1 Introduction
SWT | Aim | Target conditions | Artificial recharge | Saltwater interception | Well type | Water treatment |
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ASR-coastal | Temporal storage | Brackish aquifers | yes | no | MPPW | Pre-treatment |
Freshkeeper | Protect wellfields | Stratified groundwater quality | no | yes | PP, MPPW | Optional post-treatment |
Freshmaker | Temporal storage | Freshwater lenses | yes | yes | HDDW | Pre-treatment |
2 Materials and Methods
2.1 Field-Testing of Subsurface Water Technologies (SWT)
2.1.1 ASR-Coastal
2.1.2 Freshkeeper
2.1.3 Freshmaker
2.2 Broader Evaluation of the Efficiency of Subsurface Water Technologies (SWT) to Improve Freshwater Management
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Brackish water upconing (Reilly and Goodman 1987): resorting from shallow abstraction from a stratified aquifer (i.e., freshwater overlying brackish/saline water);
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Seawater intrusion (SWI; Werner et al. (2013): ‘the landward incursion of seawater’ via the subsurface;
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Bubble drift during aquifer storage and recovery (Ward et al. 2009): ‘injected freshwater trying to “float” upwards through the aquifer while the denser native groundwater sinks down and inwards, contaminating the well at the bottom’;
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Thin target aquifer for abstraction / storage: this may imply a low yield per well, requiring placement of expensive well galleries with many wells and pumps;
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Saline seepage in deep polder areas (de Louw et al. 2010): with ongoing land subsidence, sea-level rise, and occasionally (former) peat excavations, saline seepage is a increasing problem in delta areas as it causes salinization of inland surface waters.
3 Results
3.1 Field-Testing of Subsurface Water Technologies (SWT)
3.1.1 ASR-Coastal
3.1.2 Freshkeeper
3.1.3 Freshmaker
3.2 Broader Evaluation of the Efficiency of Subsurface Water Technologies (SWT) to Improve Freshwater Management
Hydrological problem | ASR-coastal | Freshkeeper | Freshmaker |
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Brackish water upconing | +/- | + | + |
Seawater intrusion | +/- | + | + |
Bubble drift during ASR | +/- | +/- | + |
Thin target aquifer for abstraction / storage | - | - | + |
Saline seepage | +/- | + | + |
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Brackish water upconing: this process can be delayed by ASR-coastal, but full elimination cannot be guaranteed as upconing may still occur during recovery, especially when storage periods are long and deeper water is saline, which is similar to scenario A and B of the Freshmaker in Section 3.1.3. When brackish water is not desalinated or discharged from the groundwater system but only re-injected, upconing may also still threaten shallow abstraction wells in the Freshkeeper case, as recently demonstrated by Alam and Olsthoorn (2014). The Freshkeeper and Freshmaker in their presented form (injecting membrane concentrate in a deeper confined aquifer or discharging abstracted saltwater to sea) can sufficiently eliminate upconing, as demonstrated by field monitoring (Section 3.1.2) and model scenarios C and D (Section 4.1.3). For both the Freshkeeper and Freshmaker it is required that freshwater abstraction rates and saltwater interception rates are coupled: overabstraction of freshwater during limited interception of saltwater will result in upconing;
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Seawater intrusion: ASR-coastal may only prevent saltwater intrusion if the net injection exceeds the saltwater intrusion, making it a freshwater hydraulic barrier (e.g. Luyun et al. 2011; Mahesha 1996), with deep injection at the optimal aquifer interval (Abarca et al. 2006). One may expect, however, that the Freshkeeper and the Freshmaker can even prevent saltwater intrusion, provided that the well placement and their abstraction rates are such that the entire intruding saltwater wedge is intercepted and disposed of or desalinated. This was confirmed using density-dependent transport modelling in combination with an optimization model (Abd-Elhamid and Javadi 2011), which indicated that coupled interception and abstraction (ADR: abstraction, desalination, recharge) can then be considered most (cost-) efficient;
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Bubble drift during aquifer storage and recovery: ASR-coastal can reduce the freshwater during ASR, but it was shown by Zuurbier et al. (2014) that it will not lead to 100 % recovery of injected freshwater. Like for conventional ASR: the more saline the aquifer is, the lower the recovery efficiency will be. A Freshkeeper may help to recover a larger part of injected freshwater during ASR, but will also not make the system render 100 % recovery of injected freshwater (Van Ginkel et al. 2014). The Freshmaker concept appeared able to recover a volume equal to the injected freshwater volume in a saline environment (without depleting the natural freshwater lens), given that an existing freshwater lens is enlarged and natural recharge is occurring;
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Thin target aquifer for abstraction / storage: ASR-coastal and the Freshkeeper will be hard to apply in thin aquifers. However, the use of HDDWs in the Freshmaker case may make thin aquifers viable for abstraction/storage, since a single, high-capacity well is feasible. Approriate design of the HDDW (length, diameter, pumping rates, amongst others) is however crucial to attain a uniform distribution of the abstraction along the HDDW screen (Sun and Zhan 2006; Wang et al. 2014). At the Freshmaker trial, this was ensured by the relatively limited length of the HDDWs (70 m) and confirmed by the observed lowering of the freshwater-saltwater interface along full length the wells;
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Saline seepage in deep polders: ASR-coastal may freshen the diffusive seepage component sourced by shallow groundwater in the upper aquifer, but is less effective in counteracting seepage of deeper, saline groundwater via boils, which can be the largest salt contributor in polder areas (de Louw et al. 2010). The Freshkeeper concept was previously suggested as a suitable technique to counteract saline seepage (Olsthoorn 2008; Stuyfzand and Raat 2010), although it was considered unviable when all abstracted water is directly re-injected in deeper aquifers (De Louw et al. 2007) because of hydrological effects in the surrounding areas and the required high pumping rates. This underlines that disposal or concentration of the abstracted brackish water is desirable. The Freshmaker can decrease the saline seepage in polder areas based on the modelling performed for the Ovezande field pilot (Section 3.1.3). The current set-up does not contribute to a reduction in salt load to the local surface water system, as the intercepted saltwater is disposed of at a local water course here,. Disposal of intercepted brackish-saline groundwater and membrane concentrate is therefore expected to be a key element in coastal freshwater management.