Freshwater storage in brackish-saline aquifers for irrigation water supply: a bottomless pit or a fountain of gold?
KWR Watercycle Research Institute
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Aquifer storage and recovery (ASR) can be successful in storing and recovering freshwater via wells for irrigation water supply. It is attractive thanks to the limited space requirements above ground and the generally successful conservation during storage. However, the ASR recovery efficiency (‘RE’, the fraction of the injected water that can be recovered with a satisfying quality) can be limited in brackish–saline aquifers. This is due to the lower density of the injected freshwater with respect to the ambient brackish or saline groundwater, which causes early contamination with ambient groundwater at lower parts of the ASR well. Successful recovery remains unattainable and temporal freshwater surpluses are then injected in a ‘bottomless pit’. However, recent advances in hydrological engineering allow mitigation of buoyancy effects and an increase of RE. In this way, ASR can offer a fountain of gold in water-stressed coastal areas, which typically have shallow brackish-saline groundwater and therefore a lack of suitable ASR target aquifers. To test the ability of the hydrological engineering solutions to improve RE, two dedicated ASR set-ups were recently implemented in coastal areas of The Netherlands. The first used low-cost, independently operated multiple partially penetrating wells (MPPW) in a single borehole. An extra 20-40% of the injected water could be recovered at MPPW-ASR systems storing greenhouse roofwater in confined, brackish aquifers, thanks to preferential deep injection in the target aquifer and recovery at the aquifer’s top. Interception of brackish water via the deepest well screen further increased the RE (10-20%). Detailed hydrochemical monitoring also highlighted relevant water quality changes occurring during MPPW-ASR, mainly a Na-enrichment due to cation exchange. Also, the deeper aquifer had a dominant impact on the final water quality due to the preferential deep injection and the geochemical composition of this interval. In the second set-up, horizontal directional drilled wells (HDDWs) were implemented in an ASR-system for the first time. Two 70m long, superimposed, low-cost HDDWs were assembled in a ‘Freshmaker’ system to enlarge a shallow freshwater lens by simultaneous infiltration (shallow HDDW) in the lens, and deep abstraction of underlying saltwater (deep HDDW). In dry periods, the shallow HDDW was used for abstraction of freshwater from the lens, while the deep HDDW prevented upconing of deep saltwater by continuation of the abstraction. A maximum yearly volume of around 6,000 m3 of surface water could be successfully injected, followed by successful abstraction of an equal freshwater volume for irrigation at a fruit orchard. The presented innovative ASR set-ups were successfully tested in the horticultural sector to provide irrigation water. The estimated costs per m3 are 0.3 to 1.5 US $/m3, and can compete with the local (yet less sustainable) alternatives (piped water, brackish water desalination). In the freshwater management strategy of the National Deltaprogramme, the innovative ASR solutions were therefore embraced to attain local ‘self-reliance’.