Coastal aquifers and the deeper subsurface are increasingly exploited. The accompanying perforation of the subsurface for those purposes has increased the risk of short-circuiting of originally separated aquifers. This study shows how this short-circuiting negatively impacts the freshwater recovery efficiency (RE) during aquifer storage and recovery (ASR) in coastal aquifers. ASR was applied in a shallow saltwater aquifer overlying a deeper, confined saltwater aquifer, which was targeted for seasonal aquifer thermal energy storage (ATES). Although both aquifers were considered properly separated (i.e., a continuous clay layer prevented rapid groundwater flow between both aquifers), intrusion of deeper saltwater into the shallower aquifer quickly terminated the freshwater recovery. The presumable pathway was a nearby ATES borehole. This finding was supported by field measurements, hydrochemical analyses, and variable-density solute transport modeling (SEAWAT version 4; Langevin et al., 2007). The potentially rapid short-circuiting during storage and recovery can reduce the RE of ASR to null. When limited mixing with ambient groundwater is allowed, a linear RE decrease by short-circuiting with increasing distance from the ASR well within the radius of the injected ASR bubble was observed. Interception of deep short-circuiting water can mitigate the observed RE decrease, although complete compensation of the RE decrease will generally be unattainable. Brackish water upconing from the underlying aquitard towards the shallow recovery wells of the ASR system with multiple partially penetrating wells (MPPW-ASR) was observed. This “leakage” may lead to a lower recovery efficiency than based on current ASR performance estimations.
Confined and semi-confined aquifers are increasingly being used for
storm water and (Ferguson, 1990), brine disposal (Stuyfzand and Raat,
2010; Tsang et al., 2008) and storage of freshwater (aquifer storage and
recovery or ASR; Pyne, 2005), heat (aquifer thermal energy storage or
ATES; Bonte et al., 2011a), and CO
The perforation of aquifers and aquitards accompanying the subsurface activities imposes an additional risk by the potential creation of hydraulic connections (“conduits”) between originally separated aquifers or aquifers and surface waters. This risk is plausible, as estimations indicate that about two-thirds of the wells worldwide may be improperly sealed (Morris et al., 2003), although the attention for this potential risk is limited (Chesnaux, 2012). Additionally, many of the new concepts to use the subsurface (e.g., ATES, ASR, brine disposal) require injection via wells, which may cause fractures, even when the annulus is initially properly sealed, by exceedance of the maximum-permissible injection pressure (Hubber and Willis, 1972; Olsthoorn, 1982). The soil fractures are undesirable for most groundwater wells in the relatively shallow subsurface, since they create new connections between originally separated aquifers.
The resulting short-circuiting or leakage process has been studied at
laboratory (Chesnaux and Chapuis, 2007) and field scale
(Jiménez-Martínez et al., 2011; Richard et al., 2014), and for
deep geological CO
The lack of proper design and regulation of subsurface activities using wells can be partly caused by the lack of clear field examples of how well-intentioned use of the subsurface for sustainability purposes can fail thanks to earlier activities underground. This lack can be caused by the fact that short-circuiting may not be easy to observe (Santi et al., 2006), or because failing or disappointing projects often do not make it to public or scientific reports. Therefore, we present in this study how short-circuiting via a deeper borehole led to failure of freshwater recovery during ASR in a coastal aquifer. The objective of this paper is to demonstrate and characterize the potential consequences of perturbations for coastal ASR systems. Additionally, the use of deep interception of saltwater to improve shallow recovery of freshwater upon ASR was assessed. The Westland ASR site in the coastal area of the Netherlands served as a demonstration and reference case.
Cross section of the Westland ASR site to schematize the geology, ASR wells, ATES well, and the typical hydrochemical composition of the native groundwater. Horizontal distances not to scale.
The Westland ASR system is installed to inject the rainwater surplus of
270 000 m
Depth of the various well screens.
Summary of the ASR operation.
Locations of ASR (AW), ATES, and monitoring wells (MW).
The ASR wells used a 3.2 m high standpipe to provide injection pressure,
whereas the ATES well used a pump to meet the designed injection rate of 75 m
The target aquifer for ASR (Aquifer 1) was found to be 14 m thick and
consists of coarse fluvial sands (average grain size: 400
The groundwater is typically saline, with observed Cl concentrations ranging
from 3793 to 4651 mg L
All ASR and monitoring well screens were sampled prior to ASR operation
(November and December, 2012). MW1 and MW2 were sampled with a high frequency
during the first breakthrough of the injection water at MW1 (December 2012,
January 2013), while all wells were sampled on a monthly basis
(Table 3). In all, 3 times the volume of the well
casing was removed and stable field parameters were attained prior to
sampling. The injection water was sampled regularly during injection phases.
All samples were analyzed in the field in a flow-through cell
for electrical conductivity (EC) (GMH
3410, Greisinger, Germany), pH and temperature (Hanna 9126, Hanna
Instruments, USA), and dissolved oxygen (Odeon Optod, Neotek-Ponsel,
France). Samples for alkalinity determination within 1 day after sampling
on the Titralab 840 (Radiometer Analytical, France) were stored in a 250 mL
container. Samples for further hydrochemical analysis were passed over a
0.45
Sampling rounds at the Westland ASR site (2012–2014). “IN” is injection water.
Hydrogeological properties of the geological layers in the Westland SEAWAT model.
Combined electrical conductivity, temperature, and pressure transducers (CTD) divers (Schlumberger Water Services, Delft, the Netherlands) were used for continuous monitoring of conductivity, temperature, and pressure in the target aquifer at MW1 and MW2. Calibrated, electronic water meters were coupled to the programmable logic controller (PLC) of the ASR system to record the operation per well screen.
Groundwater transport modeling was executed to validate the added value of
the MPPW setup under the local conditions. In the later stage of the
research, the groundwater transport model was used to test potential
pathways for deeper groundwater to enter the target aquifer and explore the
characteristics of a potential conduit via scenario modeling. Correction
for groundwater densities in the flow modeling was vital, due to
significant contrast between the aquifer's groundwater and the injected
rainwater. In order to incorporate variable density flow and the transport
multiple species, SEAWAT version 4 (Langevin et al., 2007)
was used with PMWIN 8 (Chiang, 2012) to simulate the ASR operation.
A half-domain was modeled to reduce computer runtimes (Fig. 4). Cells of 1
Equal constant heads were imposed at two side boundaries of the aquifers,
the top of the model (controlled by drainage) and at the base of the model.
No-flow boundaries were given to the other two side boundaries of the model.
Initial Cl concentrations were based on the results of the reference
groundwater sampling at MW1. SO
The recorded pumping rates of the ASR wells and the ATES K3-b well during
two ASR cycles were incorporated in the SEAWAT model. The ASR operation was
modeled with a properly sealed and an unsealed ATES borehole. In the latter
case, a hydraulic conductivity (
The collected data on the aquifer characteristics in the SEAWAT groundwater
model were used to analyze the future performance of the MPPW-ASR system for
the current (with leakage) and a “normal field site” (without leakage from
deeper aquifers via a perturbation, or after sealing of the perturbation).
The SEAWAT model was used to simulate three consecutive ASR cycles with the
representative operational characteristics from
Table 5 for the Westland site (Zuurbier
et al., 2012). Once the recovered Cl concentration exceeded 50 mg L
Setup of the modeled, representative ASR cycle for the Westland subsurface without short-circuiting of deeper saltwater.
Cumulative grain size contents observed at MW1 (at 5 m from ASR well 1) in this study. S1–S3 mark the depth intervals of the ASR well screens.
Setup of the Westland ASR groundwater transport model (half-domain).
The first ASR cycle started in December 2012. The first recovery started
halfway January 2013. Despite the abstraction with only the shallow wells of
the MPPW, a rapid and severe salinization was found within the first days of
recovery, after injecting freshwater for about 1 month
(Fig. 5). It was expected that due to mixing and
buoyancy effects during ASR, MW2 would salinize first, followed by MW1, and
finally the ASR wells (AW1 and AW2) towards the end of the recovery phase,
with each time the deepest well screens salinizing first. This salinization
would then be caused by the replacement of freshwater by ambient groundwater
(very low-SO
Pumping of the ASR system during cycle 1 (2012/2013), EC
observations at MW1 (5 m from AW1), and the EC in the recovered water at AW1
and AW2. MW
The SEAWAT model underlined that tilting of the freshwater–saltwater
interfaces at the fringe of the ASR bubble did not cause the early
salinization observed, as this would have led to a much later salinization
(Fig. 6) without enrichment of SO
Modeled (solid lines) and observed (data points) SO
Modeled (solid lines) and observed (data points) SO
The hydrochemical observations and model outcomes of Cycle 1 indicated that the source of the early salinization was the intrusion of saltwater from Aquifer 2. Considering the lithology, thickness, and continuity of Aquitard 2 (confirmed by grain size analyses and cone penetrating tests on the site), leakage via natural pathways through this separating layer was unlikely. According to the rate and sequence of salinization, the leakage could well be situated at the ATES K3-b well close to AW1.
Cycle 2 started with the injection of 66 178 m
Pumping of the ASR system during cycle 2 (2013/2014), EC
observations at MW1 (5 m from AW1), and the EC in the recovered water at AW1
and AW2. AW2.1 and AW2.3 were used for freshwater recovery (12 324 m
In order to re-enable recovery of freshwater, the deepest wells of the MPPWs
(AW1S3 and AW2S3) were transformed to interception wells or “Freshkeepers”
(Stuyfzand and Raat, 2010; Van Ginkel et al., 2014), abstracting the
intruding saltwater and injecting this in a deep injection well in Aquifer 2
at of distance 200 m from the ASR site. This way, an acceptable water quality
(Cl < 50 mg L
Modeled and observed SO
Modeled and observed Cl concentrations at the most relevant well screens.
The SEAWAT model with leakage via the borehole of K3-b was able to
reasonably simulate the water quality trends regarding SO
Deep saltwater intrusion via ATES K3-b borehole during shallow recovery of injected freshwater at the Westland ASR site at the start of Cycle 2.
Calculated leakage flux
Modeling of Cycle 2 demonstrated that salinization during recovery was independent of the injected freshwater volume. Salinization occurred after recovery with the same rate as in Cycle 1, despite a 4 times larger injection volume. Analysis of the modeled concentration distribution and pressure heads showed that injected freshwater could not reach deep into the deeper saline aquifers since the freshwater head in the leaky ATES borehole during injection was more or less equal to the freshwater head in the deeper saltwater aquifer. In other words, little freshwater was pushed through the conduit into the deeper aquifer. Further on, the freshwater that did reach the deeper aquifer got rapidly displaced laterally as a result of buoyancy effects (Fig. 11).
A significant head difference (
An analytical solution was presented by Maas (2011) to calculate the
vertical leakage via a gravel or sand pack. In this solution, it is presumed
that an aquitard was pierced during drilling and the annulus was filled up
with sand or gravel without installing a clay seal. The leakage is then
calculated as function of the different hydraulic conductivities, pressure
difference, and the radius of the borehole and well screen:
Calculating the leakage flux using the
Modeled recovery efficiencies at the Westland ASR site without short-circuiting using different pumping strategies. The relative pumping rate per MPPW well screen is given for each particular screen.
The SEAWAT model was used to evaluate the ASR performance at the Westland
field site with three different ASR strategies
(Table 7), with and without the saltwater leakage.
During the 120 days of recovery it was aimed to recover as much of
the freshwater (marked by Cl < 50 mg L
Recovery with conventional, fully penetrating ASR wells will be limited to around 30 % of the injected freshwater in a case without the saltwater leakage. For the case with leakage, freshwater recovery will be impeded by the short-circuiting during the storage phase; the wells will produce brackish water already at the start of the recovery phase. The use of a MPPW for deep injection and shallow recovery has a limited positive effect due to the limited thickness of the aquifer: one-third of the injected water can be recovered in a case without leakage. The improvement of recovery efficiency (RE) by introduction of the MPPW is limited in comparison with the conventional ASR well since some saltwater from Aquitard 2 was found to move up to the shallower recovery wells of the MPPW system (“upconing”) rapidly after the start of recovery. The slight increase in Cl concentrations caused by this process is sufficient to terminate the recovery due to exceedance of the salinity limit. Before the fringe of the freshwater bubble reached the recovery wells, recovery was already terminated. In the case of saltwater leakage, salinization occurred within 2 days, limiting the RE to only 1 %.
The introduction of the Freshkeeper to protect the shallow recovery wells by
interception of this deeper saltwater significantly extended the recovery
period, enabling recovery of 40 % in the first year for direct use.
Ultimately, this will yield a RE of almost 50 % of virtually unmixed
(Cl < 50 mg L
When this ASR operational scheme with the Freshkeeper was applied to the
field pilot, where short-circuiting saltwater hampered freshwater recovery,
approximately one-third of the injected freshwater could be recovered. The
ASR well close to the leaking borehole (AW1) was unable to abstract
freshwater in this case. Only AW2 could be used for freshwater recovery, in
the end only via the shallowest well (AW2S1). The freshwater loss by
short-circuiting cannot be eliminated completely since a large volume of
unmixed freshwater is abstracted together with intruding saltwater during
the required interception. The RE will therefore remain lower than in an undisturbed
geological setting (RE: 48 %). At the same time, the required interception
of brackish-saline water will be higher (Table 7),
with a total volume of more than 30 000 m
In this study, the first focus was on the causes for the significantly lower
observed freshwater RE of the system. This RE was initially less than a few
percent, whereas recovery of around one-third of the injected water was
expected. The hydrochemical analyses clearly indicated that the observed
salinization was caused by unexpected intrusion of deeper saltwater, as
marked by substantially higher SO
Knowing the source of the salinization, several transport routes can be
presumed. First of all, intrusion of deep saltwater may occur when Aquitard 2
has a significantly lower
The potential effects of short-circuiting induced by deep perturbation on ASR in a shallower coastal aquifer were subsequently explored. In this case of freshwater storage in a confined, saline aquifer, pressure differences induced by the difference in density between injected freshwater, and native groundwater provoked intrusion of native groundwater in the injected freshwater bubbles via the presumed conduit. It is illustrated that a complete failure of the ASR system can occur when the short-circuiting via such a conduit occurs close to the ASR wells and little mixing with ambient saltwater is allowed.
The negative effects of short-circuiting on ASR on coastal aquifers are mainly related to the hydraulics around the conduits. First, freshwater is not easily transported downwards through the conduits into a deeper aquifer, while it is easily pushed back into the shallower aquifer when injection is stopped or paused. Second, the freshwater reaching a deeper aquifer is subjected to buoyancy effects and migrates laterally in the top zone of this deeper aquifer. Finally, during storage and especially during recovery, the pressure differences in combination with a high hydraulic conductivity rapidly induce a strong flux of saltwater from the whole deeper aquifer into the shallower ASR target aquifer, where a relatively low hydraulic head is present. This short-circuiting induced by such a pressure difference is hampered by the low permeability of the aquitard in a “pristine situation”. A continuous, undisturbed aquitard is therefore indispensable for the success of ASR in such a setting, as intrusion of deeper saltwater is not desired.
With an increasing distance between the ASR wells and a nearby conduit, the proportion of mixed saltwater in the recovered water decreases while the arrival time increases. When the conduit is situated outside the radius of the injected freshwater body in the target aquifer, a decrease in RE is not expected.
The Westland field example highlights how design, installation, and operational aspects are vital in the more-and-and more exploited subsurface in densely populated areas. First of all, old boreholes are unreliable and their presence should better be avoided when selecting new ASR well sites (Maliva et al., 2016). Second, installation and operation of (especially injection) wells should be regulated by strict protocols to prevent the creation of new pathways for short-circuiting. Finally, it is important to recognize that similar processes may occur in unperturbed coastal karst aquifers, where natural vertical pipes can be present (Bibby, 1981; Missimer et al., 2002).
In order to mitigate the short-circuiting and improve the freshwater recovery upon aquifer storage under these unfavorable conditions, several strategies can be recognized. Obviously, sealing of the conduits would be an effective remedy. However, it may not be viable to (1) locate all conduits, for instance when the former wells are decommissioned or when the confining clay layer is fractured upon deeper injection under high pressure, and (2) successfully seal a conduit at a great depth. This is underlined by the fact that only limited reports of successful sealing of deep conduits can be found.
Apart from sealing, one can also try to deal with these unfavorable conditions. MPPW were installed at the Westland ASR site, for instance, enabled interception of intruding saltwater by using the deeper well screens as “Freshkeepers”. After this intervention, about one-third of virtually unmixed injected freshwater becomes recoverable. This way, the RE is brought to a level similar to the level obtained by an MPPW-equipped ASR system without the Freshkeeper interception and without short-circuiting, while the RE would otherwise remain virtually null. It does require interception of a significant volume of brackish-saline groundwater, however, which must be injected elsewhere or disposed of. The addition of a Freshkeeper will therefore inevitably increase the investment costs (additional infrastructure for re-injection/disposal) and operational costs (electricity required for pumping).
A significant part of the unmixed freshwater is blended with saltwater in the Freshkeeper wells, such that the freshwater recovery becomes lower than in the situation in which the Freshkeeper is applied and saltwater intrusion via short-circuiting is absent. At the Westland field site, this is compensated by desalinating the intercepted brackish-saline groundwater, which is a suitable source water for reverse osmosis (RO) thanks to its low salinity. The freshwater (permeate) produced in this process is used for irrigation, while the resulting saltwater (concentrate) is disposed of in Aquifer 2. The resulting RE increase is plotted in Fig. 12. Even when no unmixed freshwater is available, desalination of injected water mixed with groundwater can be continued with this technique to further increase the RE. In comparison with conventional brackish water RO, this leads to a better feed water for RO (lower salinity) while salinization of the groundwater system by a net extraction of freshwater is prevented by balancing the freshwater injection and abstraction from the system.
Recovery efficiencies at the Westland ASR site with and without the borehole leakage resulting from the SEAWAT groundwater transport model for a conventional ASR well (one well screen, fully penetrating), deep injection and shallow recovery via multiple partially penetrating wells without a “Freshkeeper” (scenario MPPW), for a MPPW in combination with a “Freshkeeper” (scenario Freshkeeper), and for a scenario in which RO is applied on the intercepted brackish water to produce additional freshwater (50 % of the abstracted brackish water).
In case of a strict water quality limit and relatively saline groundwater, brackish groundwater upconing from the deeper confining aquitard toward shallow recovery wells is a process to take into account, apart from the buoyancy effects in the target aquifer itself. This was shown by the SEAWAT model runs without short-circuiting, which showed a small increase in Cl concentrations at the ASR wells prior to the full salinization caused by arrival of the fringe of the ASR bubble. The SEAWAT model indicated that the (sandy) clay/peat layer (Aquitard 2) below the target aquifer was the source of upconing brackish-saline groundwater. Although this layer has a low hydraulic conductivity, it is not impermeable and salinization via diffusion can occur in this zone, while brackish pore water can physically be extracted from this aquitard. The transport processes in this deeper aquitard are comparable with the borehole leakage water via conduits in this aquitard: freshwater is not easily pushed downwards during injection, but brackish water is easily attracted during recovery. After the recovery phase this zone salinizes until the next injection phase starts, so a gradual improvement in time is limited. Brackish water may also be attracted from the upper aquitard (“downconing”), but this process is counteracted by the buoyancy effects and did not lead to early termination of the freshwater recovery in the Westland case.
The release of brackish water from the deeper aquitard in coastal aquifers can be relevant when quality limits are strict, the native groundwater is saline, and the native groundwater in the target aquifer is displaced far from the ASR wells. The performance of ASR may then be much worse than is predicted by existing ASR performance estimation methods (e.g., Bakker, 2010; Ward et al., 2009), which assume that impermeable aquitards confine the target aquifer. Even in the first MPPW field test (Zuurbier et al., 2014), this process was not observed, due to a smaller radius of the freshwater bubble, resulting in earlier salinization due to buoyancy effects. The upconing water can optionally be intercepted by a (small, deep) Freshkeeper well screen to extent the recovery of unmixed freshwater, likewise the interception of intruding saltwater at the Westland site.
Finally it should be noted that the ASR system analyzed in this study had very strict water quality limits (practically no mixing allowed) and that a buffer zone (Pyne, 2005) between the injected freshwater and the relatively saline ambient groundwater was not realized before starting the ASR cycles. The boundary conditions for ASR were therefore already unfavorable. Also, the potential improvement after more than three cycles was not explored. The performance of this ASR system should therefore not be considered the typical performance of ASR in a brackish-saline aquifers, which controlled by a complex interplay of geological conditions and operational parameters (Bakker, 2010), well design (Zuurbier et al., 2014, 2015), and the formation of a buffer zone prior to starting the ASR cycles (Pyne, 2005).
This study shows how short-circuiting negatively affects the freshwater
recovery efficiency (RE) during aquifer storage and recovery (ASR) in
coastal aquifers. ASR was applied in a shallow saltwater aquifer (23–37 m b.s.l.)
overlying a deeper saltwater aquifer (> 47.5 m b.s.l.)
targeted for aquifer thermal energy storage. Intrusion of deeper saltwater
was marked by chemical tracers (mainly SO
Field observations and groundwater transport modeling showed that interception of deep short-circuiting water can mitigate the observed RE decrease, although complete compensation of the RE decrease will generally be unattainable since also unmixed freshwater gets intercepted. At the Westland ASR site, the RE can be brought back to around one-third of the injected water, which is comparable to the RE attained with an ASR system without the Freshkeeper in the same, yet undisturbed, setting. With the same Freshkeeper, the setup would be able to abstract around 50 % of the injected water unmixed, if the setting would be undisturbed. This underlines the added value of such a interception well for ASR. Finally, it was found that brackish water upconing from the underlying aquitard towards the shallow recovery wells of the MPPW-ASR system can occur. In case of strict water quality limits, this process may cause an early termination of freshwater recovery, yet it was neglected in many ASR performance estimations to date.
The data used in this manuscript can be obtained by contacting the authors.
The authors declare that they have no conflict of interest.
The authors would like to thank the funding agents of the studies discussed in this paper: Knowledge for Climate, the Joint Water Research Program of the Dutch Water Supply Companies (BTO), the EU FP7 project “Demonstrate Ecosystem Services Enabling Innovation in the Water Sector” (DESSIN, grant agreement no. 619039), and the EU Horizon2020-project “SUBSOL” (grant agreement no. 642228). Edited by: B. Berkowitz Reviewed by: D. Pyne and one anonymous referee