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Enabling Successful Aquifer Storage and Recovery
of Freshwater Using Horizontal Directional
Drilled Wells in Coastal Aquifers
Koen G. Zuurbier1; Jan Willem Kooiman2; Michel M. A. Groen3;
Bas Maas4; and Pieter J. Stuyfzand5
Abstract: Aquifer storage and recovery (ASR) of freshwater surpluses can reduce freshwater shortages in coastal areas during periods of
prolonged droughts. However, ASR is troublesome in saline coastal aquifers as buoyancy effects generally cause a significant loss of injected
freshwater. The use of a pair of parallel, superimposed horizontal wells is proposed to combine shallow ASR with deep interception of
underlying saltwater. A shallow, fresh groundwater lens can thereby be enlarged and protected. This freshmaker setup was successfully
placed in a coastal aquifer in The Netherlands using horizontal directional drilling to install 70-m-long horizontal directional drilled wells
(HDDWs). The freshmaker prototype aims to inject a specific volume of freshwater and abstract the same volume of water (consisting of
injected water and ambient native groundwater) within the targeted water quality. Groundwater transport modeling preceding ASR operation
demonstrates that this set up is able to abstract a water volume of 4,200 m3equal to the injected freshwater volume without exceeding strict
salinity limits, which would be unattainable with conventional ASR. This is the first study to demonstrate the potential benefits of HDDWs for
a field ASR application. The model outcomes indicate that the feasibility perspectives of ASR in coastal aquifers worldwide require revision
thanks to recent developments in hydrologic engineering. DOI: 10.1061/(ASCE)HE.1943-5584.0000990.© 2014 American Society of Civil
Engineers.
Author keywords: ASR; Horizontal wells; HDDW; Coastal aquifers; Horizontal directional drilled wells; Recovery efficiency; Aquifer
storage and recovery; Freshwater management.
Introduction
Freshwater supply in coastal areas worldwide is under pressure due
to salinization, increasing droughts, and/or increasing freshwater
demands (Werner et al. 2013). With drinking, industrial, and agri-
cultural water supply at stake, efficient exploitation of any available
freshwater surpluses is essential to avoid serious shortages. Above-
ground storage of such surpluses can be inefficient as the water is
prone to evaporation, or because a vast and/or expensive surface
area is required. Aquifer storage and recovery (ASR) is defined as
“the injection of water surpluses by a well and recovery by the same
well in times of demand”(Pyne 2005), and it may be an efficient
technique to bridge the period in between surplus and demand,
without claiming surface area aboveground.
ASR is successfully applied in freshwater aquifers, but storage
of freshwater in saline aquifers is troublesome due to mixing and
displacement by buoyancy effects in ambient brackish or saline
groundwater. Although the loss by mixing can be eliminated by
preinjection of a certain volume to form a buffer zone (Pyne 2005),
buoyancy effects may continuously cause freshwater losses (Ward
et al. 2009;Zuurbier et al. 2013). In such cases, the difference in
density between injected freshwater (low density) and ambient
saline groundwater (high density) will induce upward movement
of freshwater. The conventional ASR setup, which uses a single
vertical well for injection and recovery, will therefore generally fail
in saline coastal aquifers, as the lower part of the ASR well rapidly
abstracts ambient saline groundwater (Esmail and Kimbler 1967).
Use of upscaling or multiple partially penetrating wells may coun-
teract the freshwater loss by this effect in brackish, confined aqui-
fers, but is presumably insufficient for small-scale ASR in saline
aquifers (Zuurbier et al. 2013,2014), especially when they are thick
and unconfined.
Recent development of horizontal directional drilled wells
(HDDWs; Cirkel et al. 2010) may initiate successful ASR in coastal
aquifers. Previous studies show that by spreading shallow abstrac-
tion of freshwater from a small freshwater lens over a large area,
for instance by a HDDW, a larger volume of freshwater can be ab-
stracted (Oude Essink 2001;Stoeckl and Houben 2012). Instead of
a single HDDW, a parallel, superimposed HDDW pair is proposed
in a more advanced setup to enable both shallow injection and ab-
straction of freshwater in such a freshwater lens, as well as inter-
ception of underlying saltwater. The fresh–salt interface can be
actively managed this way to enlarge natural fresh groundwater
lenses during injection (the freshmaker concept), storing large vol-
umes of freshwater in the process. During subsequent storage and
1KWR Watercycle Research Institute, P.O. Box 1072, 3430 BB
Nieuwegein, Netherlands; and Critical Zone Hydrology Group, VU Univ.
Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, Netherlands (cor-
responding author). E-mail: koen.zuurbier@kwrwater.nl
2KWR Watercycle Research Institute, P.O. Box 1072, 3430 BB
Nieuwegein, Netherlands.
3Critical Zone Hydrology Group, VU Univ. Amsterdam, De Boelelaan
1085, 1081 HV Amsterdam, Netherlands.
4Critical Zone Hydrology Group, VU Univ. Amsterdam, De Boelelaan
1085, 1081 HV Amsterdam, Netherlands.
5Professor, KWR Watercycle Research Institute, P.O. Box 1072, 3430
BB Nieuwegein, Netherlands; and Critical Zone Hydrology Group, VU
Univ. Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, Netherlands.
Note. This manuscript was submitted on October 29, 2013; approved on
February 21, 2014; published online on March 19, 2014. Discussion period
open until December 8, 2014; separate discussions must be submitted for
individual papers. This paper is part of the Journal of Hydrologic Engi-
neering, © ASCE, ISSN 1084-0699/B4014003(7)/$25.00.
© ASCE B4014003-1 J. Hydrol. Eng.
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abstraction, the enlarged freshwater lens can be protected by con-
tinuing the deeper abstraction of saltwater. The first freshmaker
prototype was successfully installed in 2013 in a shallow coastal
aquifer in the province of Zeeland (the Netherlands, Fig. 1).
The aim of this study is to verify and quantify the prospective
benefits of this innovative ASR configuration based on ground-
water transport modeling preceding its first field operation, yet
taking into account the local hydrogeological settings. Moreover,
the aim was to demonstrate that even in (unconfined) coastal aqui-
fers with saline groundwater, ASR can be a viable freshwater man-
agement technique thanks to recent developments in hydrologic
engineering. The latter may have large implications for ASR fea-
sibility worldwide.
Material and Methods
Study Area
The study area is located in the southwest of the Netherlands, in the
coastal province of Zeeland. Freshwater is scarce in the study area
due to the surrounding Scheldt estuaries (Fig. 1) and saline seepage.
Current freshwater resources are therefore limited to rainwater,
local fresh groundwater lenses in sandy creek ridges (Fig. 1), and
some inland river water transported by a pipeline. Because of the
large irrigation-water demand but limited rainfall in summers,
freshwater shortages occur in the local agricultural and horticultural
sector, causing a considerable loss of revenue especially for the
fruit-production sector. By contrast, large freshwater surpluses are
collected by drainage systems and discharged to sea to control the
groundwater levels especially in winters, when precipitation rates
are high and water use and evapotranspiration are low.
It is aimed to store a part of the local fresh drainage water, which
is otherwise discharged to sea in a shallow, fine-sand aquifer in one
of the creek ridges using the freshmaker in a field trial. The field
site is situated on a sandy, relatively young, 5-km wide creek ridge
near the village of Ovezande (Fig. 1). The ridge reaches 0–2m
above sea level (m-ASL) and is surrounded by (older) peat and clay
deposits [0–1.5 m-below sea level (m-BSL)]. The creek-ridge aquifer
consists of fine to medium fine sands. Draining water courses
on the creek ridge are deep, and have controlled water levels of
0.6–0.7 m-BSL. They quickly salinize during dry summers, when
electrical conductivities increase to approximately 5,000 μS=cm.
The thickness of the fresh groundwater lens in the creek ridge is
dependent on surface elevations and the surrounding drainage level
(de Louw et al. 2011). In general, their thickness is less than 15 m,
which legally prohibits abstraction from these reserves for irriga-
tion purposes to prevent salinization.
Set Up of the Freshmaker Pilot and Planned Operation
At the field site, the local surface level varies from 0.1 to 0.5 m-ASL.
Horizontal directional drilling was used to create two open boreholes
with a diameter of approximately 300 mm. The targeted aquifer in-
tervals for the boreholes were based on cone-penetration tests to en-
sure that the HDDWs were placed in sections with a relatively high
permeability, without intervening clay layers. The depth profile of
the boreholes was recorded in the field using a directional drilling
locating system (DigiTrak, U.S.) and global positioning system.
A bentonite SW drilling fluid (HDD Drilling Fluids, Schoonebeek,
the Netherlands) was used to lubricate the drilling, to dispose the
cuttings, and to provideborehole stability. A 70-m-long HDDW with
an inner diameter of 75 mm and four rows with 10-mm holes at
10-cm intervals was wrapped with geotextile. It was then installed
in a borehole at a depth of 13.35–14.38 m-BSL (Fig. 2) to act as the
interception well. A perforated casing with an inner diameter of
125 mm and eight rows of open holes of 10 mm at 10-cm intervals
over a length of 70 m surrounded this HDDW during placement for
protection and was left around the HDDW. A second, shallow
HDDW (ASR well) with the same properties was installed for arti-
ficial recharge and recovery offreshwater surpluses in a second bore-
hole, right above the interception well at 6.68–6.93 m-BSL. At this
HDDW, a nonperforated casing was used for protection during
placement, which was removed after the HDDW was in place. Once
the HDDWs were in place, a dispersant was injected, after which
500 m3was abstracted to remove the drilling fluid.
During the field pilot, freshwater surpluses from a water course
will be stored in a basin (approximate volume of 4,000 m3)to
enable intake of large volumes of freshwater in periods with the
highest discharge of fresh surface water in the water course. After
settlement of fine particles in the basin, water pumped from the top
of the basin will be injected by the upper HDDW, using a 3-m high
standpipe to provide the pressure for injection. Abstracted saltwater
from the deep HDDW will be discharged to the local watercourse,
with a permitted maximum of 40 m3=day (Fig. 3). The recovered
freshwater by the freshmaker is used for irrigation in the growing
season at an orchard, where a maximum chloride concentration of
250 mg=L is allowed.
Characterization of the Target Aquifer
The target aquifer was characterized using a 40-m-deep bailer
drilling at the center of the HDDWs (MW1, Fig. 2), with samples
taken every 1 m. Grain-size distributions of these samples were
derived using a HELOS/KR laser particle sizer (Sympatec GmbH,
Germany), after preparation using the method of Konert and
Vandenberghe (1997). It was found that the aquifer is relatively
homogeneous and consists of fine to medium fine sand, with a
mean grain size of 150–200 μm (Fig. 4).
At three locations (MW1, 2, and 4; Figs. 2and 3), electrical
conductivities in the aquifer were recorded by geophysical borehole
logging using a Robertson DIL-39 probe. The exact location of the
fresh–salt interface was foundthis way. Continuous vertical electrical
Fig. 1. Depth of fresh–salt interface (i.e., chloride concentration ¼
1,000 mg=L) indicating natural freshwater lenses found on the island
of Zuid-Beveland (Zeeland, the Netherlands), and the location of the
Ovezande freshmaker trial
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soundings (CVESs) were conducted to map the lateral extent of the
freshwater lens. The CVES results indicated the presence of
a freshwater lens with a thickness of 0–10 m. This thickness
was controlled by the elevation of the surface level and the seepage
of saline groundwater toward the draining water course (Fig. 5).
Based on EM-39 measurements, the freshwater lens had a thickness
of approximately 9 m and a mixing zone of approximately 6 m
at the location of the HDDWs. Below this mixing zone, the high
conductivities indicated the presence of groundwater with salinity
equal to local seawater, which has chloride concentration of ap-
proximately 16,800 mg=L.
Modeling of the Freshmaker Benefits
A two-dimensional (2D) SEAWAT version 4 (Langevin et al. 2007)
model was built before the installation of the HDDWs to analyze
the efficiency of the freshmaker setup and estimate the required
pumping rates during operation. A simple slice consisting of only
one row comprising 10 m of the HDDW pair was simulated to limit
model runtimes (Fig. 6). Edge effects on the outer ends of the
HDDWs were therefore neglected. Hydraulic conductivities were
estimated based on the grain-size distributions using the procedure
suggested by Bear (1972) and typical values were matched for local
Fig. 2. Cross section of the freshmaker setup at the Ovezande trial; MW = monitoring well
Fig. 3. Plan view of the freshmaker setup at the Ovezande trial;
MW = monitoring well
Fig. 4. Grain size distribution in the target aquifer at MW1; c = clay,
s = silt, vfs = very fine sand, fs = fine sand, mcs = medium coarse sand,
cs = coarse sand; mean grain size is indicated in a dashed line; the depth
position of HDDW1 and HDDW2 is marked with arrows
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creek-ridge sediments (approximately 5–10 m=d). Draining water
courses close to the freshmaker well pair were simulated using
MODFLOW’sriver package. Topography was taken from local
elevation measurements. At 850 m from the HDDW pair, a constant
head boundary was placed. The initial chloride concentration was
produced by simulating 100 years with a realistic recharge of
200 mm=year (Royal Netherlands Meteorological Institute 2013).
The conductance of the river beds was modified until the simulation
produced the salinity distribution of the reference CVES results
(Fig. 5). A small longitudinal dispersivity of 0.1 m was required
to reproduce the mixing zone recorded by borehole logging.
The outcomes of the initial model were used as initial conditions
before the installation of the freshmaker HDDW pair in the model.
The HDDWs were simulated by normal single-cell wells with
a fixed discharge per stress period in the slice at 6.75 and
14.25 m-BSL. Discharge of each well during 5 years (Table 1,
Scenario D) was based on the estimated water availability and
the minimal well capacity. For each year, five stress periods were
simulated: an injection phase, a first recovery phase (sprinkling
against frost damage), a storage phase, a second recovery phase
(drought irrigation), and an idle period awaiting new freshwater
surpluses.
Three additional scenarios were modeled to verify the benefits
of freshmaker configuration. These three scenarios included the
following:
•Scenario A: Normal ASR operation (scenario ASR), using only
the upper HDDW for injection and abstraction of freshwater.
No interception of saltwater;
•Scenario B: ASR operation as in Scenario A, which is preceded
by an additional stress period of 120 days to inject an extra
volume equal to the targeted abstraction volume and develop a
buffer zone (scenario ASR, buffer zone). This may significantly
reduce freshwater losses during subsequent ASR cycles, as
demonstrated by Pyne (2005);
•Scenario C: Scenario in which no water was injected by the shal-
low HDDW, but still deeper saltwater was abstracted in winter
Fig. 5. CVES results at the Ovezande field site; positions of the HDDWs are marked white (upper) and black (deeper)
Fig. 6. Schematization (not to scale) of the 2D SEAWAT model to evaluate the performance of the freshmaker; VANI = vertical anisotropy ratio
Table 1. Modeled (Yearly) ASR Scheme for the Ovezande Freshmaker
Trial (Scenario D)
Period t¼ðdÞ
Qin (m3=day)
(HDDW1,
fresh)
Qout (m3=day)
(HDDW1,
fresh)
Qout (m3=day)
(HDDW2,
saline)
Winter (infiltration) 120 35 0 35
Spring (recovery 1) 30 0 70 35
Spring (storage) 60 0 0 35
Summer
(recovery 2)
60 0 35 35
Idle 95 0 0 0
Total (m3)—4,200 4,200 9,450
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before as well as during the abstraction of freshwater in summer
[coded freshkeeper, as such a set up is similar to the (vertical)
freshkeeper setup proposed by Stuyfzand and Raat (2010)].
Results
Estimated Freshmaker Performance by SEAWAT
Groundwater Transport Modeling
SEAWAT modeling gave firm insights into the potential perfor-
mance of the freshmaker. The results indicate that the modeled
freshmaker was able to lower the fresh–salt interface by approxi-
mately 5 m, down to the level of the deep HDDW (Scenario D).
The thickness of the freshwater lens was increased up to a distance
of 30 m away from the HDDW. A targeted freshwater volume of
4,200 m3became available for abstraction in this process. During
abstraction phases, the fresh–salt interface moved up again, how-
ever, not threatening the freshwater quality at the upper HDDW
(Fig. 7, Scenario D), and a freshwater volume of 4,200 m3was
abstracted. When only the deepest HDDW was actively inter-
cepting saline groundwater in this scenario (storage phases), the
fresh–salt interface was lowered and stabilized. The modeled chlo-
ride concentrations at HDDW1 indicated that the abstracted water
in spring (recovery 1, Table 1) was merely injected surface water
(approximately 100 mg=L Cl), whereas in summer (recovery 2)
native groundwater from the freshwater lens was abstracted
(40 mg=L Cl), which was mixed with some upconing saltwater
at the end of Cycle 1. In subsequent cycles of Scenario D this
upconing was limited and did not impose a risk for the chlorinity
of the abstracted water. The results show that the aquifer was slowly
freshening, which was underlined by a decrease in saline seepage
toward the local water course.
Because of the simultaneous injection of freshwater and abstrac-
tion of deeper saline groundwater in Scenario D, the predicted ef-
fects increase of the phreatic groundwater level by the model were
less than 5 cm during injection. A maximum phreatic drawdown
of 7 cm was predicted by the model above the HDDW pair dur-
ing abstraction, indicating that the hydrological effects remained
limited. This highlights another important advantage of the use of
HDDWs over vertical wells; hydraulic effects are distributed along
the length of the HDDWs, preventing major local drawdowns.
Potential negative consequences of the ASR operation such as re-
duced water availability near the plant roots and/or land subsidence
are therefore expected to be negligible.
Benefits of the Freshmaker Concept over Conventional
ASR Concepts
Significantly less freshwater was found attainable when only a
single HDDW (Scenario A: ASR) was installed at the depth of
HDDW1 (6.75 m-BSL), compared with the full freshmaker setup
in Scenario D. This was evidenced by a firm increase in chloride
concentrations during both abstraction phases (Fig. 7, Scenario A),
exceeding the local maximum chloride concentration for irrigation
water after abstraction of a volume, which was approximately 50%
of the injected volume. The last cycles indicated that no further
improvement in the ASR performance could be expected. The
results show that buoyancy effects are significant, which causes
lateral spreading of injected freshwater during injection, and up-
coning of saline groundwater early during the abstraction phase.
The introduction of a buffer zone (Scenario B) in this particular
setting did not lead to a significant increase in freshwater abstrac-
tion. This is demonstrated by the modeled chloride concentrations
in Cycle 5 (Fig. 7, Scenario B), which were more or less equal to a
case without the injection of a freshwater surplus for the buffer zone
formation. This points out that a buffer zone is not maintained in
between the HDDWs and cannot provide the desired prolonged
protection from underlying saltwater.
Importance of Freshwater Injection: Comparison
with a Freshkeeper Operation
When a freshmaker was installed, but no water was injected
(Scenario C: Freshkeeper), a satisfying volume of freshwater could
be abstracted from Cycle 5 onward, due to the almost continuous
interception of saltwater by the deep HDDW, increasing infiltration
of freshwater, and decreasing seepage to the surface water. These
results suggest that injection of freshwater is not a requirement for
the abstraction of a same volume of freshwater, and that continuous
interception of saltwater preceding freshwater abstraction may be
sufficient. The latter was confirmed by modeling of an additional
scenario in which the first freshwater abstraction was preceded
by 1.3 years of deep interception (35 m3=day) of saltwater. In the
Fig. 7. Chloride (Cl) concentrations at the upper HDDW for scenarios
without the interception of saline groundwater by a (a and b) deep
HDDW; (c) without injection; (d) freshmaker (HDDW2)
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following cycles, the volume of 4,200 m3could be recovered with
chloride concentrations not exceeding 140 mg=L. A somewhat
larger drawdown (13 cm) was observed, however, which might
necessitate additional local irrigation near the HDDWs to prevent
water shortage in the root zone.
Discussion and Conclusions
Abstraction of freshwater during ASR operation in coastal aquifers
is generally troublesome due to buoyancy effects. A freshmaker
setup to enlarge thin fresh groundwater lenses in shallow aquifers
by the use of two parallel, superimposed HDDWs was tested using
2D SEAWAT groundwater transport modeling preceding its field
operation. A shallow HDDW was used to inject and abstract fresh-
water surpluses and a deeper HDDW to intercept deeper saline
groundwater. The SEAWAT transport modeling showed that a
freshwater volume equal to the injected volume could be abstracted
yearly, and that the abstracted water consisted of both injected
water and native, fresh groundwater. Because of the use of the
HDDWs, regional hydrological effects can be expected to be
limited.
A prerequisite is continuous interception of deeper saline
groundwater by the deeper HDDW. Conventional ASR setups
(although using HDDWs) predicted limited freshwater abstraction
(approximately 50% of the injected volume). The origin of this
significant reduction in ASR performance compared with the fresh-
maker setup can be found in the basics of a freshwater lens in saline
groundwater. As with natural freshwater lenses in these relatively
homogenous aquifers, the depth of the freshwater lens relative to
the local drainage level is controlled by the Ghyben–Herzberg
relation. This means that the extending head in the freshwater
lens compared with this drainage level and the density difference
between the fresh and saline groundwater control the thickness of
the lens (Verruijt 1968). The modeling results showed that the head
increase in the freshwater lens compared with the reference situa-
tion in the ASR scenario is limited due to the low injection rates.
However, a much higher head increase in this phreatic aquifer will
lead to root deterioration of orchard trees or even groundwater
exfiltration. Furthermore, in storage phases (with neither freshwater
availability nor demand), the relative head increase cannot be
maintained, resulting in thinning of the freshwater lens and loss of
freshwater. Upconing of saltwater is favored in the abstraction
phase due to the abstraction from a shallow freshwater lens, under-
lain by saline groundwater and is found in various studies (Aliewi
et al. 2001;Asghar et al. 2002;Oude Essink 2001;Reilly and
Goodman 1987;Schmork and Mercado 1969;Werner et al. 2009,
2013). To safely increase the thickness of the lens in limited time,
prevent losses during storage, and abstract a large freshwater vol-
ume in periods of demand, abstraction by the deeper HDDW
proves to be indispensable.
The depth of the intercepting, deeper HDDW is a relevant
design parameter as (1) this HDDW can abstract costly freshwater
when it is installed too shallow or (2) provide insufficient protec-
tion of the upper HDDW when it is installed too deep. SEAWAT
modeling with an extra fictitious, conservative tracer in the injec-
tion water showed that no injected freshwater should enter the top
of the deeper HDDW in the Ovezande trial for the operational
parameters of Scenario D. Only a part of the brackish mixing
zone was abstracted in the model, as shown by the modeled chlo-
ride concentrations at HDDW2 (Fig. 8). SEAWAT modeling also
showed that placement of the deeper HDDW at 20 m-BSL led
to early salinization of upper HDDW in the first 2 years. Lowering
of the fresh–salt interface was less below HDDW1 in this case
and extended laterally. Although the target aquifer was modeled
as being homogeneous and anisotropic, intervening clay layers
may further decrease the functionality of the deeper, intercepting
HDDW in field applications. This emphasizes the need for a priori
aquifer characterization, for instance using cone-penetration tests.
Altogether, the modeling results indicate that appropriate place-
ment depths were chosen for the Ovezande field trial.
SEAWAT modeling also showed that under the local hydrolog-
ical conditions simulated (freshwater lens, net recharge, controlled
drainage levels), the interception of deep saltwater eventually
enables abstraction of the targeted freshwater volumes, even with-
out injection of freshwater surpluses. However, this operation may
affect the local hydrology stronger than the proposed freshmaker
operation, or cause mining of the existing freshwater lens. In ad-
dition, beneficial effects on the produced freshwater are anticipated
with the injection by the freshmaker, such as subsurface iron re-
moval from the otherwise iron-containing natural freshwater lens
by injection of oxygen-rich water (Antoniou et al. 2013;van Halem
et al. 2010).
This study confirms the theoretical feasibility of the fresh-
maker principle, preceding the first-field application. The findings
suggest that a robust ASR configuration is available for coastal
areas with similar hydrological settings worldwide, where ASR
was previously considered unviable. This means that for the first
time, valuable freshwater surpluses can be stored in these relevant
areas without claiming vast surface areas aboveground due to re-
cent developments in hydrologic engineering. With this increased
freshwater availability at hand, coastal areas can remain (or be-
come) interesting for agriculture, industries, and inhabitants. Future
studies should focus on field verification of the outcomes of this
study, as well as observation and modeling of HDDW edge effects,
potential aquifer heterogeneities, water-quality changes from aqui-
fer reactivity, and potential well clogging.
Acknowledgments
This study was funded by the Dutch national research program
Knowledge for Climate and the parties involved in GO-Fresh
(Geohydrological Opportunities Fresh Water Supply). Maatschap
Rijk-Boonman, Bos Grijpskerke, and Meeuwse Handelsonderming
Goes are thanked for their contribution in the Ovezande field trial.
Three anonymous reviewers are thanked for their comments to the
earlier version of the manuscript.
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