under certain conditions, although earlier buoyancy-driven conceptual models of riparian lenses appear to remain valid where the thickness of the unsaturated zone is sufficiently large.

Nature area Rammegors, which has recently been transformed from a fresh inner-dyke nature area to a salt tidal area. Due to this transformation, salt water is infiltrating in a fresh waterlens. This salinisation process is investigated in more detail by two- and three dimensional models together with mearsurements in the area. Zeeland project FRESHEM has provided detailed isohaline maps of the area and Deltares is making transient isohaline maps based on measurements made by an ERT-cable which is situated in Rammegors. These data has been and will be used to investigate which factors; bathymetry, lithology, tides or regional groundwater flow, will have the largest impact on the salinization process in Rammegors. This investigation shows that discretization size has an influence on the speed and spatial distribution of salt plumes. Lithology has the largest influence on the salinization process, followed by bathymetry. Spring and neap tides do differ from the normal tides situation only when bathymetry is not taken into account.

<p>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 12.000m<sup>3</sup>/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.</p>

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