The Efficiency and Economics of the Demonstrated SWT
The presented subsurface water technologies (SWT) highlight a recent trend in hydrological engineering driven by new drilling techniques, water treatment, and automation of water supply facilities using sophisticated programming and sensors. The SWT-examples (ASR-coastal, Freshkeeper, and Freshmaker) show that despite the increasing complexity, these technologies can realize a significant increase in freshwater availability in coastal areas for a competitive cost price.
SWT will not fully overcome all the mentioned hydrological problems in all coastal zones, but can generally improve freshwater production, or as found in this study: a reduction of freshwater losses during storage of several tens of percents or the complete prevention of freshwater upconing. This has economical relevance. For instance, the greenhouse owner’s water demand at the Nootdorp ASR-coastal field site requires an average recovery efficiency of approximately 40 % of the injected water. It was demonstrated that this was not feasible with a conventional ASR well (<20 %) or a shallow partially penetrating well (<35 %). The use of a MPPW at this site (with only minor additional costs for PVC pipelines, standpipes, and valves) boosted the freshwater recovery up to more than the owner’s demand (55 %). Instead of investing in more expensive and less sustainable freshwater sources (in this case: desalinated or piped water), there is now a valuable freshwater surplus that can be sold to neighbouring companies with a higher demand.
In the case of the Freshkeeper (Noardburgum) an entire well field was closed and replaced following salinization in 1993. SEAWAT modelling scenarios suggest that installation of only six Freshkeeper wells in a circular set-up is sufficient to prevent salinization of the entire well-field in future (Oosterhof et al. 2013; Van der Valk 2011). Since Vitens Water Supply was looking for additional drinking water in this region, this is a cost-reducing outcome. A recent study has shown the potentials of Freshkeeper to abate salinization problems in Florida (USA), and to guarantee the long-term drinking water supply there (Ross et al. 2014). In the Florida case, a Freshkeeper was found economically much more feasible than alternative water supply options such as full-scale brackish water reverse osmosis. The exact economic benefits of SWT for other cases may vary and likewise for normal MAR-techniques, they are often hard to assess a priori due to feasibility uncertainties and the chance of under-performance (Arshad et al. 2014; Maliva 2014). However, the SWT ability to counteract reductions in freshwater production resulting from unsuitable aquifer conditions will mitigate the increase of operational expenditures, potentially compensating for higher capital expenditures.
Other SWT Examples
SWT are not limited to the field test examples presented in this paper. For instance, Van Ginkel et al. (2014) proposed an elegant concept to store freshwater in an Egyptian saline aquifer by combining freshwater storage with saltwater abstraction from below the injected freshwater, which has similarities with and can further improve the ASR-coastal concept. Alam and Olsthoorn (2014) proposed to discharge a part of the intercepted brackish water by deep Punjab scavenger wells to achieve a net freshening effect (comparable to elements of both the Freshkeeper and the Freshmaker). In 2013, the Baton Rouge Water Co. (U.S.A.) has installed a brackish water scavenger well that, similar to the Noardburgum Freshkeeper, should prevent brackish water upconing to the overlying freshwater production wells. The pumped brackish water is disposed of to the Mississippi river (Tsai 2011). Olsthoorn (2008) and Stuyfzand and Raat (2010) proposed a Freshkeeper at a polder scale, using the abstracted brackish water for drinking water production and simultaneously solving various environmental problems caused by upward seepage of nutrient-rich brackish groundwater at the same time. However, no Freshkeeper is currently operating for this purpose.
Wider Scope of Application
The SWT development and studies mentioned above suggest that although the field-tested SWT are all situated in the Netherlands, they potentially have a much wider scope of application. This is underlined by the evaluation of the SWT in this study, which shows SWT can be used to reduce or overcome very common hydrological problems in coastal zones, which are amplified by an expected exacerbation of saltwater intrusion in coastal zones by sea-level rise and changes in both recharge and evaporation due to global climate change (Oude Essink et al. 2010), which will require a more enhanced management of coastal aquifers (Werner et al. 2013). SWT fulfills the demand for more advanced management tools to deal with coastal groundwater salinization and the demand for increased freshwater storage.
Elements of the SWT discussed in this paper may also be combined. For instance, a Freshkeeper was recently added to a new field ASR-coastal system to protect shallow recovery wells and produced additional freshwater via RO-treatment. In this field pilot, clogging of the RO-membranes is monitored with large interest, since these receive a feedwater, which is a mixture of infiltrated fresh, oxic rainwater with saline, anoxic groundwater. In general, abstracted water quality is a relevant aspect when RO-treatment is involved in SWT since the chemical and physical (suspended fines, temperature) quality of water used for RO, which is abstracted close to the freshwater–saltwater transitions may vary significantly over time due to freshening, salinization, and changes in redox conditions, especially upon artificial infiltration of fresh, oxic water. Membrane selection and prevention of membrane clogging are, therefore, critical aspects when desalination via RO is incorporated in the selected SWT.
It should be noted that all current SWT examples are being tested in sandy aquifers in The Netherlands, which are dominated by intergranular flow. However, limestone aquifers are also frequently found in coastal zones, and are targeted for freshwater supply worldwide. Transport processes may differ significantly in such aquifers due to dual-porosity (Bibby 1981). This can lead to underperforming ASR-systems due to early salinization via preferential flow paths (e.g., Maliva and Missimer 2010; Missimer et al. 2002; Pyne 2005). In the same way, this may reduce the effectiveness of SWT, since flow patterns are less predictable and preferential flow paths may hamper for instance the interception of brackish-saline water by the Freshkeeper and the Freshmaker.
Disposal of concentrate produced upon desalination as applied in the Freshkeeper example may be another obstacle, as this is often not allowed on surface waters or sewage systems. Re-injection in deeper aquifers on the other hand may induce (local) groundwater salinization and is therefore under discussion. Important prerequisites for this disposal are often the salinity of the receiving aquifer and the required separation of abstraction and injection well screens by aquitards because local salinization and short-circuiting must be prevented. Since desalination in combination with deep disposal of concentrate does not directly add an additional salt mass to the groundwater system, it may be more relevant to evaluate the regional consequences of this net abstraction of H2O from the groundwater body. A key question is then if this net abstraction is compensated by either intrusion of more saline groundwater (negative) or by recharge of freshwater (naturally or artificially). The latter is often the case when inland brackish or saline groundwater originates from former transgressions in coastal zones that are currently recharged by freshwater, while seawater intrusion is generally limited to areas close to the shore.
The Future of SWT
SWT provide a coupled solution of a natural ecosystem service with a technological approach that allows for an enhanced protection and utilization of the freshwater resources in coastal areas. The SWT described in this paper have all been developed within public-private partnerships of innovators in the water market. SWT are gaining more-and-more interest from early adopters in the Netherlands. Following the pilot described in this paper, authorities in western Netherlands now consider ASR-Coastal as an important tool serving their regional water governance, and are stimulating greenhouse owners to increase their water self-supportiveness by applying this technique. Recently, a group of farmers in southwest Netherlands have inquired for a Freshmaker feasibility study to improve the irrigation water supply in their orchards. Vitens Water Supply in the North of the country just started a follow-up Freshkeeper pilot study that should be the final step towards full-scale application in the near future.
Despite the growing interest for SWT, further uptake inside and outside The Netherlands is slowed down by a number of non-technical barriers, including a lack of: (1) demonstration of long-term viability, (2) an analysis of their hydrological effects in their surroundings, (3) knowledge of new technologies and the ability to construct and operate them, (4) capabilities upon making investment decisions, and (5) inherent conservatism due to a lacking local track-record of successful implementation of SWT. As a consequence, more expensive and potentially unsustainable but proven technologies are chosen for freshwater management, such as seawater desalination or restrictions on water delivery. We plea for prolonged SWT testing in the current pilots, replication of SWT pilots in other areas worldwide, and the development of technical and non-technical support tools that can facilitate potential end-users in investment decision making and SWT implementation. Such an approach will accelerate acceptance and implementation of subsurface water technologies as robust answers to freshwater resources challenges in coastal areas.