Groundwater flow in the vicinity of two artificial recharge ponds
in the Belgian coastal dunes
A. Vandenbohede & Emmanuel Van Houtte & Luc Lebbe
Abstract Since July 2002, tertiary treated wastewater has
been artificially recharged through two infiltration ponds in
the dunes of the Belgian western coastal plain. This has
formed a lens of artificially recharged water in the dunes’
fresh water lens. Recharged water is recovered by extraction
wells located around the ponds. Hydraulic aspects of the
artificial recharge and extraction are described using field
observations such as geophysical borehole loggings and a
tracer test. Borehole logs indicate recharged water up to 20m
below surface, whereas the tracer test gives field data about
the residence times of the recharged water. Furthermore, a
detailed solute transport model was made of the area
surrounding the ponds. Groundwater flow, capture zone,
residence times and volume of recharged water in the aquifer
are calculated. This shows that the residence time varies
between 30days and 5years due to the complex flow pattern.
The extracted water is a mix of waters with different
residence times and natural groundwater, assuring a relatively stable water quality of the extracted water.
Keywords Groundwater recharge/water budget . Capture
zone . Residence times . Numerical modeling . Belgium
Introduction
The phreatic aquifer of the dunes in the Belgian western
coastal plain is important as a source of fresh water such
as that used for the local water supply. In the surrounding
Received: 5 November 2007 / Accepted: 27 May 2008
Published online: 26 June 2008
© Springer-Verlag 2008
A. Vandenbohede ()) : L. Lebbe
Research Unit Groundwater Modelling,
Department of Geology and Soil Science,
Ghent University,
Krijgslaan 281 (S8), 9000, Gent, Belgium
e-mail: [email protected]
Tel.: +32-(0)9-2644652
Fax: +32-(0)9-2644653
E. Van Houtte
Intermunicipal Water Company of the Veurne Region (IWVA),
Doornpannestraat 1, 8670, Koksijde, Belgium
Hydrogeology Journal (2008) 16: 1669–1681
areas (sea, shore and polder), mainly brackish to salt water
is found. Otherwise, fresh-water lenses present in the
phreatic polder aquifer contain very limited fresh-water
resources (Vandenbohede and Lebbe 2002). Tertiary sediments below the Quaternary phreatic aquifer consist
mainly of low permeable sediments and/or contain
brackish water or salt water. Consequently the fresh-water
lens under the dunes forms the main groundwater reserve
which can be exploited. Groundwater exploitation for the
production of drinking water started in 1947 at the water
extraction St-André by the Intermunicipal Water Company
of the Veurne Region (IWVA; Fig. 1). The IWVA is
responsible for the distribution of drinking water in the
western part of the Belgian coastal plain. To remediate
decreasing water levels and also to guarantee current and
future water extraction possibilities, alternative exploitation methods were studied during the 1990s (Van Houtte
and Verbauwhede 2005; Vandenbohede et al. 2008). As in
many other water extractions (e.g. Asano 1992; Van
Breukelen 1998; Bouwer 2002; Greskowiak et al. 2005;
Massmann et al. 2006), it was opted to artificially recharge
the aquifer using superficial ponds. In July 2002, the
IWVA started with artificial recharge of the phreatic dune
aquifer using treated wastewater effluent. This effluent
undergoes an extra (tertiary) treatment using ultra filtration
and reverse osmosis prior to recharge in the dune aquifer.
Recharge is realised by means of two shallow interconnected ponds. The water is extracted using a well
battery (well battery two on Fig. 1) located around the
ponds. A mixture of artificially recharged and native dune
water is extracted. In this way, it is possible to reduce the
net groundwater extraction, whereas the total volume of
extracted water has increased. Consequently, the result is a
sustainable water extraction in an area with limited freshwater reserves (Vandenbohede et al. 2008).
The purpose of this paper is to discuss hydraulic aspects
of the interaction between recharge ponds and extraction
wells. This is done by integrating two approaches. Firstly,
results of a number of field measurements and tests are
available. Geophysical borehole loggings (electromagnetic
conduction) give information of the extension of the
recharged water whereas a tracer test makes it possible to
derive residence times. Secondly, a groundwater flow model,
using the MOCDENS3D computer code (Oude Essink
1998), is developed in which all available information
(recharge and extraction rates, geological data etc.) is
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Fig. 1 The St-André water extraction is located in the Belgian western coastal dunes near the French-Belgian border (square in index
map). The water extraction consists of two well batteries and the artificial recharge ponds are located between the wells of well battery 2
summarized. This results in a more detailed and threedimensional insight into the groundwater flow in the
vicinity of the recharge ponds. Calculation of residence
times of the recharged water and capture zone of well
battery 2 are important tools necessitating the use of the
MOCDENS3D computer code (Oude Essink 1998).
St-Andre infiltration project
The dunes of the Belgian western coastal plain are very well
suited for the extraction of water. The phreatic aquifer
consists of fine medium sands in which lenses of silty or
clayey fine sand can occur. A shallow silty layer exists, for
instance, under the western pond and this layer acts as a
semi-permeable layer. The influence of this layer will be
discussed further in the report. The aquifer has a thickness
of about 30 m; the width of the dunes (from high water line
to polder) is 1.75 km. The substratum of the aquifer is
formed by the clay of the Kortrijk Formation, Ieper Group.
This clay is of Eocene age and is considered here as an
impermeable boundary. Landward from the dunes, a polder
(artificially drained low-lying area) is located.
A fresh-water lens is found in the dune aquifer. In the
polder, brackish water to salt water is found, whereas a saltwater lens occurs above fresh water under the shore. More
Hydrogeology Journal (2008) 16: 1669–1681
details on the origin and evolution of the fresh-water–saltwater distribution under the shore and in the dunes and polder
can be found in Lebbe (1978), Vandenbohede and Lebbe
(2006) and Vandenbohede et al. (2008). This fresh-water–
salt-water distribution means that the only relatively important fresh-water reserves are situated in the dune aquifer.
Extraction of water from the dunes for the production of
drinking water started in St-André in 1947. The extraction
rate steadily increased from about 0.5 million m3/year during
the 1950s to 1.75 million m3/year during the 1960s. From
then on, extraction rates remained 2 million m3/years up to
the second millennium. Initially, the water extraction started
with one well battery with 109 wells (well battery 1 in
Fig. 1). In 1968, a second well battery was put in use with
54 wells. Extraction rates of both well batteries were more or
less the same. Extraction wells had screens between 6 and
10 m below surface; in the 1980s new wells were drilled
with screens between 12 to 16 m below surface. In 2002, 70
wells were active in conjunction with the first well battery
and 112 wells in conjunction with the second well battery.
To partially restore hydraulic heads and natural
groundwater flow in the dunes around the St-André
extraction and to ensure water production which meets
the demands, it was decided in the mid 1990s to
artificially recharge the dune aquifer. After considering
different options, the artificial recharge project started in
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July 2002. The system which is used now consists of two
recharge ponds surrounded by wells (well battery 2). The
ponds are interconnected through a subterranean pipe.
Effluent, from a nearby waste-water treatment plant, is
used for recharge after additional treatment using ultra
filtration (UF) and reverse osmosis (RO) (Van Houtte and
Verbauwhede 2005). The recharged water is thereafter
extracted via the wells of well battery 2 situated north and
south of the ponds (Fig. 2). Figure 2 shows, instead of the
112 wells, only 110 wells. This is because Fig. 2 shows
the well positions in the finite difference grid of the model
whereby 2×2 wells are located in the same finite
difference cell in two occasions. Wells 1–10 have
submersed pumps, whereas wells 11–110 are connected
via a siphon to a central collector from which water is
extracted. The artificial recharge ponds are located in a
dune slack. The recharge ponds are in direct connection
with the saturated zone, there is no unsaturated zone
between the bottom of the ponds and the water table.
The artificial recharge project loops the water cycle:
extracted water goes to the users and their waste water is
purified. This water, after extra treatment, is then again reused
to recharge the dune aquifer. Up to 2.5 million m3/year can be
recharged and the same amount of water can be extracted.
An additional amount of 1.7 million m3 natural dune water
can be extracted (1 million by well battery 2 and 0.7 million
by well battery 1). This means that the capacity of the water
extraction is more than doubled (from 2 million to
4.2 million m3/year), whereas the net amount of water
extracted from the dune aquifer is reduced from 2 million to
1.7 million m3/year.
Figure 3 shows the total dissolved solids content (TDS,
mg/l) of the recharge and extracted water as a function of
time. From the start of the artificial recharge project until
May 2004, TDS in the recharged water was approximately
100 mg/l. This water consisted of 90% RO and 10% UF
water. The latter is added to the former to mineralise the
recharged water to a certain extent. This mineralisation is
required before recharging the dune aquifer. From May
2004, NaOH is used to mineralise and adjust the pH of the
recharged water instead of UF water and TDS of the
recharged water became from then on smaller, approxi-
Fig. 3 Total dissolved solids (TDS, mg/l) of the recharge and
extraction water as function of time
mately 50 mg/l. The extracted water is a mixture of
recharged water (80%) and native dune water (20%). The
recharged water becomes also mineralised by for instance
chalk dissolution in the aquifer (unpublished data). This
means that the TDS of the extracted water is substantially
higher than that of the recharged water. The TDS in the
extracted water before the start of the artificial recharge
project was approximately 800–900 mg/l. This is larger
than the TDS of shallow dune water which is approximately 550 mg/l due to the fact that water originating from
deeper parts of the dune aquifer is also flowing to the
extraction wells. This deeper water has a slightly higher
TDS. TDS of the extracted water decreased steadily in the
first year of the project to about 325 mg/l or slightly more
than 3 times the TDS of the recharged water. Change in
composition of the recharged water in May 2004 is also
visible in the extracted water since the TDS decreased to
about 225 mg/l or 4.5 times the TDS of the recharged
water. Notice that during the second part of 2002, TDS of
the extracted water increased to 800 mg/l. During this
period, recharge rates were very low (70,000 m3/month
instead of 175,000 m3/month) due to start-up problems
with the production of recharged water. The extraction rate
remained, however, unaltered, meaning that more native
dune water was extracted. Therefore, TDS of the extracted
water increased temporally.
Field observations
Geophysical borehole measurements
Fig. 2 Model grid centred over the two ponds and position of the
extraction wells of well battery 2. Well numbers of individual wells
are indicated as are the observation wells (WP)
Hydrogeology Journal (2008) 16: 1669–1681
A focussed electromagnetic induction tool (EM39, Geonics)
is used for geophysical borehole logging. The EM39 is
specially designed for use in wells encased with electrical
non-conductive materials. This is a major advantage in
comparison with older electrical methods (long normal and
short normal) which could only be performed in open
boreholes or in fully screened wells. EM39 employs a small
internal transmitter coil energised with an audio-frequency
current to induce eddy currents in the soil surrounding the
well. These eddy currents generate an alternating secondary
magnetic field which can be observed by small receiver coils
located at some distance from the transmitter. The small
secondary magnetic field is linearly proportional to the
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electrical conductivity of the surrounding material and the
device can be calibrated to read the terrain conductivity
directly (McNeill 1986). The arrangement of coils provides a
relatively large lateral range and a high degree of vertical
resolution which makes it very suitable for hydrogeological
research. EM39 measures the electrical conductivity of the
surrounding sediments within a distance range from 20 to
100 cm from the well axis while being insensitive to
conductivity of the borehole fluid and disturbed material
situated near the well axis. The vertical resolution is a few
decimetres.
EM39 measurements were made in wells WP6.1, WP7.1,
WP24, WP23, WP21 and WP22 (see Fig. 2 for locations) in
August 2006. Earlier measurements (August 2002) were
also performed in all but WP7.1. Vertical scale is expressed
in mTAW, whereby 0 mTAW is the Belgian reference level
(2.36 m below mean sea level). WP7.1 is located south of
well battery 2, which means that native dune water is
expected in this location. Mean electrical conductivity of the
dune aquifer at WP7.1 (Fig. 4) is 20 mS/m corresponding
with a TDS of the pore water of 550 mg/l. There are two
zones with a slightly higher conductivity representing layers
or horizons with slightly increased silt or clay content.
Conductivity increases also below –20 mTAW, due to the
higher TDS values of the pore water at the bottom of the
phreatic aquifer and due to the transition of the Quaternary
sediments to the clay of the Kortrijk Formation. WP6.1 is
located close to the ponds and the EM39 profile can be
subdivided in two parts. A low conductivity of 6 mS/m is
seen in the first 15 m of the August 2006 measurement. This
corresponds with a TDS of 175 mg/l and is the recharged
water which has already undergone mineralisation because
of the passage through the upper part of the dune sediments.
A lens or horizon with higher conductivity is present around
–10 mTAW, which is due to an increase in silt or clay
content of the sediments. Below –13 mTAW, conductivity
increases to 27 mS/m, which is due to the fact of the native
dune water becoming more mineralised deeper in the dune
aquifer. Notice that there is an important difference between
the 2002 and 2006 measurement. The August 2002
measurement was made only 1 month after the start of the
artificial recharge project. Also, water with a higher TDS
was recharged than is currently the case (Fig. 3). Although
the distinction between recharge and native dune water can
be seen on the 2002 measurement, conductivity of the
former water is still larger than in the 2006 measurement.
WP24 is located between the western pond and the
extraction wells but has only a casing in the upper part of
the aquifer. Mean conductivity is 6 mS/m in 2006, indicating
the presence of recharged water. A higher conductivity was
again measured in 2002. Notice also that the recharged water
is observed from the top of the profile onwards. EM39
measurements in WP23, WP21 and WP22 show the same
results as WP24 and are not shown here.
Tracer test
A tracer test was performed to derive the residence time of
the artificially recharged water in the dune aquifer. Therefore, NaF was dissolved in the recharged water whereby
fluoride was used as a tracer. After addition of the tracer,
fluoride concentration in the eastern pond measured
0.95 mg/l, whereas this was 1.33 mg/l in the western pond.
Fluoride concentrations were measured in wells WP21,
WP22, WP23, WP24 and in the extracted water. The wells
WP are nested wells with three screens, respectively at a
depth of 3(XX.4), 7 (XX.3) and 11.5 (XX.2) m below
surface level. Distance of WP21 and WP24 to the ponds is
10 m, whereas this is 30 m for WP22 and WP23. Fluoride
was measured every two or three days in these wells.
Figure 5 shows the fluoride measurements in the
observation wells and in the extracted water. In the wells
located closest to the ponds (WP24 and WP21), breakthrough of fluoride was observed after 11 days since the start
of the test. Increased fluoride concentrations were measured
in WP23 and WP22 after 19 days. However, no samples
where taken before day 19, so time of breakthrough is
uncertain in these cases. There is an important difference
between wells WP23–WP24 and wells WP21–WP22.
Notice that the deepest screen of WP24 (WP24.2) shows
larger concentrations than the shallower screens. This is also
the case for WP23, albeit that the difference in concentration
Fig. 4 EM39 measurements giving the electrical conductivity (EC) in function of depth in WP7.1, WP6.1 and WP24
Hydrogeology Journal (2008) 16: 1669–1681
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Fig. 5 Fluoride concentration in function of time for the different observation wells WP21, WP22, WP23, WP24 (a–d) and of the
extracted water (e). The time scale is in the number of days since start of the test
between the different screens of WP23 is much less than for
WP24. In WP21, the largest concentrations are observed in
the shallowest screen. The reason for this difference is
aquifer heterogeneity. As indicated before, a shallow semipermeable layer is present below the western pond but not
below the eastern pond. Consequently, there is an important
lateral flow component above the semi-permeable layer near
the western pond. Therefore shallow observation wells of
WP21–WP22 have a larger fluoride concentration than
deeper wells. Near the eastern pond, however, recharged
water can more easily flow deeper in the groundwater
reservoir and the largest fluoride concentrations are observed
in the deepest observation wells.
MOCDENS3D model
A three-dimensional density-dependent groundwater
flow model was needed to obtain a more detailed view
of groundwater flow around the artificial recharge
ponds, determine capture zones, travel and residence
times and the geometry of the volume of recharged
water. For this simulation MOCDENS3D (Oude Essink
1998), which is based on the three-dimensional solute
transport code MOC3D (Konikow et al. 1996), was
chosen and adapted for density differences and Visual
Hydrogeology Journal (2008) 16: 1669–1681
MOCDENS3D (Vandenbohede 2007) was selected for
the visualisation of model results and the calculation of
captures zones and residence times.
The MOCDENS3D model of the recharge process is
based on an existing regional scale model simulating the
groundwater flow and fresh-water–salt-water distribution
in the dunes, polders and shore of the area surrounding the
St-André water extraction (Vandenbohede et al. 2008).
This regional scale model spans an area of 4,200×4,500 m
and includes the evolution of the water extraction from its
start in 1947 until the end of 2006. First step was,
however, the simulation of the current fresh-water–saltwater distribution. Therefore, the replacement of salt by
fresh water because of land reclamation and the formation
of the dune belt was simulated. Subsequently, the
evolution of the water extrication from 1947 to June
2002 was simulated using stress periods of 2 or 3 years.
The model was calibrated with hydraulic head observations and water quality data. Stress periods of 2 months
were used from the start of the artificial recharge project
(July 2002) until the end of 2006 to calculate the effect of
the artificial recharge on the groundwater flow and freshwater–salt-water distribution in the whole dune aquifer.
The model described here zooms in on the 975×525-m
area surrounding the ponds (Fig. 6) and consists of 105
rows and 195 columns. Dimension of every finiteDOI 10.1007/s10040-008-0326-x
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Fig. 6 a Horizontal cross-section through the regional flow model at the level of the screens of the extractions wells (Vandenbohede et al.
2008). The rectangular box shows the detailed model around the ponds. The grey scale represents TDS (mg/l) of the pore water, black lines
are lines of equal fresh-water heads and arrows indicate the groundwater flow. b Cross-section at the same level through the detailed model
showing the chloride concentration (mg/l) and the groundwater flow by means of the flow vectors and lines of equal fresh-water head. Grey
scale in b is the reverse from grey scale in a
difference cell is 5×5 m and 12 layers are considered, each
with a thickness of 2.5 m, which means that the complete
phreatic aquifer is considered in the model and that the
clay of the Kortrijk Formation determines the impermeable lower boundary. The north, south, east and west
boundaries of the model are constant head boundaries.
The values for these constant heads are derived from the
regional scale model and are altered after every stress
period. Stress periods of 2 months are used, coinciding
with the stress periods of the regional scale model. After
Hydrogeology Journal (2008) 16: 1669–1681
every stress period, the recharge and extraction rates
change as does the concentration of the recharged water if
necessary. The solute concentration in the model is that of
the chloride concentration of the pore water.
Horizontal hydraulic conductivity of the dune sediments
is 12 m/d for every layer. Ratio of the horizontal to the
vertical conductivity is 20. This anisotropy is 40 between
layer 1 and 2 under the western pond, simulating the
increased hydraulic resistance of the semi-permeable layer
present at this location. This layer is identified in a number
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of drilling descriptions located in the western part of the
model domain, but the exact lateral extension is not known.
Therefore, it is conceptualised that this layer is present in
the western part (from column 1−118) of the model.
Anisotropy is also 40 for layer 10−12 because the lower
part of the aquifer is less permeable. Horizontal hydraulic
conductivity is also smaller (8 m/d). Natural recharge is
280 mm/year, whereas the longitudinal, horizontal transverse and vertical transverse dispersivity are respectively
0.1, 0.01 and 0.001 m. Effective porosity is 0.38.
Influences of boundaries are minimised in the model in
two ways. First issue is the distance of the boundaries from
the wells. From the larger model, the zone of influence of
well battery two is known. Based on this, position of
boundaries is chosen so that the drawdowns are minimal.
Secondly, a combination of constant head and no flow
boundary is applied. A constant head boundary assumes the
presence of imaginary wells mirroring the pumping wells
or injection wells in a number of injection or pumping wells
respectively. At the position of the boundaries, the
superposition of the pumping or injection (in the model)
and the imaginary injection or pumping wells cancel each
other resulting in a constant head. However, between the
pumping or injection wells and the boundary, the drawdown is influenced because of the imaginary wells.
Because of the presence of four constant head boundaries,
the model wells result in four sets of imaginary injection
and pumping wells. These are also mirrored by the
boundaries and so on, resulting in a number of imaginary
injection and pumping wells of first order, second order,
etc. The higher the order, the farther these wells are located
from the boundaries and the smaller their influence is.
These effects can be partially cancelled out by halving the
hydraulic conductivities of the first and last rows and the
first and last columns of each layer. These values must be
regarded as the mean of the expected conductivity values
and a conductivity of zero, the latter being an impermeable
boundary. Impermeable boundaries are realised in the
model by imaginary pumping or injection wells, respectively for pumping or injection wells in the model.
Consequently, by the combination of constant head
boundaries and the adjustments of the hydraulic heads,
the odd numbered imaginary wells (and most important the
first order wells) are deleted.
Model results
Groundwater flow and water-quality distribution
Figure 6b shows a horizontal cross-section through the
aquifer at the level of the extraction wells for the end of
2006. The grey scale represents chloride concentration,
whereas the arrows show direction and magnitude of
groundwater flow. Figure 7 shows two vertical crosssections through the aquifer. Figure 7a, c are cross-sections
according to column 80, which is through the western
pond; whereas b and d are cross-sections according to
column 145, which is through the eastern pond. Distribu-
Fig. 7 Vertical cross-sections through the dune aquifer. a and c Cross-sections according to column 80 which is through the western pond;
b and d Cross-sections according to column 140, which is through the eastern pond. Chloride concentration is shown at the end of 2004 (a
and b) and at the end of 2006 (c and d). Arrows show the direction and magnitude of the groundwater flow and lines represent the freshwater heads. The position of the extraction wells is indicated with black vertical bars
Hydrogeology Journal (2008) 16: 1669–1681
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tion of chloride is shown at the end of 2004 (a and b) and at
the end of 2006 (c and d). Arrows show the direction and
magnitude of the groundwater flow and lines represent the
fresh-water heads. In these figures, differences between the
two ponds are visible. The area with recharged water in
the cross-section is smaller for the eastern than for the
western pond. Additionally, the presence of recharged
water is confined to the area between the pond and the
extraction wells for the eastern pond. This is not the case
for the western pond. Here, water is present beyond the
limits of the extraction wells. This is also visible in the
horizontal cross-section in Fig. 6b. There are two reasons
for these differences. First of all, the eastern pond is smaller
than the western pond. This means that less water recharges
and that the volume of recharged water present under the
eastern pond will be smaller than the volume of recharged
water present under the western pond. Secondly, aquifer
heterogeneity is of importance. The presence of a semipermeable layer under the western pond means that
recharged water encounters a larger hydraulic resistance
for vertical flow. The result is that recharged water will
spread more laterally from the western pond. Of note is, for
instance, the flow towards the wells located south of the
pond. Recharged water flows at first mainly laterally away
from the pond before flowing towards the wells; thus, the
area where recharged water is present extends south of the
extraction wells. This means that the recharged water first
flows over the position of the extraction wells before
flowing back towards them.
The influence of the semi-permeable layer in the
simulation agrees very well with the conclusions of the
tracer tests. Further, recharged water is found to a depth of
approximately 25 m under the western pond and to
approximately 18 m under the eastern pond. This agrees
with the EM39 measurements in WP6.1, which is situated
between both ponds. In WP6.1, recharged water is
observed to a depth of approximately 20 m.
Capture zones
Figure 8 shows a horizontal cross-section through the
capture zone of the extraction wells. This cross-section is
made at the top level of the model giving the time
recharged water (natural and artificial) needs to travel
from the recharge point to the extraction wells; or,
conversely, it gives the residence time of the recharged
water (natural and artificial) in the aquifer. Figure 9 shows
two vertical cross-sections through the capture zone
according to columns 80 and 140. Capture zones are
calculated by means of a particle-tracking algorithm
within Visual MOCDENS3D. A number of particles are
placed in the model grid and their paths are calculated
using the x, y and z velocity components of the nodes of
the finite difference grid by means of three-dimensional
interpolation. The time after which particles are removed
from the model grid through the wells is registered. A
contour plot of these travel times gives the capture zones.
A number of different conclusions can be made from
Figs. 8 and 9. For water recharging in the pond, it takes in
general less than 50 days to reach the extraction wells.
There are, however, a number of exceptions. For instance,
a water divide exists in the centre of both ponds. Water
recharging south of the divide flows to the southern wells
of well battery 2, whereas water recharging north of the
divide flows towards the northern wells. This divide is
clearly visible on the vertical cross-sections through the
capture zone in Fig. 9 as well as in Figs. 7 and 8. Water
recharging close to the water divide needs a longer time to
reach the extraction wells because it is involved in a
longer flow cycle towards these wells. Whereas water
Fig. 8 Horizontal cross-section through the capture zone of the extraction well. Grey scale represents the time (in days) needed for water
recharging in the first layer of the model to reach the extraction wells. Times larger than 350 days are not shown here. Position of ponds and
extraction wells are indicated by black lines
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Fig. 9 Vertical cross-sections along column 80 (a) and column 140 (b) of the capture zone of the extraction well. Time is in days; times
larger than 350 days are not shown. The position of the extraction wells are indicated by black vertical bars
recharging close to the edges of the ponds flows more or
less directly to the wells, water recharging in the centre of
the ponds flows downwards in the aquifer. When this
latter recharged water reaches the bottom of the recharged
water lens, it flows upwards and towards the wells. The
flow pattern results in the characteristic lobate crosssection through the capture zone with the ponds in the
centre as can be seen for the eastern pond (Fig. 9b). The
western pond shows a more complex situation because of
the location of wells 1–10. These wells are located farther
from the pond. Figure 8 shows that wells 1–10 receive
recharged water from the north-western part of the western
pond but this takes significantly longer than 50 days. The
capture zone of these wells is also clearly delineated from
the rest of the northern wells (11–40) or from the nearest
southern wells (41–50). Figure 9a shows a third lobate
zone which is a cross-section through the capture zone of
wells 1–10. Notice, for example, that water recharging just
north of the water divide does not flow towards the closest
wells but towards the wells 1–10. The time to reach the
extraction wells is larger than the 350 days shown on the
Hydrogeology Journal (2008) 16: 1669–1681
figures. Wells 41–50 which are not located exactly around
the pond receive only a small amount of recharged water.
Moreover, the most distant wells from these extract
mainly naturally recharged water.
The flow vectors in Figs. 6 and 7 and the cross sections
through the capture zones indicate that most of the
artificially recharged water flows towards the extraction
wells. This is evidently due to the fact that the ponds are
firmly surrounded by extraction wells. The fact that no
extraction wells are located west and east of the ponds has
no effect on this. At the western side of the ponds,
recharged water flows ultimately towards wells 1–10 or
41–50. The largest zone where no wells are placed is
situated northeast of the eastern pond. Water which
recharges in the eastern part of the eastern pond, however,
flows towards wells 30–40 or 100–110.
Residence times
Residence times of the artificially recharged water in the
dune aquifer are discussed more in detail here also using a
DOI 10.1007/s10040-008-0326-x
1678
particle-tracking algorithm of Visual MOCDENS3D. A
large number of particles (13 per finite difference cell) are
placed in all cells of the model representing the ponds.
The movement of these particles is calculated and
followed until they reach an extraction well. The time to
travel the distance from the ponds to an extraction well is
the residence time of the particle in the aquifer. Thereafter,
a statistic analysis of the residence times of all particles is
made.
Figure 10a shows a time-frequency distribution of the
residence times of water recharged in the ponds, whereas
Fig. 10b shows the cumulative distribution. These figures
can be regarded in different ways. First, it indicates when
water recharging at the same point in time reaches the
extraction well. Eight percent of the recharged water
reaches the extraction wells after 27 days; after 36 days,
this percentage increases to 25%. Half of the amount of
water which was recharged at the same point in time is
recovered after 55 days, whereas this is 117 days for 75%.
Total recovery of the artificially recharged water is
achieved after 1,813 days or almost 5 years. To ensure
clarity of the figures, only residence times up to 350 days
for the time-frequency plot and up to 500 days for the
cumulative distribution are shown in Fig. 10. Notice that
the time-frequency distribution of Fig. 10a is highly
asymmetric, which is due to the complex nature of the
capture zone of the extraction wells and the high variety of
flow paths as indicated in the previous section. About 50%
of the water flows relatively quickly to the extraction
wells, which it reaches after less than 60 days. Water
recharging, for example, in the vicinity of the water divide
in the ponds flows in a deep and much longer flow cycle
towards the extraction wells and is spread over a longer
time span. This contributes to the tailing of the distribution
in Fig. 10a. Another issue is the position of the extraction
wells. In all, 75% of the wells are located at a distance
which is less than 60 m from the ponds—other wells are
between 60 and 100 m. Water flowing from the western
pond towards wells 1–10 will have in general a larger
residence time because of the larger distance. The same
accounts for water flowing from the eastern part of the
eastern pond towards wells 100–110. Consequently, water
recharging in the ponds at the same point in time reaches
the wells at different times. However, 50% of the water
reaches the extraction wells within 60 days, which refers
to water flowing to the extraction wells via shallow flow
Fig. 10 Frequency–residence time plot (a) and cumulative frequency–residence time plot (b) of the residence time of the artificially
recharged water in the dune aquifer
Hydrogeology Journal (2008) 16: 1669–1681
DOI 10.1007/s10040-008-0326-x
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paths or water flowing to the nearest extraction wells. The
other 50% of the water flows in deeper (and slower) flow
paths to the extraction wells or to more distant extraction
wells. In the latter case, the result is an increase of
frequency on the general decreasing frequency-residence
time plot of Fig. 10. Notice for instance the small increase
in frequency around a residence time of 90 days and
between 150 and 200 days.
Figure 10a can also be considered to be the equivalent
of a breakthrough curve of a tracer and can thus be
compared with the results of the tracer test, more
particularly with Fig. 5e. This shows enlarged fluoride
concentrations between 30 and 45 days, thus coinciding
with the highest frequencies of the time-frequency
diagram of Fig. 10a. Consequently, calculations are in
good agreement with the results of the tracer test. Figure 9
can also be seen as a distribution of the residence times of
the water which enters the extraction wells at one time.
Thus, it is derived that 50% of the extracted water
simultaneously has a residence time of 55 days and so
on, whereas 50% of the water has a longer residence time
of up to 5 years. This means that the extracted water is a
mix of water with different residence times from about
30 days up to 5 years.
A number of particles will have flow paths ending in
the same well. A statistical analysis was made for each
individual well. This is visualised by means of box plots
(Fig. 11) for each well of well battery 2. Box plots show
lower quartile, median and upper quartile values. The
whiskers are lines extending from each end of the box to
show the extent of the rest of the data. Outliers (+) are
data with values beyond the ends of the whiskers.
Some interesting trends can be seen from Fig. 11.
About 40% of the wells (wells 10–20, 50–75, 90–100)
have a lower quartile between 25 and 30 days and a
median between 30 and 40 days. This means that, as
shown in Fig. 10, about 50% of the recharged water is
recovered in less than 60 days. Residence time of water
extracted in the other 60% of the wells is larger and spread
over a larger time span, which is, as already explained,
due to the combination of longer flow cycles of the
recharged water and position of extraction wells. Water
extracted from wells 1–10, for instance, has a longer
residence time which is partly due to the larger distance to
the western pond. Also, the amount of water reaching
these wells follows deep and longer flow cycles as already
shown in Fig. 9. Water reaches wells 42–49 also after a
longer time—note that there is a clear trend from well 42
to well 49, which is mainly due to the position of the
wells. Moreover, no recharged water is extracted from
well 41, which is located furthest from the pond. A similar
observation is made for wells 101–110 which are located
southeast of the eastern pond. Wells 107–110 are located
too far away from the pond to extract recharged water.
For wells 25–40, there is a general trend of residence
time increasing from wells 25 to 31 and decreasing from 32
to 40. The same can be said for wells 75–90. The increase
of residence time, however, is not as large as for instance
wells 1–10, 41–50 or 100–110. The reason for this is the
position of these wells, which are located between the two
ponds or north or south of the smallest part of the eastern
pond. The combination of the facts that less water infiltrates
here with the larger distance between pond and extraction
wells (mainly to the southern wells) makes the residence
times of water entering these wells larger.
Of note is also that, in general, the larger the residence
time is of water entering a particular well, the larger the
difference between lower and upper quartile values is.
This difference is smallest for the wells 10–20, 50–75, 90–
100, which have the shortest residence times. The increase
of the difference with increasing residence time is very
well illustrated by, for instance, wells 100–110, wells 25–
40 or wells 75–90. Longer residence times are found for
wells which are positioned relatively far from the
infiltration ponds. These wells also receive recharged
water taking part in the long and deep flow cycle, which
Fig. 11 Box plots of the residence time of infiltration water reaching the different wells of well battery 2. The whiskers are lines extending
from each end of the box to show the extent of the rest of the data. Outliers (+) are data with values beyond the ends of the whiskers
Hydrogeology Journal (2008) 16: 1669–1681
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means that the spectrum of residence times will be much
larger than for wells located close to the ponds. Therefore
the difference between the lower and upper quartile values
will be larger for these former wells.
Volume of infiltration water
Figure 12 shows the evolution of the volume of recharged
water present in the dune aquifer. This is subdivided into
total volume of recharged water and volume of recharged
water present under the eastern and western pond. During
the first half year of the artificial recharge project (until the
end of 2002), the volume of recharged water is increasing
relatively quickly. Afterwards, this volume is still increasing
but the rate of increase is decreasing. At the end of 2007, the
volume of the recharged water continues to increase. This
means that, although the recharge and extraction rates are
more or less fixed during the last 3 years, the volume of
recharged water present in the aquifer was still increasing. At
the end of 2006, a volume of 1.1502 106 m3 of water is
calculated as being present in the aquifer. Taking into
account a porosity of 0.38, this means a volume of 3.0268
106 m3 of dune aquifer. About 70% of the recharged water is
present under the western pond.
To study how these volumes will evolve in the near
future, the groundwater model was extended for 5 years,
whereby recharge and extraction rates varied as in 2006.
Between 2007 and 2012 there is still an increase in
volume of recharged water, although this increase
becomes very small. The fact that this volume of
recharged water will not be at a dynamical equilibrium,
even after 10 years of recharge is not unexpected. This is
the same as, for instance, the formation of fresh-water
lenses where a dynamical equilibrium is reached after
more than 100 years (Vandenbohede and Lebbe 2002).
Obviously, the increase in volume of recharged water will
become very small within the next 10 years.
Conclusions
Since July 2002, tertiary treated wastewater has been
artificially recharged in the phreatic dune aquifer of the
Belgian western coastal plain. This was done to realise a
sustainable water extraction in an area where fresh-water
resources are limited. Hydraulic aspects and storage and
recovery of the recharged water were studied using
borehole measurements, results of a tracer test and a
three-dimensional solute groundwater flow model. Capture zones and residence times were studied to identify
characteristics of the system.
Because of the artificial recharge, a lens of recharged
water has formed in the native fresh dune water lens.
Dimensions of this lens were derived from borehole
measurements and the modelling. Borehole measurements
show a clear distinction between recharged and native dune
water, the former having a lower electrical conductivity
because of the lower TDS. Recharged water is found up to a
depth of 20 m below surface at WP6.1, located almost
between the two ponds. Modelling shows that the depth of
the recharged water lens under the eastern pond is slightly
less than 20 m, whereas this is slightly more than 20 m for
the western pond. Because of its larger surface, more
recharged water is present under the western than under the
eastern pond. The volume of recharged water in the aquifer
is, 5 years after the start of the recharge project, still
increasing, although this increase will become very small
in the next 5–10 years. Groundwater flow and flow paths
show that the occurrence of recharged water is not restricted
to the zone between the ponds and the extraction wells,
especially around the western pond. This is due to a shallow
semi-permeable layer present below the western pond and
illustrates the effects of aquifer heterogeneity. The importance of the semi-pervious layer was also indicated by the
tracer test. All recharged water is, however, recovered as is
indicated by the capture zones.
Fig. 12 Volume of recharged water present in the dune aquifer in function of time. The total volume of both ponds is given, as is the
volumes of water present under the eastern and western ponds
Hydrogeology Journal (2008) 16: 1669–1681
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Analysis of residence times has indicated that 50% of
the water recharged at the same point in time reaches
extraction wells in less than 60 days. It takes up to almost
5 years for the last water recharged at the same point in
time to reach the extraction wells. These calculations are
confirmed by the results of the tracer test. The distribution
of residence times indicates that the extracted water is a
mix of water with residence times ranging from slightly
less than 30 days up to almost 5 years. Large residence
times are the result of water flowing in a longer and
deeper flow cycle towards the extraction wells. Extraction
of this recharged water is therefore spread over a longer
time span. There are distinct patterns in the distribution of
residence times of water reaching each individual well.
This distribution is clearly influenced by the position of
the well and the distance of this well to the recharge
ponds. A larger distance for instance means that the mean
residence time is longer. It also means that the spectrum of
residence times of the extracted water is larger. The spread
of the residence times, together with the fact that also
native dune water is extracted (80% recharge and 20%
native dune water) makes the water quality of the total
extraction water stable throughout the year. This is, for
example, confirmed by the constant chloride concentration
of the extracted water (Fig. 3).
Acknowledgements This research was done as part of a research
project funded by Institute for the Promotion of Innovation by
Science and Technology in Flanders (IWT-Flanders), grant IWT/
OZM/050342. The first author was also supported by funding from
IWT-Flanders. The authors thank two anonymous reviewers and the
editors for their constructive comments.
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