Worldwide, land reclamations are constructed for the urban expansion of coastal megacities. Freshwater supply plays an important role in their sustainable development, especially in the light of climate change and depletion of natural water resources in the hinterland. The subsurface of these new lands offer opportunities for subsurface freshwater storage and recovery. Moreover, the design from scratch and construction makes it possible to not only manage the mixing and buoyancy of a freshwater volume in a saline aquifer operationally, but also to create physical properties of the subsurface to reach high recovery efficiencies. In this dissertation, three concepts have been identified that allow managing the mixing and density stratification of a freshwater volume in saline aquifers. These are: 1) the properties of these man made islands that reduce mixing and density stratification, 2) vertical flow barriers of limited depth that prevent the volume of fresh water from expanding radially, speeding up the formation of the freshwater stock, and 3) saltwater extraction from below the freshwater stock, which prevents the freshwater volume for floating up by counteracting buoyancy. Secondly, insight has been given in the internal structure of the porous media and its hydraulic properties of five land reclamations that were constructed by bottom dumping, rainbowing and pipeline discharge. The increasing number of land reclamations that result from the ongoing worldwide urbanisation of coastal areas, for which a robust freshwater supply must be guaranteed, make the results of this thesis widely applicable.@en

Spatially constant porosity and hydraulic conductivity are usually applied in hydrological studies related to land reclamations. However, the grain sorting and the degree of compaction within land reclamations differ per placement method. A study area at Maasvlakte II, the Netherlands, and the four other land reclamations that could be found in the literature are considered that were constructed by a combination of bottom dumping, rainbowing and discharging the sand-water mixture by pipeline. The structures of the porous media are derived for each placement method and validated by comparison with semi-variograms of cone-penetration tests. It is found that all placement methods lead to some degree of heterogeneity, so that the hydraulic conductivity in these land reclamations is not constant. This is due to the degree of segregation of the grain sizes that differs between placement methods. Segregation even varies within a specific placement method because of its characteristics and site-specific circumstances such as settling depth, grain-size distribution and angularity resulting from grain type. If land reclamations are considered for aquifer storage and recovery for freshwater supply, it should be considered that the recovery efficiency will be affected by both the properties of the material in the borrow area and by the placement methods including their spatial configuration as applied during construction of the reclamation.

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Regular aquifer storage recovery, ASR, is often not feasible for small-scale storage in brackish or saline aquifers because fresh water floats to the top of the aquifer where it is unrecoverable. Flow barriers that partially penetrate a brackish or saline aquifer prevent a stored volume of fresh water from expanding sideways, thus increasing the recovery efficiency. In this paper, the groundwater flow and mixing is studied during injection, storage, and recovery of fresh water in a brackish or saline aquifer in a flow-tank experiment and by numerical modeling to investigate the effect of density difference, hydraulic conductivity, pumping rate, cyclic operation, and flow barrier settings. Two injection and recovery methods are investigated: constant flux and constant head. Fresh water recovery rates on the order of 65% in the first cycle climbing to as much as 90% in the following cycles were achievable for the studied configurations with constant flux whereas the recovery efficiency was somewhat lower for constant head. The spatial variation in flow velocity over the width of the storage zone influences the recovery efficiency, because it induces leakage of fresh water underneath the barriers during injection and upconing of salt water during recovery.

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