ORIGINAL PAPER

CCD camera image analysis for mapping soluteconcentrations in saturated porous media

Stefanie Jaeger & Markus Ehni & Christina Eberhardt &Massimo Rolle & Peter Grathwohl & Guenter Gauglitz

Received: 19 May 2009 /Revised: 9 July 2009 /Accepted: 13 July 2009 /Published online: 4 August 2009# Springer-Verlag 2009

Abstract This paper presents an optical approach, basedon the use of a low-cost charge-coupled device (CCD)camera, for the quantitative determination of solute con-centrations in saturated porous media. The method isapplied to evaluate tracer experiments carried out in alaboratory model tank. The CCD photos deliver RGBvalues which are transferred into concentrations for theevaluation of vertical concentration profiles over the wholetank area. A specially developed evaluation procedure,including internal referencing for noise reduction, considersthe colour of the adjacencies of the evaluated spots andscattering effects. The CCD data evaluation technique isaccompanied by conventional sampling and absorptionmeasurements and by numerical flow and transport simu-lations. This non-invasive technique allows a directmapping of the concentration distribution without anydisturbance of the solute plume. Therefore, it turns out tobe an important tool for a detailed investigation offundamental processes (e.g. transverse dispersion) deter-mining the solute (e.g. contaminant) transport in porousmedia.

Keywords Optical sensor . Imaging . Porous media . Colourtracer . Numerical modelling

Introduction

Understanding the fate and transport of contaminants ingroundwater systems is a significant challenge and hasbecome an active field of applied environmental research.Complex physical, biological and geochemical processes,such as advection, dispersion, volatilization, sorption,chemical and biological reactions determine the fate ofcontaminants in the subsurface. The difficulty to obtaindetailed and comprehensive data sets directly in the fieldhas led to the development of well-controlled laboratorybench-scale experiments as a useful tool to investigatefundamental processes. In particular, flow-through tankexperiments have been extensively used since the 1960s tostudy solute transport in porous media [1–3]. Recently,these experimental set-ups have been developed to studymore complex systems including mixing-controlled abioticand biological reactions [4–8].

Image analysis techniques represent an attractive tool fora detailed monitoring of laboratory experiments at a veryhigh spatial and temporal resolution. These techniques arenon-invasive and allow mapping concentration distributionof colour tracers without disturbing the plume dynamics.The two mainly used techniques are laser-induced fluores-cence and absorption imaging. Oates et al. [9] investigateda colorimetric reaction by using a digital imaging techniquebased on light absorbance in order to study mixingprocesses. They developed a method to relate lightabsorbance to product concentrations by measuring sixknown mixtures of both educts. Schincariol et al. [10]carried out flow-tank experiments with a dense NaClplume coloured with rhodamine and recorded black andwhite photographs of the entire tank. After scanning thenegatives, the pixels, representing dye intensities, weretransformed into concentrations. However, the scanned

S. Jaeger (*) :M. Ehni :G. GauglitzInstitute of Physical and Theoretical Chemistry,University of Tuebingen,Auf der Morgenstelle 18,72076 Tuebingen, Germanye-mail: stefanie.jaeger@ipc.uni-tuebingen.de

C. Eberhardt :M. Rolle : P. GrathwohlCentre for Applied Geoscience, University of Tuebingen,Sigwartstr 10,72076 Tuebingen, Germany

Anal Bioanal Chem (2009) 395:1867–1876DOI 10.1007/s00216-009-2978-3

images had to be corrected via computer processingregarding light non-uniformity, heterogeneous media,different grain sizes and therefore, different dye intensi-ties. Hence, this entirely manual procedure could only berealistically applied for simple compositions of porousmedia. Successively, McNeil et al. [11] concentrated onthe investigations of heterogeneous porous media andtheir impact on the dye intensity. Their tank module wasilluminated by four halogen lights and image positiveswere scanned with a cooled camera. The intensity of thegreen colour channel was examined due to its highsensitivity towards the dye. As the authors only used onecolour channel, the determined concentration profilesshowed an improvable signal-to-noise ratio, when com-pared to techniques using more than one colour channel.The approach of Huang et al. [12] was based on animaging technique in conjunction with a fluorescent dyetracer. The tracer emitted visible light under UV illumina-tion and the intensities were recorded with a charge-coupled device (CCD) camera. The use of UV lightreduced the image noise. The backside illumination of theset-up resulted in light dispersion errors caused by theglass beads. The authors proposed to use a fluidreflectively matched to the porous media to correct forthese errors. Bridge et al. [13] used UV-excited fluorescentsolute and colloid tracers examined by reflective UVfluorescence imaging. Both, light source and CCDdetector were placed in front of a flow cell chamber filledwith quartz sand. The same technique was used by Rees etal. [14] to quantify transport and biodegradation processesin flow reactors containing quartz sand colonised with abiofilm. Gramling et al. [15] also introduced a CCD-basedapproach monitoring a reactive colour tracer in porousmedia. To avoid a complex calibration in sand or glassbeads, they used Na3AlF6 as porous medium whoserefractive index is very similar to that of water. Zinn etal. [16] examined solute transport in different grain-sizedglass beads by measuring light transmission intensities. Toequalise intensity differences between small and big grainsize areas, they used a filter with circular holes whichblocked the light entering the large beads while notreducing the light transmission through the small glassbeads. In their complex set-up, the light transmissionmeasurement required an expensive liquid-cooled CCDcamera and diverse lens filters.

In this study, we present an image analysis techniqueapplied to the evaluation of the behaviour of a conservativetracer plume in a saturated porous medium. In particular,the attention is focused on the high-resolution monitoringof solute vertical transverse dispersion. This parameter is ofutmost importance for contaminant transport in groundwa-ter, since it controls the extent of mixing and mixing-controlled reactions at the narrow fringes of contaminant

plumes, therefore determining the plume length. Theapproach of the developed image analysis technique isbased on RGB values instead of intensities as done in[9–15]. Consequently, the simple CCD camera set-up doesnot depend on expensive optical devices such as used in[9–16]. The laboratory set-up includes a light foil forhomogeneous illumination from the reverse of the tank,reducing problems such as light reflection. The CCDapproach, which can be understood as simple opticalsensor, provides a tool which allows to quantify theconcentration of the colour tracer at each area of interestwithout disturbing the flow through the porous medium.The use of RGB values offers the opportunity of internalreferencing for noise reduction. Scattering effects andchanging background colour are considered by a self-developed evaluation routine. This image analysis tech-nique allows a direct mapping of the concentrationdistribution inside the tanks, therefore complementing andimproving the information provided by absorption measure-ments at the outlet ports.

Experimental

Set-up

The experiments are performed in a quasi two-dimensionalflow-through tank with inner dimensions of 77.9 cm×15 cm×1.1 cm (Fig. 1). The tank is equipped with 11 portsat the inlet and at the outlet. Two peristaltic pumps (IPC-N12, Ismatec, Glattbrugg, Switzerland), operating with thesame flow rates, control the flow through the tank andestablish constant flow conditions, resulting in a steady-state plume. The pumps are connected with the portsthrough tygon tubes with an inner diameter of 0.57 mm andstainless steel capillaries with an inner diameter of0.75 mm. In front of the tank, a low-cost non-cooledDSLR CCD camera is fixed on a guide rail and can bemoved along the tank at a fixed distance from the tank wall.The CCD camera is a DSLR Olympus E-1 with a resolutionof 5 megapixels. As a lens, an Olympus Zuiko ED 50 mm/2.0 Macro is used. The camera is controlled by a PC usingthe Olympus Studio Software.

The tank is irradiated from the reverse with the help ofa 109 cm×25 cm light foil (EL-Technik, Leingarten,Germany) delivering homogeneous light over the wholetank area. Glass beads of uniform grain size (0.25–0.3 mm, Sartorius AG, Gottingen, Germany) are selectedas porous medium in order to allow a good-qualityillumination of the flow-through system. The tank is filledup to a height of about 14 cm and a thin unsaturated zoneis maintained above the water table in order to avoid anyshortcut of the tracer.

1868 S. Jaeger et al.

A conservative colour tracer is continuously injectedthrough the central (sixth) inlet port while bi-distilled wateris injected from the surrounding inlet ports. The red azo dyeNew Coccine (also called Acid Red 18 or E 124) is chosenas conservative solute and purchased from Sigma-Aldrich,Seelze, Germany (dye content 75%). This chemical isparticularly suitable for tracer tests since it is soluble inwater, chemically stable and photostable and does not reactwith the porous medium. As the CCD camera records theRGB values and therefore characterises the absorption ofthe dye, the tracer must not show any fluorescence and onlylittle scattering. Previous experiments were carried outusing the commonly used groundwater tracer fluoresceinwhich has the advantage of a very low detection limit influorescence spectroscopy. However, the results proved thatthe fluorescence influences the RGB values mainly byreabsorption effects. For this reason, in the experimentspresented in this study, New Coccine is selected as a colourtracer despite the higher LOD in the absorption measure-ments and injected in the flow-through system at aconcentration of 75 mg L−1. The established groundwaterlinear velocity in the tank (1.1 m day−1) is in the typicalrange of seepage velocities in unconsolidated sandy sedi-ments [17, 18] and determines a travel time of the solute inthe tank of 17.5 h. After the tracer plume has reached thesteady state (i.e. exchange of two pore volumes), absorptionsamples are taken at all outlet ports. After adequatedilution, they are measured at the maximum absorptionpeak of λmax=510 nm using a UV/Vis spectrophotometer(Varian Cary 50 Bio). The detection limit for New Coccineis 0.02 mg L−1.

Photos of the tracer plume are taken with the CCDcamera using an aperture of 22 and an exposure time of13 s. The format of the photos is an uncompressed Tiff andall camera-integrated noise filters are disabled. In order to

get a photo of the entire tank, it is necessary to take fivepictures and combine them with Adobe Photoshop CS2.The resulting pictures have all the same size of 8,842×1,599 pixels. The evaluation of the colour values is donewith a specially self-developed automated evaluationprocedure programmed in LabVIEW 8.0.

The RGB values of the CCD photos can be translatedinto colour tracer concentrations by a computer programme.The information from the CCD data is, successively, usedas input for the numerical simulations.

Referencing and calibration

The CCD camera delivers RGB values. In the special caseof the red azo dye New Coccine as colour tracer, the redcolour channel does not change with changing tracerconcentrations. In contrast, the blue colour channel carriesthe most important colour information with the highestchanges. The green colour channel carries similar informa-tion but because of an earlier saturation of the green colourvalue, it cannot cover the same concentration range as theblue channel. By referencing the blue colour value to thered colour value it is possible to improve the signal/noiseratio considerably (Fig. 2). The red colour value is used asan internal reference. For the quantification of the concen-tration, the B/R values are evaluated.

Before starting the plume experiments, it is necessaryto do a calibration for the tank. There are two reasonswhy a calibration is required. Firstly, there is not a linearbut an exponential relationship between the colour valuesB/R and the concentration of the tracer due to theLambert–Beer law. Secondly, the glass beads themselvesshow a colour effect due to the packing of the porousmedium. Some of the glass beads are not 100%transparent. That means that, even in a tank without a

Fig. 1 Scheme of the tank set-up. The CCD camera is fixed ona guide rail in front of the tank.From the reverse, the tank isilluminated by a light foil. Theflow is controlled by peristalticpumps at the inlet and outletports. Absorption samples aretaken at the outlet ports. Thecolour tracer is injected inthe central port

CCD camera image analysis for mapping solute concentrations 1869

plume, there are some areas that seem to be greyer thanothers. As a consequence, it is not possible to affirm thatone colour value corresponds to one concentration in theentire tank. That is why the calibration functions varyfrom one evaluated area to another. With a singlecalibration function for the whole tank, the error wouldbe very large, especially for high New Coccine concen-trations representing the interesting part for the plumeevaluation.

A commonly used calibration method which is basedon the injection of different colour tracer concentrationsin the entire tank proved not to deliver the expectedresults. There are several different factors influencing thecolour values such as the individual packing of the glassbeads and resulting colour effects, the concentration ofthe tracer, the plume width and the distance of theevaluated areas from the colourless glass beads in theadjacencies which scatter the light of the light foil intothe colour plume. A thin plume is more affected by thelarge number of colourless and scattering glass beads inthe surrounding area than a broad plume. Figure 3illustrates the effect of different plume widths on thecolour value of a single spot. The colour value changesconsiderably, especially for thin plumes. When injecting acolour tracer into the middle port, the camera will not beable to detect the same colour value as will be observed byinjecting the same concentration into all ports because ofthe amount of light which is scattered into the colour ofthe New Coccine plume by the adjacent colourless glassbeads. The determined concentrations are thereforealways calculated too low. A calibration which is basedon the injection of different New Coccine concentrationsthrough all injection ports cannot consider these scatter-ing effects. Therefore, a special calibration procedure isnecessary. The approach is based on several tracerexperiments performed by injecting the tracer from adifferent number of inlet ports. In this way, differentplume widths are established in the flow-through system

and the dependence of the scattering effect on the plumewidth can be investigated. As this effect also depends onthe concentration of New Coccine, the experiments werecarried out for concentrations of 0.5, 1, 2.5, 5, 10, 20,50, 75 and 100 mg L−1.

At the height of the middle port, 5 cm after the inlet port,an area with a size of 11×11 pixels is chosen and itsreferenced colour value is evaluated. Here, the expectedconcentration is still equal to the injected one. Additionally,the colour value of the surrounding is investigated bycalculating the mean colour value in a radius of 250 pixels(≈2.2 cm) around the chosen area. This radius is chosenbecause experimental data proved that the scattering doesnot play any role once the distance between the evaluatedpixels and the nearest colourless glass beads is more than250 pixels. The distance-dependent intensity of the scatter-ing light attenuates exponentially [19]. For each concentra-tion, the referenced colour value of the small square area is

Fig. 3 Influence of the plume width on the referenced colour value B/R of a single spot (at the height of port 6, 2 cm from the tank inletboundary)

Fig. 2 Reduction of signal/noise-ratio by referencing; left unreferenced blue colour channel, right referenced colour profile of B/R values

1870 S. Jaeger et al.

then plotted versus the mean referenced colour value of thecircle. The plot shows a linear dependence for all concen-trations proving an explicit dependence of both evaluatedcolour values. With the help of the linear regression data, atable is created containing all possible colour values for the11×11 pixel spots for all circle colour values between1.0000 and 0.0000 in steps of 0.0001. This detailedevaluation is necessary because of the exponential relation-ship between colour values and concentrations which cancause a big difference in the concentration despite a verysmall difference in the colour.

In a next step, the B/Rspot values of one possible B/Rcircle

value are taken and graphed against the correspondingconcentrations. This plot shows the expected exponentialdependence according to the Lambert–Beer Law. With thehelp of the exponential fits for all B/Rcircle values, the self-developed LabVIEW routine is now able to calculate theconcentration of a B/Rspot value found in the tracerexperiment with its corresponding B/Rcircle value of theadjacent area. As each colour channel delivers 256 colour

values, concentrations covering a range of two orders ofmagnitude can be determined.

All these steps are performed using a self-developed, fullyautomated evaluation programme based on LabView. Thecalibration procedure is schematically illustrated in Fig. 4.

Based on this calibration procedure, every area ofinterest can be evaluated subsequently, without definingan experimental pattern or fixed spot size. The calibrationdata are valid for every newly packed tank with the samefilling medium. This is a great advantage compared to thecommonly used calibration method which had to berepeated for every new tank filling.

The plume experiments are assessed by evaluatingdifferent vertical profiles. In each profile, the size of oneevaluated area is 11×11 pixels. The distance between theevaluated spots is 25 pixels. The evaluation softwarecalculates the mean B/R value of those 11×11 pixels areasand the standard deviation. With the evaluation routine,those B/R values can then be transformed into concen-trations with the help of the calibration set.

Fig. 4 Schematic illustration of the calibration procedure

CCD camera image analysis for mapping solute concentrations 1871

Solute transport

Transport of a conservative solute in a three-dimensionalporous medium can be described by the advection-dispersion equation:

@c

@t¼ @

@xiDij

@c

@xj

� �� @

@xivicð Þ with i; j ¼ 1; 2; 3 ð1Þ

where c is the solute concentration, t is the time, xi and xjare the spatial Cartesian coordinates, Dij is the hydrody-namic dispersion coefficient tensor and vi represents thelinear average velocity in the xi direction.

For a continuous injection from a line source in a two-dimensional uniform flow field, under steady-state con-ditions, Domenico and Palciauskas [20] provided thefollowing analytical solution:

c

c0¼ 1

2erf

x2 þ w=2ð Þ2 x1DT=vð Þ1=2

" #� erf

x2 � w=2ð Þ2 x1DT=vð Þ1=2

" #( )

ð2Þwhere x1 and x2 are the Cartesian coordinates in thelongitudinal and transverse direction, respectively. The flow

velocity v is oriented in the longitudinal direction, DT

represents the transverse hydrodynamic dispersion coeffi-cient, c0 is the source concentration and w is the width ofthe source.

In the quasi two-dimensional experimental set-up, thesimplifying assumption of uniform parallel flow could beconsidered valid (except in the vicinity of the inlet andoutlet ports) and Eq. 2 is used to evaluate the experimentaldata. In order to determine the transverse dispersioncoefficient, a fitting procedure, based on the minimizationof the squared residuals, is applied to the CCD camera dataevaluated at different vertical profiles.

Numerical simulations

The image analysis and the measurement of tracer concen-trations at the tank outlet are accompanied by numerical

Fig. 5 Tank with a New Coccine plume under constant flow conditions. New Coccine is injected in port 6 with a concentration of 75 mg L−1. Themarked five profiles are evaluated and quantified. Below, the concentration profile of the adsorption measurements at the sampling ports is shown

Fig. 6 Height of evaluated areas in the tank plotted against referencedcolour values B/R for the five vertical profiles shown in Fig. 5 whichare, a profile 1, located 5 cm from tank start, b profile 2, 19.4 cm fromtank start, c profile 3, 38.75 cm from tank start, d profile 4, at a lengthof 58.1 cm, and e profile 5, located 3 cm from tank end. On the rightside, the concentration profiles of the CCD are compared to the modelprofiles

b

1872 S. Jaeger et al.

CCD camera image analysis for mapping solute concentrations 1873

flow and transport simulations. The model domain isdiscretised into a spatially variable grid of 190 columnsand 187 layers. The steady-state flow field is simulated withthe finite difference code MODFLOW [21], using fluxboundary conditions corresponding to the inlet and outletwater fluxes injected and extracted by the peristaltic pumps.The hydraulic conductivity of the porous mediumK ¼ 6:14� 10�4ms�1ð Þ is determined with permeametertests and the porosity (n=0.42) is calculated from the specificdischarge and the mean arrival time of the tracer at thecentral outlet port of the tank.

The transport code MT3DMS [22] is selected to simulatethe conservative tracer experiments. Dispersion is describedaccording to the standard linear model for saturated porousmedia [23]. Therefore, the transverse dispersivity iscalculated as:

aT ¼ DT � DP

vð3Þ

where v is the linear average velocity in the longitudinaldirection, DT is the hydrodynamic transverse dispersioncoefficient determined from the fitting of the experimentaldata provided by the CCD camera image evaluationtechnique. DP represents the effective pore diffusioncoefficient that can be defined, after Grathwohl [24], asDP=Daqn, where Daq is the aqueous diffusion coefficient ofNew Coccine (Daq ¼ 3:6� 10�10m2 s�1at T=20 °C) andn [−] is the porosity.

Results and discussion

After establishing a steady-state plume in the flow-throughsystems, five vertical profiles have been evaluated. Asshown in Fig. 5, these profiles are distributed over theentire tank length at a distance from the inlet of 5, 19.4,38.75, 58.1 and 75 cm, respectively. Each profile is

composed of 51 spots, with a size of 11×11 pixels, spacedat 25 pixels. In the same figure, duplicate adsorptionmeasurements from sampling at the outlet ports are plottedas normalised New Coccine concentration.

As the calibration data (depending on the different plumewidths and concentrations) cannot be valid for a concen-tration of 0 mg L−1, a photo of a tank not containing anycolour tracer plume is used to determine a thresholdconcentration to correct for the background. In this photo,a large number of spots are evaluated with the evaluationroutine and the same calibration data set, and the highestcalculated concentration is defined as threshold value. Allconcentration values below that concentration are set tozero. This concentration limit is 1 mg L−1.

The results for all five profiles are shown in Fig. 6. Onthe left side, the position of the evaluated areas,corresponding to the height in the tank, is plotted againstthe referenced colour value B/R including the standarddeviations. In addition, the position of the inlet or outletports is marked. On the right-hand side, the colour valuesare translated into concentration profiles and plotted asnormalised concentrations with respect to the injected value(c0) of 75 mg L−1. The CCD data are compared to theresults of the numerical simulations.

Profiles located directly at the inlet or outlet ports cannotbe evaluated. The tank wall causes a disturbance of thecolour values, therefore the profiles would not be represen-tative for the tank volume.

By injecting New Coccine into port 6, the colour tracersource can be assumed as a linear source as describedabove. At a distance of 5 cm from the inlet ports, thisinjection results in the colour distribution shown in Fig. 6a.This profile is quite thin but not symmetric in reference tothe injection port 6. The colour plume moves upwards closeto the inlet boundary of the tank. This effect was observedin comparable tank experiments [7] and is a result of thespecific flow field in the tank, as confirmed by flow andparticle tracking numerical simulations. As the concentra-

Fig. 6 (continued)

1874 S. Jaeger et al.

tion values behave conversely to the referenced colourvalues, the maximum concentration of profile 1 corre-sponds to the lowest colour value of 0.2203. Due to theexponential relationship between colour values andconcentration, a small error in the lower region of thecolour values can cause a considerably high error in theconcentration value. The results of the numerical mod-elling predict that the maximum concentration has notyet changed compared to the injected one. The CCD datashow a higher concentration as injected, but thedeviation is in the range of the standard deviations givenby the colour value evaluation, which is 5% at themaximum of the profile.

Over the first 19.4 cm into the tank, the profile hasalready changed. Profile 2 is therefore wider (Fig. 6b).However, the colour value of the profile’s maximum doesnot show a significant difference compared to profile 1. TheB/R value is 0.2266. The maximum relative concentrationdetermined by the CCD data evaluation and predicted bythe model is 0.95. Additionally, the upward movement ofthe plume can be observed in both profiles, since themaximum is now located between ports 6 and 7.

The following profiles (3 and 4) show a similarbehaviour with progressively decreasing normalised con-centrations due to the effect of transverse hydrodynamicdispersion. The maximum values determined from the CCDdata are 0.83 and 0.72, respectively (Fig. 6c and d).

The fifth and last profile is located 3 cm from the outletports (Fig. 6e) and can be compared with the absorptionmeasurements after conventional sampling. At this crosssection, for the first time, the referenced blue colour valueis clearly higher than that of the previous profile. A B/Rvalue of 0.3085 can be observed. Due to the impact of thepumping at the outlet ports, the plume is drifted back to thetank centre. Its maximum returns to nearly the height ofport 6. The pumping at the outlet ports also causes flowfocusing so that the colour tracer plume again gets thinnercompared to profile 4. This flow focusing together with theslightly different locations can explain the difference of themaximum normalised concentration at profile 5 (0.60) andthe absorption value measured at the central outlet port(0.52).

The change in the B/R values is considerably smallerthan the change in concentration. In particular, for profiles 1to 4, the colour values are between 0.2203 and 0.2591, witha relative difference of 15%. The difference of normalisedconcentration is >28%. This proves that the appropriateconcentrations can only be determined by considering thebackground colour and scattering.

The CCD data provide the colour distribution in the tankand, after the evaluation procedure, the tracer concentration.The plots of Fig. 6 also show that the CCD method resultsonly in very small standard deviations in spite of the

difficulties associated with the light behaviour (e.g. scatter-ing, etc.) in the glass beads porous medium.

To allow a direct comparison of the five evaluated profiles,Fig. 7 shows the results of the CCD data as line plots. Theordinate shows again the height and position of the evaluatedareas in the tank. The x-axis represents the normalised NewCoccine concentrations. Additionally, it can be observed theupward shift of the plume after leaving the inlet ports. Closeto the outlet, the plume moves back towards the tank centreas shown by profile 5. The plume behaviour could also becaptured by the numerical simulations reproducing the flowfield and the solute transport in the experimental tank. Asdiscussed above, the most sensitive input parameters of themodel are obtained experimentally. Of particular relevance isthe determination of the transverse dispersion coefficient,which is based on the CCD data evaluated at the differentvertical profiles. An average value of DT ¼ 4:04�10�10m2 s�1 is calculated and the corresponding transversedispersivity, aT ¼ 1:85� 10�5m, calculated according toEq. 3 is used in the model. A good agreement is obtainedbetween the model and the CCD data profiles, both showinga characteristic Gaussian distribution, as expected for fluidparticles travelling through an isotropic and homogeneousporous medium [25, 26]. The deviations between observed(CCD data) and simulated results are in the range of thestandard deviations of the colour value determination.

Conclusions and outlook

In this study, we presented an image evaluation technique,based on the data of a CCD camera, and its application tolaboratory flow-through experiments in saturated porousmedia.

The method, including the developed evaluation proce-dure, offers the opportunity of directly mapping solute

Fig. 7 The referenced colour values are transformed into concen-trations with the help of the calibration data. This plot shows theconcentration profiles of the five vertical profiles in direct comparison

CCD camera image analysis for mapping solute concentrations 1875

concentrations in the entire tank. Therefore, it results inconsiderable advantages compared to the standard analyt-ical measurements at the outlet ports. In the specific case ofour experimental study, only 11 ports are present and can besampled, whereas the CCD data can be taken at asignificantly finer resolution.

The method includes internal referencing to reduce thenoise and a specially developed calibration and evaluationprocedure to take into account scattering effects andbackground information.

Consequently, the CCD approach represents a low-cost,simple and valuable method to provide a large amount of datanecessary for high-resolution investigation of solute transportin porous media. It also contributes to the improvement ofnumerical models which can be validated with extensivedatasets and successively adapted to describe transportprocesses at a larger scale (i.e. field investigations).

This simple, optical sensor method can also be used toinvestigate contaminant distribution in heterogeneous porousmedia that can be reproduced in laboratory set-up similar tothe one of this study, by using glass beads of different sizes. Insuch a way, tracer experiments could be conducted in a systemcharacterised by zones with different permeability. The CCDmethod can also be adopted to examine colour tracer plumeunder transient flow conditions (e.g. time-dependent distribu-tion of contaminants in the subsurface following strong rain ordryness periods). Typically, transient plumes cannot beinvestigated with the commonly used methods based onsampling and absorption or fluorescence spectroscopy.

Based on these investigations, interesting sampling areascan be found which can be examined not only in transparentporous media but also in realistic media such as quartz sand,using sensors for detection of real pollutants. Such sensors, e.g. chemical sensors based on polydimethylsiloxane, havealready been developed for use in reflectometric interferencespectroscopy for detection of volatile organic compounds andchlorinated hydrocarbons in water [27, 28], and represent anadequate instrument for the non-perturbing detection oforganic pollutants in contaminated soil.

Acknowledgment This work was funded by the DeutscheForschungsgemeinschaft DFG within the research group “Reactionin Porous Media” (FOR 525).

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