Journal of Hydrology X 2 (2019) 100008

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Journal of Hydrology X

journal homepage: www.elsevier .com/locate /hydrolx

Review papers

An innovative tool for groundwater velocity measurementcompared with other tools in laboratory and field tests

https://doi.org/10.1016/j.hydroa.2018.1000082589-9155/� 2018 The Author(s). Published by Elsevier B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Abbreviations: DVT, direct velocity tool; PFM, passive flux meter; PVP, pointvelocity probe; K, hydraulic conductivity; BHD, borehole dilution test.⇑ Corresponding author.

E-mail address: elyess.essouayed@ensegid.fr (E. Essouayed).

E. Essouayed a,⇑, M.D. Annable b, M. Momtbrun a, O. Atteia c

a INNOVASOL, Bordeaux INP ENSEGID, University of Bordeaux Montaigne, EA 4592 Georessources et Environnement, 1 Allée Daguin 33600 Pessac, FrancebDepartment of Environmental Engineering Sciences, University of Florida, Gainesville, FL 32611, USAcBordeaux INP ENSEGID, University of Bordeaux Montaigne, EA 4592 Georessources et Environnement, Carnot ISIFoR, 1 Allée Daguin, 33600 Pessac, France

a r t i c l e i n f o

Article history:Received 27 May 2018Revised 27 November 2018Accepted 28 November 2018Available online 06 December 2018

Keywords:Direct velocity toolDarcy flux measurementLaboratory and field comparisonInnovative

a b s t r a c t

An innovative solution for groundwater velocity measurement in wells, the Direct Velocity Tool (DVT), isdeveloped and tested. The DVT allows measurement of Darcy flux at centimeter scale vertical resolutionrequiring only a few minutes for each measurement (typically 5–10 min). Results are generated in realtime through the use of an equation. The theoretical functional Darcy flux range is between 1 and 40 cm.-day�1 for an aquifer with a hydraulic conductivity, K, less than 10�3 m.s�1. Laboratory tests showed a lin-ear response between Darcy flux imposed and Darcy flux measured with the DVT, with a high correlationcoefficient (R2 = 0.99). The DVT was tested for velocities ranging from 5 to 30 cm.day�1 where a higherstandard deviation was observed for higher velocity (±6.46 cm.day�1 for 30 cm.day�1). At a hydraulicallycontrolled field site, tracer tests, borehole dilutions, Passive Flux Meter (PFM) and DVT were tested underconstant flow conditions. The PFM, tracer test and DVT technologies measured similar Darcy flux rangingfrom 10 to 17 cm.day�1. The DVT and PFM had uncertainties of 2.70 cm.day�1 and 2.50 cm.day�1, respec-tively. At a contaminated field site, velocities measured with the PFM range between 5 and 9 cm.day�1,while the DVT provided velocities of 4 to 8 cm.day�1. Velocity measurement showed similar results at thecontaminated field site between PFM and DVT. The DVT offers an innovative solution for Darcy flux mea-surement and can be deployed easily. It provides the capability to characterize the vertical distribution ofthe horizontal velocity for a well in a period of 1–2 h.� 2018 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND

license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Velocity measurements are useful for a number of purposes inhydrogeological studies. These measurements can be used to quan-tify recharge, estimate clean-up times, design permeable reactivebarriers, characterize both groundwater and contaminant move-ment in aquifer systems (Devlin et al., 2009). Velocity can be rep-resented by the Darcy flux ‘‘q” as an apparent groundwater velocityor pore water velocity. Both are quite important for site manage-ment. The velocity referenced in this study corresponds to theDarcy flux.

Darcy flux measurements have been increasingly used for thecalculation of contamination mass flux at polluted sites andprovide an important metric to understanding the transport

mechanisms near source zones and to support remedial design(SERDP, 2010).

Darcy flux can be calculated using Darcy’s law, based onhydraulic conductivity K [L T�1], the hydraulic gradient i [dimen-sionless]. However a large uncertainty remains on the value ofhydraulic conductivity and the spatial distribution of K (Ballard,1996). The most direct method for quantifying groundwater veloc-ity estimation is a tracer test. With this method, several wells ormonitoring points are necessary to observe tracer migration inorder to estimate groundwater velocity. This approach sees limitedapplication due to the time and cost required (Labaky et al., 2009).

For the reasons stated above, several tools and methods havebeen developed for Darcy flux measurement at a single point (in-well or direct push). The first method, based on in-well measure-ment found in the literature, is the Borehole dilution test (Drostet al., 1968; Halevy et al., 1968) where the undisturbed groundwa-ter flow in the aquifer is quantified based on the dilution of a tracerintroduced into a screened well. Assuming steady groundwaterflow and a homogeneous distribution of the tracer in the well, a

2 E. Essouayed et al. / Journal of Hydrology X 2 (2019) 100008

dilution rate is calculated. In this approach, the presence of verticalwater flow in the well can disturb the mixing between tracer andgroundwater. Thus maintaining a reliable measure of tracer con-centration declines without disturbing the flow field is challenging(Drost et al., 1968). A borehole dilution test was developed in anisolated section of a well in order to avoid the influence of verticalflow (Palmer, 1993) and keeping the section well mixed. Due to thedifficulties and limits of the borehole dilution test, Brouyère et al.(2008) proposed a new method based on a tracer dilution calledthe Finite Volume Point Dilution Method (FVPDM). The FVPDM iseasy to implement in the field and the method can be conductedto provide temporal monitoring of Darcy flux. Jamin et al., (2015)compared the FVPDM to the basic borehole dilution test in a frac-tured aquifer. They showed that the borehole dilution test pro-vided a good Darcy flux estimation if the mixing volume wasprecisely known. The FVPDM method gave a better measurementof the Darcy flux but required a longer duration measurement.

In the literature, other in-well tools with a direct Darcy fluxmeasurement of groundwater have been developed including theGeoFlometer (Kerfoot and Massard, 1985) which is based on ther-mal transmission in the open portion of the well screen. The toolcreates a heat pulse in the center of the well screen and movementof water induces a thermal signal, proportional to the Darcy flux.The unit is best deployed with velocities greater than 50 cm.day�1.The Colloidal Borescope (Kearl, 1997; Kearl et al., 1999) is a toolemploying direct observation of particles in the well screen. Thesemeasured velocities are averaged to obtain a mean Darcy flux. TheColloidal borescope is more effective for high velocities from thecentimeter per day scale up to 2400 m.day�1 (Kearl et al., 1992,1999; Kearl, 1997; Korte et al., 2000). The Laser Doppler Velocimeter(Momii et al., 1993) measures the particle movement at the cross-ing point of two laser beams. The velocity range is similar to theGeoFlometer. GeoFlometer, Colloidal Borescope and Laser DopplerVelocimeter have a variable measurement range (from 1 cm.day�1

to 100 000 cm.day�1) but have limitations at low velocity particu-larly due to vertical currents in the well similar to the boreholedilution test.

The Passive Flux Meter (Hatfield et al., 2004) (PFM) is a perme-able cylinder that sorbs dissolved organic and inorganic contami-nant and intercepts groundwater flow. The detailed principle ofthe PFM will be presented later. The PFM requires an exposuretime in the field that can last from 1 to 4 weeks depending onthe well diameter and groundwater velocity. Regarding in-wellmeasurements, according to our best knowledge currently themost widely employed tools are the Borehole Dilution Test to quan-tify Darcy flux. The PFM is increasing in use with more focus onmeasuring contaminant mass fluxes.

Other tools do not require the installation of a well since thevelocity is measured through direct contact with the aquifer mate-rial, during a direct push investigation. The In Situ Permeable FlowSensor (Ballard et al., 1996) uses a thermal perturbation technique.This tool is a cylindrical heated device that is in contact with theporous media at the velocity measurement point. If the water fluxis uniform in the porous media, groundwater fluxes causes a tem-perature distribution on the surface of the cylinder varying as afunction of the direction and magnitude of groundwater flowvelocity (range from 0,5 to 90 cm.day�1). The Point Velocity Probe(PVP) (Devlin et al., 2009; Labaky et al., 2007, 2009), is composedof two half cylinders one is hollow and contains the injection/detection system. The other one half is solid and contains a grooveto stabilize the injection line. A small volume of tracer is intro-duced through the injection system and the velocity of the traceris estimated based on arrival of the tracer at a detector.

One limit of the direct-push methods is compaction of the aqui-fer materiel during installation, changing the natural characteristicof the porous media. Also, penetration of the tool could be difficult

depending on the type of aquifer material. A new device has beendeveloped for applications in screened wells (Osorno and Devlin,2017) and is called the In Well Point Velocity Probe (iW-PVP).Labaky et al. (2009) compared the PVP with other tools for ground-water measurement. The different tests were carried out in a nat-ural aquifer confined by a sheet pile wall bounded cell (appliedDarcy flux of 20 cm.day�1). Four methods were compared: PVP,GeoFlometer, Colloidal borescope and the Borehole dilution test. Theintercomparison showed that the PVP and GeoFlometer had similarmeasurement results, with a better value for the PVP. The chal-lenges noted with the PVP measurement were due to porous med-ium alteration during hammering installation that caused anegative bias for groundwater velocity measurement. The Boreholedilution experiment and Colloidal borescope overestimated thegroundwater velocity by an order of magnitude or more. Bothmethods are useful at higher velocities.

The main objective of this paper is to present an innovative toolfor groundwater velocity measurement, the Direct Velocity Tool(DVT) which intends to provide:

– an in well test;– a measurement without limitation due to vertical currents;– a direct natural gradient velocity measurement for relativelyheterogeneous rates that are common in alluvial aquifers(5 cm.day�1 to 100 cm.day�1);

– a short measurement time of 5–10 min.

In this paper, the DVT principle and method of application arefirst described. The tool is then validated in the laboratory in asmall 3D test box. The tool is then compared to a Tracer test, aPFM and a Borehole dilution test at a small scale field site test.Finally several measurements conducted at contaminated fieldsites are compared to PFM data.

2. DVT development and validation

2.1. Theory

The DVT principle is based on measuring a combined homoge-neously mixed flow of a known tracer solution and the natural gra-dient groundwater flux. The DVT is constructed using a PVC tube(reduced down to one third of the tube circumference) that ispushed in close contact with the well screen through the use ofpneumatic actuators (Fig. 1a, b) thus creating an intake windowisolating a portion of the well screen. More specifically, the intakewindow is created using a thicker neoprene seal (dark part on theDVT in Fig. 1b) which is the only part pressed against the wellscreen. The intake window makes it possible (i) to isolate a portionof the screen for measurement and (ii) to focus the groundwaterflow toward the measurement sensor (Fig. 2). To optimise theDVT measurement, the screen must be located in the well. How-ever, it’s difficult to have this information so at first the DVT is ori-ented upgradient relative to the natural gradient flow direction.Four directions (north, east, south and west) are recommendedfor testing, in order to estimate the highest Darcy fluxmeasurement.

2.1.1. Mixing principleAs shown in Fig. 2, water from the aquifer flows through the

intake window and exits at point A. The groundwater flow travelsthrough a short section of tubing before exiting at point B. Near thetubing inlet, an injection port is installed at point C wheregroundwater is mixed with an injected tracer solution. The mixedsolution is then measured at the exit, point B. Adopting a simplemass conservation and assuming the two flow streams are

Fig. 1. (a) Picture of the DVT in a transparent tube. The DVT, maintained against thescreen (neoprene part) creates the intake window in order to isolate a portion of thewell screen. (b) Schematic top view of the DVT in a well.

Fig. 2. Schematic 2D section of the DVT with the inlet system. Tracer andgroundwater homogeneous mixture permit Darcy flux calculation.

E. Essouayed et al. / Journal of Hydrology X 2 (2019) 100008 3

homogeneously mixed, the flow of water from the aquifer can becalculated:

QaquiferCaquifer þ QtracerCtracer ¼ Qaquifer þ Qtracer

� �Cmixing ð1Þ

where Q is flow rate [L3 T�1], C is concentration (or electrical con-ductivity if the tracer is a saline solution) [M L3 or mS cm�2].Qaquifer can be calculated as Caquifer is measured before the test,Ctracer and Qtracer are known and Cmixingis measured during the test.

Qtracer is very low (between 2 and 4 mL.min�1) to avoid a coun-terbalancing of Qtracer in Qaquifer . Depending on the site, calibrationof the tracer flux is modified when the outlet electrical conductiv-ity is too close to that in the aquifer.

2.1.2. Physical configurationThe DVT is composed of a one third section of PVC tube (63 mm

diameter) with two separate parts, (i) the Convex part which facesagainst the screen and includes a rubber layer around the perime-ter in order to create the intake window and (ii) the Concave partwhich forms the inlet system and device for velocity measurementemploying an electrical conductivity sensor. The intake windowhas a surface of 101 cm2 and a thickness of 2 mm.

The exit of the intake window is directly connected to the inletof the tube component of the system. The tube system wasdesigned to produce a homogeneous tracer mixture which is mea-sured at the end of the system.

From the top of the well casing, the tracer solution is injectedcontinuously using a peristaltic pump through a thin tube (4 mminner diameter), until the system reaches a steady state mixed con-centration measured with an electrical conductivity meter thusallowing aquifer flow rate calculation (Eq. (2)).

QDVT ¼ QtracerCmixing � Ctracer

Caquifer � Cmixing¼ qDVTSIntake window ð2Þ

where qDVT is the Darcy flux from the DVT [L T�1] and S is the intakewindow area [L2].

2.2. Accuracy of the DVT

2.2.1. Concept of flow distortionFor a homogeneous permeable circular element in a locally

homogeneous aquifer, the contrast of hydraulic conductivity leadsto a convergence or divergence of the aquifer flow (Strack andHaitjema, 1981). As an example, the contrast in K between a PFMand the aquifer can produce converging flow through the well,and the relation between the Darcy flux in the PFM, qPFM , and theundisturbed aquifer Darcy flux qaquifer can be defined (Annableet al., 2005):

qPFM

qaquifer¼ 2KPFM

KPFM þ Kaquifer¼ a ð3Þ

whereKaquifer is the hydraulic conductivity of the aquifer andKPFM the hydraulic conductivity of the PFM [L T�1]. This relation-ship between apparent Darcy flux and undisturbed Darcy flux isthe factor a defined by Drost et al. (1968). This factor takes intoconsideration the flow field distortion by the well as influencedby the tool inserted into the well.

2.2.2. DVT flow distortion modellingThe flow distortion around the well was modelled with MOD-

FLOW USG (UnStructured Grid) using a triangular mesh. Parame-ters and description of the models are presented in thesupporting information Appendix A and B.

2.2.2.1. Distortion comparison for three cases. The distortion createdby an empty well screen, a well screen with a PFM present and a

K aquifer (m/s)

1e-6 1e-5 1e-4 1e-3 1e-2

Alph

a co

effic

ient

0,0

0,5

1,0

1,5

2,0

2,5

Empiric fit Simulated value

5 cm/d

40 cm/d

4 E. Essouayed et al. / Journal of Hydrology X 2 (2019) 100008

well screen with a DVT installed was compared using the modelwith an aquifer K of 10�4 m.s�1 and a Darcy flux equal to 11 cm.-day�1. Model results are presented in Fig. 3. The simulation showsthat the DVT creates a distortion higher than both the PFM and theempty well screen. Calculation of the distortion considers the com-plete diameter of the well screen for the ‘‘empty well” and the‘‘PFM” cases. For the DVT, the distortion considers the width ofthe intake window. Indeed, the flux measured in the DVT is relatedto the size of the intake window.

2.2.2.2. Distortion calibration of the DVT. Different tests with theDVT were modelled in order to assess the limitations of the tool.Fig. 4 shows the distribution of the DVT distortion for a range ofDarcy flux in function hydraulic conductivity. An empirical fitwas developed using the following equation, where K is hydraulicconductivity of the aquifer [L T�1] for a Darcy flux range between 5and 40 cm.day�1

Distortion coefficient ¼ 0:7exp�2300K � 200K þ 1:2 ð4Þ

Fig. 3. Distortion model for three cases (an empty well screen, a PFM present in thewell screen and a DVT installed). For each case, the longest arrow shows the sectionof the flux that will enter the device and the shortest one the edge of the considereddevice. The distortion coefficient is equal to the ratio of the two arrow lengths.

Fig. 4. Distortion coefficient of the DVT as a function of hydraulic conductivity andthe Darcy flux imposed between 5 and 40 cm.day�1.

This equation was used for the calculation of the DVT distortion.With these results, the DVT has limited application when the dis-tortion is less than 1. So, for an aquifer with a hydraulic conductiv-ity higher than 2.10�3 m.s�1, the distortion coefficient decreasesrapidly which may limit the accuracy of the Darcy fluxmeasurement.

2.3. DVT calibration in laboratory tests

2.3.1. BackgroundThe DVT was first tested in a sand tank A, 68 cm long, 12 cm

wide and 50 cm height (Fig. 5). An 80 mm diameter screened wellwas installed in the centre of the sand box. The box was packedwith zones upstream and downstream using a gravel material tocreate constant head boundaries. The main portion of the boxwas filled with coarse sand. Two clay zones were added betweenthe box walls and the well screen in order to avoid water deviationaround the well (corresponding to an absence of distortion).Indeed, in the sand box there is no distortion between the appliedDarcy flux and measured Darcy flux in the well, thus DVT calibra-tion is not required. Using a peristaltic pump, different flow rateswere imposed in the system producing a homogeneous Darcy flux.DVT measurements were compared to the imposed Darcy fluxthrough the porous medium.

Another calibration was conducted in a second laboratory tanksystem B (100 cm long, 50 cm wide and 50 cm height) at only twoimposed velocities, 16 cm.day�1 and 26 cm.day�1, to provide acomparison between PVP and DVT.

2.3.2. ResultsSeveral flow rates were imposed in the laboratory sand tank A

to provide a range of Darcy fluxes. Six rates were tested with theDVT, from 5 to 30 cm.day�1 (Fig. 6). As shown in Fig. 6, measure-ments are repeatable and proportional to the flow rate imposedfor the six rates tested. However for the higher Darcy fluxesimposed, the measurements had a higher standard deviation withno simple explanation.

In the second sand tank, measured values by PVP were 23 ± 2.3and 30 ± 3 cm.day�1 (porosity = 0,40) for an imposed Darcy flux of16 and 26 cm.day�1, while the DVT velocities were 25.6 ± 1 and31 ± 6 cm.day�1 for the same imposed Darcy flux. This additionalcomparison generally supports measurements between PVP andDVT.

Fig. 5. Sand box used for DVT calibration test.

Fig. 6. DVT measurement in the sand tank for five imposed Darcy fluxes: q = 5, 10,15, 20, 25 and 30 cm.day�1. Error bars represent one standard deviation from themean of 3 measurements for each Darcy flux.

E. Essouayed et al. / Journal of Hydrology X 2 (2019) 100008 5

3. DVT field applications

3.1. Controlled flow in a natural aquifer: background

For validation, the DVT was deployed in a natural unconfinedaquifer for comparison with other velocity methods. This aquiferis localised in Bordeaux (France) on a Plio-Quaternary formationwith a thickness of 6 m. This formation is composed of variablemedium sands. The bottom of the aquifer is delimited by a clayaquitard and the water level was measured at 2.80 m belowground surface.

The test was conducted using several wells shown in Fig. 7. P1,P2, P4 and P5 are screened over a depth of 5–6 m with the bottomat the clay layer. The measurement point P3 is screened from 5 to5.60 m depth. Pumping in P5 and Injection in P1 was imposed inorder to have a controlled flow field. The flow rate for the con-trolled zone was 75 L.h�1, and was imposed continuously during

the three weeks period of testing (Tracer test, Borehole dilution test,PFM and DVT). P3 was used for measurements in the center of theimposed flow.

3.1.1. Tracer testFor the tracer test, a NaCl solution of 1 g.L�1 was used. Two tra-

cer tests pulses, 40 L and 20 L, were injected for different durations(5 and 2.50 min) for comparison. The saline solution was injectedat the bottom of the piezometer P2 at a flow rate of 8 L.min�1. Acontinuous low recirculation was set in P3 using a peristaltic pumpin order to homogenise the concentration and continuously mea-sure the electrical conductivity of the water (Consort Multiparam-eter C3010).

3.1.2. PFM setupThe PFM is a cylinder composed of granular activated carbon

(GAC) and provides both contaminant mass flux and Darcy fluxmeasurements. The PFM is inserted into a well and allows ground-water flux through the device. After an exposure time from 1 to4 weeks the PFM is removed and sampled for analysis. The PFMactivated carbon is initially impregnated with five alcohols andtheir loss ratio is proportional to groundwater flux and providesDarcy flux through calculation. The mass of all contaminant inter-cepted by the sorbent is also analysed and contaminant mass fluxis calculated. For this study only the resident alcohols are used forDarcy flux estimation.

A PFM with a length of 15 cm and diameter of 6,8 cm was con-structed for the field tests. The PFM parameters and design wereimplemented based on Hatfield et al. (2004). For the experimentaldesign, four alcohol were used, Methanol, Ethanol, Isopropyl alco-hol and n-Hexanol. To produce a 15 cm long PFM, more than 200 gof activated carbon was prepared. An initial sample of the activatedcarbon was taken in order to compare with samples exposed togroundwater flow. After PFM exposure, the relative mass is calcu-lated for Darcy flux estimation.

The PFM was inserted in P3 at 5.40 m depth for a period of6 days. All the GAC was recovered and mixed to homogenize thesamples. Initial and final GAC samples were analysed using GCMSwith a 5 sil-MS column. Isopropyl alcohol was analysed for this test

Fig. 7. Schematic view and picture of the controlled field test zone.

6 E. Essouayed et al. / Journal of Hydrology X 2 (2019) 100008

on four identical samples. The relative mass calculated with Iso-propyl alcohol was higher than 0.3 (mg.L�1/mg.L�1), which allowsuse of the simplified form for Darcy flux calculation (Annableet al., 2005):

qaquifer ¼ 1:67rhRd 1�Mrð Þ½ �=at ð5Þ

where qaquifer is Darcy flux [L T�1], r [L] is radius of the PFM cylinder,h is the water content in the PFM [dimensionless],Rd is the retarda-tion factor of the resident tracer used (isopropyl alcohol in this case)[dimensionless], Mr is relative mass of tracer remaining in the PFMsorbent [M], a the distortion coefficient and tis PFM time of expo-sure [T].

3.1.3. Borehole dilution testThe borehole dilution approach is based on a simple assump-

tion that the decreasing tracer concentration injected in the testinterval, is proportional to the apparent velocity and directly tothe Darcy flux. This method requires (i) a steady state flow, (ii) ahomogeneous mix between the tracer and the water tested, (iii)no density gradient induced by the tracer (Piccinini et al., 2016).

In the well, after homogeneous mixing, the tracer concentrationC decreases according to (Freeze and Cherry, 1979):

dCdt

¼ �AvaCW

ð6Þ

where A is the cross-section of the groundwater flow [L2],va theapparent velocity [L T�1] andW the dilution volume [L3]. The veloc-

ity in the aquifer is calculated using an appropriate a as the PFM orDVT. In this case, the conditions of the controlled field (sand) lead toa converging factor of 2 (Drost et al., 1968).

In general, the borehole dilution test is conducted within a sin-gle isolated section. Here, the piezometer used for the dilution test(P3) has a single screen with a length of 60 cm and 68 mm diame-ter. Therefore, the test satisfy this criterium. A NaCl solution wasused as a tracer with a concentration of 1 g.L�1 and an injected vol-ume of 500 mL (during 350 min) in order to increase the salinity inthe well from 50% to 200%. The solution was injected at low flowinto the screened section and a recirculation was imposed contin-uously during the test at a rate of 1 L.min�1 corresponding to 1screen-well bore volume per min (Lamontagne et al., 2002). Con-ductivity of the section was recorded with the Consort Multipa-rameter C3010.

3.2. Controlled flow in a natural aquifer: results

3.2.1. Tracer testThe results of the two tracer tests are presented in Fig. 8. For the

two tests, peaks were observed at 19 h after the injection. The dis-tance between P2 and P3 is 38.5 cm. The software TRAC was usedin order to estimate the porosity and Darcy flux (Gutierrez et al.,2013). The comparison between modelled and measured value isshown in Fig. 8 and Table 1. The presence of coarse sand and thecalibration suggest that the range of porosity may be about 20%(estimated by the software TRAC). Thus, the mean of the Darcy flux

Fig. 8. Tracer test results 1 and 2 show a peak 19 h after the tracer injection.Measured values are the dotted lines and the TRAC modelled are the continuouslines.

Table 2Parameters value for PFM in the controlled field.

Mr (�) Darcy flux (cm/day)

t [day] 6 Sample 1 0.62 12.76h [–] 0.55 Sample 2 0.68 10.74r [cm] 3.4 Sample 3 0.79 7.05Rd [–] 109 Sample 4 0.69 10.41

Mean 10.25

Time (min)

0 50 100 150 200 250 300

ln [

(C-C

b)/(C

0-C

b) ]

0

1

2

3

4

ln [ (C-Cb)/(C0-Cb) ] = 0,04 t

Fig. 9. Logarithm of the tracer ratio plotted with time. Slope is determined for zeroto 60 min.

DVT PFM Tracer test

Dar

cy fl

ux (c

m/d

)

0

5

10

15

20

25

Fig. 10. Velocity results in P3 well for DVT, PFM and the tracer test, errors bars areexplained in the text.

E. Essouayed et al. / Journal of Hydrology X 2 (2019) 100008 7

estimated is equal to 14.5 cm.day�1 with an estimated standarddeviation of 1.8 cm.day�1.

3.2.2. PFM resultsThe hydraulic conductivity of the PFM granular activated car-

bon was measured at 3.5 10�3 m.s�1 (Hatfield et al., 2004). Anaquifer pump test was conducted at the controlled field site. Usingthe Cooper-Jacob calculation, the aquifer K was estimated at 6.310�4 m.s�1. Eq. (3) permits the calculation of the a coefficient equalto 1.69. The real darcy flux mean calculated in the controlled envi-ronment was 10.25 cm.day�1 (see parameters value in Table 2).The standard deviation calculated from the four identical samplesis 0.97 cm.day�1. In an earlier field test, Annable et al. (2005),reported uncertainty of 25%. Using this value for the comparisonyields an error estimate of 2.5 cm.day�1 (Fig. 10).

3.2.3. Borehole dilution testThe borehole dilution test is considered completed when a

value lower than 25% of the initial added concentration is reached.This value is represented by the sharp change of slope observed inFig. 9. Prior to the break in slope, a continuous decrease is visiblefrom 0 to 60 min and is used in the calculation of Darcy flux. Acurve fit gave a slope of 0.04 min�1 equal to the value = 2va

pr . Theapparent velocity calculated was 290 cm.day�1. With an a of 2,the Darcy flux measured in P3 with the borehole dilution is esti-mated at 145 cm.day�1. Brouyère (2003) and Brouyère et al.(2008) defined Qin and Qcr in order to know the critical injectionflow rate of tracer where Qin is the tracer injection and Qcr the crit-ical flow rate. If Qcr < Qin, the dilution test is not valid and thetransit flow rate is cancelled (Jamin et al., 2015).

Qcr ¼ pQt ð7Þwhere Qt is the Darcy flux estimated related to the flow section. Inour case, the flow section corresponds to the screen length (60 cm)and the well diameter (6.8 cm). If we consider a Darcy flux equal to

Table 1Estimated value for the two tracer test using software TRAC.

Advective traveltime Vp (cm.day�1)

Porosity w Darcy flux q(cm.day�1)

Tracer test 1 – 40 L 63 0.21 13.24Tracer test 2 – 20 L 60 0.26 15.72

15 cm.day�1, Qcr is close to 1 L.h�1. For the borehole dilution test,500 mL of tracer was injected over 350 min, so the test is valid withregard to this point. The high results of darcy flux with boreholedilution test might be explained by tracer density effects in the well.

3.2.4. Direct velocity toolWith a permeability of 6.3�10�4 m.s�1 for the controlled field

site, the coefficient alpha is equal to 1.54 for the DVT. The meanDarcy flux calculated using the DVT was 17.8 ± 2.70 cm.day�1

(Fig. 10). The standard deviation was calculated using four identi-cal DVT tests.

Fig. 11. Map of the contaminated site with the four well used for the field comparison.

Darcy flux (cm/day)

2 4 6 8 10 12 14

Dep

th (m

)

5,5

6,0

6,5

7,0

7,5

8,0

WA1WA2WA3WA4

Fig. 12. Vertical distribution of Darcy flux for PFM results for WA1, WA2, WA3,WA4. Depth represents the distance from the surface.

8 E. Essouayed et al. / Journal of Hydrology X 2 (2019) 100008

3.3. Contaminated site test: background

The DVT and PFM were field-tested for comparison. The fieldsite selected presents a semi-confined alluvial aquifer from theQuaternary, composed of (i) heterogeneous sand deposits, fromcoarse to fine sand and (ii) local clay lenses. The water table wasmeasured 4 m below ground surface. A clay unit was located from7.50 to 9.50 m below surface and defined the bottom of the alluvialaquifer. A pumping test showed a low hydraulic conductivity withan average of 5 10�6 m.s�1. Four wells with the same characteris-tics (screened over a depth of 5–8 m and an 80 mm diameter) weretested at this site (WA1, WA2, WA3 and WA4 in Fig. 11).

3.4. Contaminated site test: results

At the contaminated study site, six PFMs each 1 m long wereused to provide a vertical distribution of Darcy flux. Two PFMswere inserted in each of the WA1 and WA4 wells, and one wasinserted in each of the WA2 and WA3 wells. In Fig. 12, PFM resultsare presented with the vertical distribution of Darcy flux for eachwell. The velocity ranges from 4 to 13 cm.day�1 with a highermean for WA1 (9 cm.day�1) and a lower for WA2 (5.3 cm.day�1).The piezometer WA1 had a high variation from 5.50 to 7.50 m witha higher velocity at a depth of 7 m.

DVT measurements were done only for one depth (6.50–7.00 mdeep) in order to test the repeatability. K of the zone was estimatedat 5�10�6 m.s�1 so DVT and PFM had respectively an a coefficientequal to 1.89 and 2. DVT and PFM data were compared at the samedepth (between 6.50 and 7.00 m). The DVT measured Darcy fluxvaried from 4 to 8 ± 1.0 cm.day�1 (standard deviation of 3 mea-surements at the same depth). At the field site, one sample for eachdepth was analysed, thus an uncertainty of 25% was consideredappropriate for these data (Annable et al., 2005). The PFM mea-sured Darcy flux from 5 to 9 ± 1.8 cm.day�1.

4. Summary and discussion

The Direct Velocity Tool described in this paper is an innovativesolution for groundwater measurement in wells. DVT allows mea-surement of the magnitude of groundwater velocity at the cen-timetre per day scale and can be done at multiple depths in asingle well. Measurements take 5 to 10 min for one measurementand the results are immediate (no chemical analysis or substantialcomputer calculations).

Contaminated sites are often located in heterogeneous zoneswith low velocity and K, thus this limit is not considered an issuefor many field applications. The first tests conducted in the labora-tory sand tank showed a linear response between Darcy fluximposed and the Darcy flux measured with a high correlation coef-ficient (R2 = 0.990). The DVT was tested for velocities ranging from5 to 30 cm.day�1 and a higher standard deviation was observed forthe higher velocities (±6.46 cm.day�1 for 30 cm.day�1). Also, PVPand DVT was compared on a second sand tank and both showedclose results.

In the controlled field, tracer test, borehole dilution, PFM andDVT were tested under the same conditions. Results are summa-rized in Table 3.

For the controlled field site, with an estimated K of 6.3�10�4 m.s�1, PFM, tracer test and DVT measured respectively velocitiesequal to 10.25 cm.day�1, 14.5 cm.day�1 and 17.8 cm.day�1. DVTand PFM have respectively an uncertainty of 2.70 cm.day�1 and2.50 cm.day�1, which is close to the one given by the tracer test(1.8 cm.day�1). The Borehole Dilution test overestimated the veloc-ity by an order of magnitude (145 cm.day�1 vs 10–15 cm.day�1).Labaky et al. (2009), presented a velocity measurement compar-ison between PVP and other tools, and the borehole dilution resultsshowed a similar overestimation of the velocity. This gap is proba-bly due to (i) the density flow induced during the injection of thetracer and (ii) a inhomogeneous mixture between the tracer and

Table 3Velocity measurement results in the controlled field and contaminated sites with the different tools used for comparison.

Controlled field Real site

Tracer test BHD PFM DVT PFM DVT

q measured cm.day�1 14.5 145 10.25 17.8 5–9 4–8Uncertainties cm.day�1 ±1.8 Unknown ±2.5 ±2.7 ±1,8 ±1.0

E. Essouayed et al. / Journal of Hydrology X 2 (2019) 100008 9

groundwater, the use of a peristaltic pump might not have pro-vided sufficient mixing.

For the contaminated field site, with an estimated hydraulicconductivity of 5�10�6 m.s�1, PFM measured velocities of 5–9 cm.-day�1. DVT measured velocities of 4–8 cm.day�1. The uncertaintiesof DVT and PFM velocity measurements were 1.0 and 1.8 cm.day�1

respectively. Velocity measurement showed similar results in thecontrolled field test with PFM, DVT and Tracer tests and at the con-taminated site with PFM and DVT (Table 3).

Although the measurement range tested covers a portion of thevelocities expected under field site conditions (Brooks et al., 2008),measurements will be conducted at other sites to evaluate anextend the Darcy flux range and thus the capabilities of the DVTtechnique. Measurement at other sites will evaluate the robustnessof the DVT and its application under different conditions. DVT dis-tortion simulation improved the definition of the limitations of thetool. For an aquifer with a high hydraulic conductivity (more than2.10�3 m.s�1) uncertainties will be greater. Further research isneeded to evaluate the inlet system head loss in order to adaptthe DVT for a higher range of Darcy flux measurement. Addition-ally, measurement using an alternate tracer could be a solutionto avoid potential density effects in the well. The DVT presentsan interesting solution for Darcy flux measurement in a well screenand can potentially provide a vertical distribution of horizontalDarcy flux, which is an important characteristic for better contam-inant site management.

Conflict of interest

The authors declare no conflict of interest.

Acknowledgements

TOTAL R&D can be thanked for their availability to performvelocity measurements at PERL facilities with Point Velocity Probe,developed by Prof. J.F Devlin from Kansas University. This workwas supported by INNOVASOL and Bordeaux INP ENSEGID, inBordeaux, France.

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.hydroa.2018.100008.

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