the waste arising from > 60 years of 
 civil and military nuclear 
 operations around the world contains long - lived radionuclides that 
 must be contained and isolated from future populations . deep geological 
 disposal facilities ( gdf ) proposed by us and european waste management 
 organisations employ an engineered multibarrier approach ( figure 1 ) to retard the release of radioactive species from the waste 
 in quantities that could be detrimental to life and the environment . the multibarrier design concept typically combines reducing conditions 
 with high ph with the purpose of limiting the solubility and mobility 
 of radionuclide species within gdf if ( or when ) primary containment 
 fails . general design features of the multibarrier 
 geological disposal 
 facility ( gdf ) concept proposed for the long - term ( 10 year ) storage of high - level nuclear wastes in the deep subsurface 
 ( bgl = below ground level ) . a significant obstacle to implementation of gdf 
 is public and political 
 concern around risks and consequences of failure against design criteria 
 over the 10 to 10 year required lifespan of 
 the facility , highlighted by several failures 
 to site gdf repositories , e.g. , in the uk and 
 at yucca mountain in the usa . should gdf 
 performance be compromised , it is possible that long - lived , mobile 
 radionuclides will be transported through the engineered backfill 
 into groundwater and pose a long - term hazard to the biosphere and 
 water resources . thus , it is critical to the safety case for the gdf 
 not only to be able to demonstrate that the design performance is 
 well understood but to show that conditions arising from design failures 
 are also accounted for and mitigated as far as possible . one 
 potentially problematic radionuclide is technetium-99 , a high - yield 
 fission product of u , which has a long half - life ( 2.1 
  10 years ) and high solubility in oxic conditions 
 as the pertechnetate anion [ tc(vii)o4 ] . while the conditions within the gdf are expected to be reducing , 
 such that insoluble tc(iv ) should be the dominant oxidation state , 
 the uk nuclear authority estimates suggest that a significant proportion 
 of the uk tc inventory is expected to be present as the 
 tc(vii ) pertechnetate species . performance 
 assessment analysis of tc mobility has shown that the potential risk 
 to future populations from tc critically depends upon 
 its oxidation state , such that tc(vii ) 
 presents a significantly greater risk than tc(iv ) over the one million 
 year lifetime of the gdf , even when reducing conditions are applied . therefore , a robust design for the engineered barrier concept should 
 be able to account for the risk arising from the presence and behavior 
 of the mobile tc(vii ) , separately from the specific probability of 
 oxic conditions occurring or persisting within any given gdf scenario . an improved understanding of the behavior of pertechnetate in proposed 
 barrier materials is also necessary to evaluate the potential of different 
 design specifications to mitigate or remove the potential hazard . understanding the spatial and temporal dynamics of geochemistry 
 within and surrounding a gdf is essential in this task . the importance 
 of ( bio)geochemical gradients on radionuclide mobility is the focus 
 of substantial current research , e.g. , refs ( 911 ) . such studies ideally require noninvasive , nondestructive 
 measurement of the distribution , migration , and chemical transformation 
 of radionuclides within a physical model of the barrier material . this should be considered over time as internal conditions respond 
 to controlled changes in boundary conditions . quantitative imaging 
 techniques offer a means of achieving this 
 information and have been developed to study reactive transport in 
 porous media for a range of materials . imaging techniques include 
 visible light transmission and fluorescence imaging , nuclear magnetic resonance ( nmr ) , and x - ray computed tomography 
 ( see ref ( 14 ) for a 
 recent review ) . gamma attenuation techniques with external americium-241 
 or cesium-137 sources have been used to determine fluid transported 
 within a column . a key methodological 
 step remains : the extraction of quantitative geochemical information 
 from image data , particularly in three dimensions and opaque materials . techniques have been developed to quantify , 
 from image data , ph and oxygen gradients in two dimensions ( 2d ) within 
 porous media and to extract transport , 
 deposition , and remobilization rate parameters from time - lapse image 
 sequences of colloidal particles in translucent quartz sand . recent work demonstrated that gamma - emitting radioisotopes can be 
 used as an effective imaging tracer within opaque sediment and mineral 
 systems , both in static batch experiments and in flow - through columns . these 
 studies utilized ultratrace concentrations of a gamma - emitting technetium 
 isotope , technetium-99 m ( commonly used in medical and industrial imaging 
 applications ) , to demonstrate qualitatively the immobilization of 
 technetium on fe(ii)-bearing sediments and minerals , via an fe(ii)-mediated 
 reduction of tc(vii ) to tc(iv ) . in this study , we report the 
 use of 2d gamma - imaging to quantify tc transport parameters 
 in a simple granular porous media 
 model . uniform , saturated one - dimensional flow through ottawa quartz 
 sand , as a model test material , is used to demonstrate the ability 
 of gamma imaging to obtain reproducible data sets at the mesoscale 
 ( millimeters to decimeters ) , which can be used to yield transport 
 parameters by fitting standard convection  dispersion models . furthermore , we apply this methodology , for the first time , to investigate 
 the feasibility of direct noninvasive quantification of radionuclide 
 migration within opaque cementitious gdf candidate material ( crushed 
 nirex reference vault backfill ( nrvb ) ) under circum - neutral and alkaline ph . the technique represents 
 a base for development of model systems for noninvasive study of radionuclide 
 migration in complex physicochemical environments , critical to establishing 
 the design specifications and safety case for future gdfs , and also 
 for application to other contaminant transport in the subsurface . replicated flow cells enabled aqueous solutions with 
 and without 
 a tc tracer to be pumped through saturated quartz sand 
 at a steady flow rate with continuous monitoring using a gamma camera . tc is a controlled radioactive substance ; therefore , experiments 
 were performed with appropriate risk assessment in specialist facilities 
 at a hospital which routinely produces and handles the material for 
 use in clinical nuclear medicine . all other chemicals used were obtained 
 from fisher scientific ( uk ) unless otherwise indicated . bench - scale flow cells were 
 constructed of two perspex plates separated by viton seals and bolted 
 tightly together ( figure 2 ) . the rear plate 
 was solid , while the front plate had an indent creating a void space 
 to hold the porous material . an upper port in the rear plate allowed 
 for input of the aqueous phase to the cell , while a lower port in 
 the same plate allowed for removal of the aqueous phase for control 
 of flow rate and sampling . flow was from top to bottom along a distance 
 of 70 mm between the ports . tc tracer was injected through 
 a needle immediately below the inlet port ( figure 2 ) . schematic of the experimental setup showing construction of the 
 flow cells , the pumping system , and image acquisition geometry . the flow cells were filled with 
 70 g of ottawa sand ( 99.5% sio2 , particle diameter 500700 
 m ) or 45 g of crushed 
 nirex reference vault backfill ( nrvb ) , sieved to 24 mm particle size , ensuring maintenance of appropriate 
 flow rates . crushing and experimental setup were conducted in air 
 several hours prior to experimentation , in which time some carbonation 
 of the exposed surfaces may have taken place . the possible chemical 
 changes that might be expected for degraded backfill are outside the 
 scope of the current study and are thus not addressed in this model 
 gdf system . the sand was washed and sonicated in ultrahigh quality 
 water ( 18 m ) five times to remove any existing impurities and 
 oven - dried for 24 h prior to use . the nrvb material was prepared by 
 mixing 130.1 g of ordinary portland cement , 49.12 g of ca(oh)2 , 143.1 g of caco3 , and 177.76 ml of water in a 
 hobart mixer , giving a w / s ratio of 0.552 . it was cured at room temperature 
 for 28 days and kept sealed prior to use . duplicate flow cells containing 
 sand were saturated with 16 ml of ph 5.7 , deionized water ( 18 m ) 
 so that the material plus aqueous phase filled the cell above the 
 inlet port . a 3 mm depth of solution was maintained above the top 
 of the material to ensure a uniform pressure head across the flow 
 field . identical flow cells were prepared using a ph 10.7 buffer solution 
 ( 0.05 m nahco3 , 0.1 m naoh ) . a flow cell containing nrvb was saturated with 25 ml of deionized water at ph 5.7 ( 18 m ) . the porosity calculated from the ratio of solution volume to total 
 saturated pack volume was 0.37 for sand and approximately 0.77 for 
 the crushed nrvb , taking into account the internal porosity of the 
 material itself ( estimated as 0.55 ) . bulk densities of the porous materials as packed were 1.68 and 1.57 
 kg dm for sand and nrvb , respectively . fully constructed , prefilled flow cells were transported to the 
 nuclear medicine department of the royal hallamshire hospital ( sheffield , 
 uk ) for imaging . flow was maintained in the cells at 0.33 ml min using a multichannel peristaltic pump ( watson marlow , 
 uk ) , yielding a calculated pore velocity equal to 4.29  10 m s for the sand and 2.05  
 10 m s for the nrvb ( due to 
 the greater porosity ) . the darcy flux in both cases was 1.5  
 10 m s. flow cells were flushed 
 with tc - free solution for 20 min to establish uniform 
 flow conditions prior to injection with tc and subsequent 
 imaging . imaging was performed on a dual - headed ge medical systems 
 infinia gamma camera ( ge medical , milwaukee , wi , usa ) fitted with 
 a high resolution collimator . a dynamic acquisition with 30 s frame 
 intervals was initiated a few seconds prior to injection of the tc into the flow cells . images were acquired with a matrix 
 size of 256  256 resulting in a pixel size of 2.2 mm . the spatial 
 resolution of the imaging system was measured at the collimator face 
 using standard nema testing techniques 
 and was found to be 4.6 mm fwhm . this equates to a spatial resolution 
 of 6 mm at the location of the flow cells . due to the low 
 spatial resolution tc as pertechnetate 
 [ tc(vii ) ] was produced on - site via saline - based elution of a ge medical 
 systems drytec tc generator . this volume gave 
 an activity of 1520 mbq at the time of the experiment . this 
 corresponds to tc concentrations of < 1 mm . the activity 
 in each syringe was accurately measured in a capintec crc-15r radionuclide 
 calibrator . following injection of the tc into the flow 
 cells as instantaneous pulses ( < 1 s injection ) , the residual activity 
 in each syringe was measured and this reading was subtracted from 
 the  full  reading to determine the exact activity injected 
 into each cell . in all cases , tc activity readings were 
 decay - corrected to the time the gamma camera acquisition was started 
 ( eq 1 , 2):12where a0 is the 
 corrected activity ( mbq ) , at is the uncorrected 
 activity ( mbq ) , k is the decay constant ( s ) , t is the time elapsed ( s ) , and t1/2 is the half - life of tc ( 21  636 
 s ) . for each flow cell , a sensitivity value ( counts per mbq ) was determined 
 so that image counts could be related directly to tc 
 activity . this sensitivity value was calculated by using region of 
 interest ( roi ) analysis to determine the image counts per frame within 
 the region of the cell , averaged over the first 4 frames following 
 injection of the radioisotope . during this early period , this averaged count value was 
 then divided by the known activity injected into the cell to yield 
 a sensitivity factor ( eq 3):3where sf is the sensitivity 
 factor , xci is the mean of the 
 counts from the 
 first four frames of image acquisition ( counts pixel ) , and a0 is the initial activity ( mbq ) . this sensitivity factor was applied to all decay - corrected gamma counts 
 throughout the experiments to yield the concentration c ( mbq pixel , normalized by the volume of pores 
 in each pixel to give mbq ml ) of tc at any location in each time step . raw image data were calibrated 
 using the sensitivity factor ( eq 3 ) to give 
 2-d planar spatial arrays of tracer concentration data at 30 s intervals 
 for up to 3 h during and after transit of the main mass of tc through the flow cell . spatial moments in the direction of travel 
 were calculated at the center of mass of the plume using imagej software . calibrated concentration maps showing contours 
 of tc mass within the flow cells were produced by interpolation 
 of the 2-d data arrays using surfer 9.0 software ( golden software , 
 ca ) . the transport of the tc through the uniform saturated 
 flow field was modeled using a one - dimensional ( 1-d ) convection dispersion 
 equation for reactive solute transport ( eq 4):4where , subject to specified 
 initial and boundary 
 conditions , c is the aqueous concentration of a tracer 
 at a given distance along the center of mass from inlet x ( m ) and elapsed time t ( s ) ,  ( s ) is a first - order decay coefficient describing irreversible removal 
 from the mobile aqueous phase , r is a retardation 
 factor describing equilibrium interaction with the solid phase , and d is a dispersion coefficient equal to the product of the 
 longitudinal dispersivity  ( m ) and mean pore flow velocity , vp ( m s ) . a numerical solution 
 to eq 4 was implemented in inverse ( parameter - fitting ) 
 mode in excel - cxtfit software to yield 
 transferable parameters describing the transport of the radionuclide 
 in the ottawa quartz sand and nrvb . figure 3a  d shows the calibrated concentration 
 distribution data for tc transport through the ottawa 
 sand at ph 5.7 , at 8 ( 0.29 ) , 16 ( 0.58 ) , 24 ( 0.87 ) , and 32 ( 1.16 ) minutes 
 after injection . values in parentheses and all time data thereafter 
 are expressed in pore volumes ( pv ) , where time is normalized by the 
 transit time of a volume of solution equal to the volume of void spaces 
 in the sand . under the conditions of these experiments , 1 pv was equivalent 
 to 1650 s ( 27.5 min ) of travel time . the tc tracer passed 
 through the saturated sand as well - defined plumes with peak concentrations 
 in the center approximately 10  2 mbq ml . figure 4 shows the total tc 
 activity measured in the sand as a function of time for experiments 
 at ph 5.7 and 10.7 . this behavior was highly reproducible 
 for experimental runs at both ph 5.7 and 10.7 . residual activities 
 measured after 2 pv ( not shown ) were less than 1% of the total activity 
 injected and were not significantly different from zero , taking into 
 account the assumed measurement error quantified by the standard deviation 
 ( 1% ) for total activity measurements made between 0.25 and 0.5 
 pv . calibrated concentration distribution from gamma camera images 
 of tc activity in ottawa quartz sand in a ph 5.7 solution 
 at ( a ) 0.29 pv ( 8 min ) , ( b ) 0.58 pv ( 16 min ) , ( c ) 0.87 pv ( 24 min ) , 
 and ( d ) 1.16 pv ( 32 min ) and in nirex reference vault backfill at 
 ( e ) 0.3 pv ( 20 min ) , ( f ) 0.6pv ( 40 min ) , ( g ) 0.9 pv ( 60 min ) , and 
 ( h ) 1.2 pv ( 80 min ) . total measured activity of tc ( normalized to input 
 activity ) as a function of time ( expressed as pv , normalized to flow 
 rate ) during transport through ottawa quartz sand s1 and s2 at ph 
 5.7 and s3 at ph 10.7 and nrvb . figure 3e  h shows calibrated concentration distributions 
 for tc transport in nrvb at 0.3 , 0.6 , 0.9 , and 1.2 pv . 
 due to the lower pore velocities in nrvb than in sand , 1 pv was equivalent 
 to 4300 s ( 71.5 min ) of travel time . the peak 
 concentration at 0.3 pv was 12 mbq ml , measured 
 at 3 cm from the injection point . after 0.6 , 0.9 , and 1.2 
 pv , the peak concentrations were measured as 10 , 9 , and 8 mbq ml , respectively , at 4 , 5.5 , and 6 cm from the 
 tracer injection point ( figure 3 ) . the 
 total tc activity measured in the nrvb as a function 
 of time is shown in figure 4 . as in sand , total 
 activity decreased broadly symmetrically around 1 pv , indicating conservative 
 transport with longitudinal dispersion . residual activity at the end 
 of the measurement period was less than 2% of the initial activity 
 and not significantly different from zero , taking into account variability 
 in the image data quantified as noted above . tc activity 
 was summed across horizontal pixel rows 
 ( normal to the vertical direction of transport through the cells ) 
 to yield concentration profiles which could be expressed as a function 
 of distance from inlet or time since tracer injection . these data 
 were fitted with the 1-d convection  dispersion model ( eq 4 ) . in individual experiment runs s1s3 and 
 n1n2 ( table 1 ) , the model was regressed 
 to concentration profiles measured at five distances from the inlet 
 simultaneously , using the linear least squared error method . the best fit model parameters for each experiment 
 condition are shown in table 1 ; irreversible 
 sorption parameter  was never larger than zero and is therefore 
 not tabulated . figure 5a compares data from 
 an individual experiment run in sand with the output from the model 
 run in forward mode for the same distance intervals , using averages 
 of the parameter values shown in table 1 . the 
 correlation between the model and data is very strong ( r = 0.98 ) . figure 5b shows equivalent 
 data and model output ( r = 0.99 ) for 
 nrvb . dispersion 
 model ( lines ) to measured tc concentrations ( data points ) 
 in ( a ) ottawa quartz sand and ( b ) nrvb material at several distances 
 from the inlet during uniform saturated flow . the parameters obtained from 
 numerical modeling of tc transport in sand ( table 1 ) strongly indicate 
 conservative transport ( r = 1 ) of the tc through the ottawa quartz sand . this conclusion is supported by 
 independent spatial moments analysis of the calibrated image data 
 ( figure 6 ) , which yields a mean velocity for 
 the center of mass of the tc plumes in sand of 4.32 
  10 m s. the transport 
 velocity for the tc was therefore not significantly 
 different from the pore velocity in the quartz sand ( 4.29  10 m s ) calculated a priori . in 
 contrast , both spatial moments and numerical modeling yielded the 
 same mean transport velocity for tc transport through 
 nrvb , 1.64  10 m s , 
 which was slower than the calculated pore velocity based on the internal 
 and boundary conditions of the experiment ( 2.05  10 m s ) . this may be due to errors in estimation 
 of the internal pore structure of the nrvb which may be discontinuous , 
 creating regions of low flow or immobile pore water . while r remained close to 1 indicating conservative transport , 
 the longitudinal dispersivity ,  , for nrvb was 0.0033 m , more 
 than three times that modeled in sand . tracer plume center of 
 mass ( com , first spatial moment in the direction 
 of flow ) plotted as a function of calculated water movement for tc transport through ottawa quartz sand and nrvb . data are averages of experimental replicates . the sorption of solutes to a solid phase is often described by 
 an equilibrium linear sorption coefficient kd ( m kg ) estimated from batch 
 experiments , which contain a known volume of solution , concentration 
 of solute , and mass of solid phase , yielding ( eq 5):5where c0 and cs are , respectively , 
 the initial solute concentration 
 in solution and final equivalent concentration on the solid phase , m ( kg ) is the mass of solid phase , and v ( m ) is the volume of the fluid . we approximated these 
 parameters by normalizing the known input activity and the observed 
 retained activity by the volume and mass of porous media in the flow 
 chamber , to obtain an estimate for kd ( we 
 denote this method m1 ) . for reactive transport through porous media 
 and assuming that surface reactions occur sufficiently rapidly relative 
 to transport that equilibrium can be achieved , kd can also be related both to the retardation factor r in the convection dispersion equation ( denoted 
 method m2 ) and to the ratio of mean transport velocities obtained 
 from spatial moments analysis ( method m3 ) by eq 6:6where  is the porosity , 
 b is the bulk density ( kg m ) 
 of the porous 
 media , and vtc is the mean velocity of 
 mass flux ( m s ) . estimated sorption coefficients for both sand and 
 nrvb were small , of the order 10 m kg , which is consistent with the transport parameters 
 obtained from the convection  dispersion modeling and the observed 
 low retention of tc in the sand after 2 pv . although 
 the model - derived errors associated with nrvb were relatively large , kd as estimated by all three methods was consistently 
 greater in nrvb ( approximately an order of magnitude ) than in sand 
 ( table 1 ) . we reiterate that kd as calculated assumes equilibrium in the underlying 
 sorption reactions ; however , we can not confirm this with the data 
 reported here and , as such , our values may be biased toward underestimation . we do note , however , the empirical observation that after a relatively 
 short period of flushing of the mobile tc plume from 
 the flow chamber , less than 12% remained suggesting that the 
 sorption that does occur within the transit time of the plume may 
 be readily and rapidly reversible when solute concentrations return 
 to zero , for both materials under these experimental conditions . we 
 are also aware of the possibility that some irreversible sorption 
 ( 12% ) may occur as it is hard to rule out this condition without 
 sorption capacity measurements . in the interpretations 
 that follow , we recognize that several assumptions and simplifications 
 have been made in the model gdf systems investigated . these have been 
 made in order to demonstrate the applicability of the gamma imaging 
 technique to radionuclide transport in a gdf and , as such , provide 
 the basis for future detailed experimentation . the limited retardation 
 ( r  1 ) interpreted using the model , very 
 low estimated kd obtained with the different 
 methods , and the minimal retention of tc in the sand 
 at the end of the experiments imply closely conservative transport 
 of tc through ottawa sand . although this is the first 
 time that this has been confirmed directly , it is not an unexpected 
 result . the point of zero charge ( pzc ) of ottawa sand is between ph 
 2 and 5 , so at the ph of these experiments 
 ( > 5.7 ) , the sand surface is negatively charged . since the pertechnetate 
 anion ( tco4 ) is also negatively charged , 
 chemical sorption is therefore impeded by repulsive electrostatic 
 interactions between the tc and sand . this indicates that tc(vii ) 
 may be transported freely in environments where the substrate has 
 only negatively charged surfaces . such pure - phase interactions are 
 a simplification of natural environments , especially where significant 
 quantities of fe(ii ) or other minerals capable of reducing tc(vii ) 
 to the less mobile and less soluble tc(iv ) are present or in environments 
 where microbially mediated reactions may take place to alter the oxidation 
 state of technetium . nevertheless , these results highlight the utility 
 of quantitative measurements of transport parameters for tc(vii ) in 
 opaque porous media , that may be applied to substrate related to gdf 
 concepts ( e.g. , clay , host rock ) . we have shown that it is possible 
 to obtain quantitative transport 
 data using the gamma imaging technique in opaque engineered backfill 
 material . pertechnetate transport in nrvb in our gdf - proxy experiments 
 was closely conservative , i.e. , our data pertaining to tc transport in this material showed no significant retardation and 
 very low sorption coefficients . previous studies have suggested low 
 sorption coefficients for tc(vii ) in batch experiments using aged , 
 crushed nrvb , and our study indicates , for 
 the first time using quantitative imaging , that such observations 
 may translate into a significant potential for transport of tc(vii ) through a backfill candidate material in a model flowing 
 groundwater system . despite the low spatial resolution of the gamma 
 images , tc transported through nrvb exhibited a greater 
 dispersivity and slower transport velocity than in sand . discontinuities in the internal 
 structure may create significant immobile ( very low flow ) zones within 
 the pore space . further work on nrvb will explore the use of a mobile - immobile 
 ( mim ) transport model ( e.g. , tang et al . ) , to better elucidate the dynamics of solute transport through this 
 material . the quantitative gamma imaging technique described 
 in this paper 
 represents a rapid and convenient method for obtaining transport data 
 for tc . the main advantage of this technique is that 
 quantitative images can be obtained in opaque media ; it is possible 
 to see the retained mass as a function of time and space , allowing 
 for a direct visual quantification of transport parameters . furthermore , 
 in experiments where the sorption can be controlled for , it may be 
 possible to visualize and quantify sorption . this multitude of information 
 is such that transport models , such as cxtfit used here to test and 
 validate the methodology , may not be required to derive transport 
 parameters . the spatial resolution presented in this 
 methodology was relatively low ( 6 mm ) , largely as an artifact of the 
 spatial constraints placed upon using a working hospital camera . however , 
 higher spatial resolution , and thus accurate dispersivity measurements , 
 should be possible , depending upon the quality of the instrument and 
 proximity to the collimator . quantitative gamma imaging has 
 several potential applications to 
 contaminant transport in opaque media . in the context of geological 
 disposal of nuclear waste , the transport of i from the 
 waste and through the engineered barrier is a key concern due to its 
 long half - life ( 15.7  10 years ) , high solubility , 
 and poor sorption . gamma imaging coupled with the -emitting i radiotracer could be used to develop an understanding 
 of iodine transport behavior and thus support engineered barrier material 
 design . because the gamma camera can detect ultratrace concentrations 
 of radionuclides , several gamma - emitting isotopes could also be used 
 to nondestructively quantify the transport of environmental contaminants 
 in soil , such as chromium ( cr ) or mercury ( hg ) . this highlights the potential versatility of the technique , 
 applicable to a wide range of scenarios as a novel tool to understand 
 the spatial and temporal dynamics of the geochemistry of a variety 
 of radiotracers in opaque media .
we 
 present a novel methodology for determining the transport of 
 technetium-99 m , a -emitting metastable isomer of 99tc , through quartz sand and porous media relevant to the disposal 
 of nuclear waste in a geological disposal facility ( gdf ) . 
 quartz sand 
 is utilized as a model medium , and the applicability of the methodology 
 to determine radionuclide transport in engineered backfill cement 
 is explored using the uk gdf candidate backfill cement , nirex reference 
 vault backfill ( nrvb ) , in a model system . 
 two - dimensional distributions 
 in 99mtc activity were collected at millimeter - resolution 
 using decay - corrected gamma camera images . 
 pulse - inputs of 20 
 mbq 99mtc were introduced into short ( < 10 cm ) water - saturated 
 columns at a constant flow of 0.33 ml min1 . 
 changes 
 in calibrated mass distribution of 99mtc at 30 s intervals , 
 over a period of several hours , were quantified by spatial moments 
 analysis . 
 transport parameters were fitted to the experimental data 
 using a one - dimensional convection  dispersion equation , yielding 
 transport properties for this radionuclide in a model gdf environment . 
 these data demonstrate that 99tc in the pertechnetate form 
 ( tc(vii)o4 ) does not sorb to cement 
 backfill during transport under model conditions , resulting in closely 
 conservative transport behavior . 
 this methodology represents a quantitative 
 development of radiotracer imaging and offers the opportunity to conveniently 
 and rapidly characterize transport of gamma - emitting isotopes in opaque 
 media , relevant to the geological disposal of nuclear waste and potentially 
 to a wide variety of other subsurface environments .