transport of carbon-14 in a large unsaturated soil … of carbon-14 in a large unsaturated soil...

Transport of Carbon-14 in a Large Unsaturated Soil Column

Mitchell A. Plummer,* Larry C. Hull, and Don T. Fox

ABSTRACT 14C released from the SDA would migrate downward,with infiltrating soil moisture, to eventually reach theWastes buried at the Radioactive Waste Management ComplexSnake River Plain aquifer. Conversely, in assessing at-(RWMC) of the Idaho National Engineering and Environmental Lab-

oratory (INEEL) include activated metals that release radioactive 14C mospheric exposures, all 14C was assumed to migrateas they corrode. To test and refine transport predictions that describe upward in the gas phase. In addition to these extremereleases to the environment with time, we conducted a series of trans- simplifications of transport processes, the assessmentport experiments with nonreactive gas- and aqueous-phase tracers also did not consider the effect of partitioning betweenand inorganic 14C species in a large unsaturated soil column filled aqueous and solid phases during liquid transport.with sediment representative of that at the RWMC. The tracer tests, To provide data for more realistic modeling of 14Chydraulic measurements, and chemical monitoring provided con-

releases from the SDA, and thereby provide a strongerstraints on physical transport parameters, water content, and aqueous–scientific basis for regulatory decisions, we conductedgas partitioning behavior. With those constraints, we estimated aa series of transport experiments with conservative gassolid–aqueous distribution coefficient for the sediment through in-and liquid tracers as well as 14C. Parameters for contami-verse modeling of the 14C transport data, using both a simple gas-

diffusion model and a multiphase flow and transport simulator nant transport modeling are typically based on labora-(STOMP). Results indicate that 14C transport in this system is well tory bench-scale experiments. In this case, to test thedescribed by a reactive gas diffusion model, with a pH-dependent ability of unsaturated flow and transport models to pre-retardation factor. Fitting transport simulations to the early-time dict 14C behavior at larger scales, and to determine flowtransport data yielded Kd ≈ 0.5 � 0.1 mL g�1, while soil samples and transport parameters from measurements at muchremoved approximately 1 yr later yielded Kd values of 0.8 to 2.4 mL larger spatial and temporal scales, we conducted theg�1. These values are consistent with those derived from smaller-scale

experiments in a large column filled with the same sedi-experiments, demonstrating that laboratory-based measurements pro-ment used as cover material at the SDA. Such mesoscalevide a valid means of estimating transport behavior at much largertransport experiments are rarely conducted because ofspatial and temporal scales. Assuming that 14CO2 migration in thethe considerable costs and long time periods involved,RWMC is dominated by gas transport, our results suggest that most

14C released from the RWMC would discharge to the atmosphere but are a valuable means of testing transport models atrather than to the underlying Snake River Plain aquifer. a scale approximating field conditions with control not

possible in the field. In this case, the transport param-eters derived from these mesoscale experiments shouldprovide a good basis for predictive modeling of 14C trans-Radioactive carbon released as 14CO2 from wastesport from the SDA.emplaced in unsaturated sediments can be trans-

ported in both gaseous and dissolved form and maythus impact the overlying atmosphere and underlying BACKGROUNDgroundwater. Analysis of 14C release rates to both those

Wastes in the SDA are emplaced in a layer of surficialreceptors is a concern at the INEEL, where radioactivesediment, approximately 2 to 9 m thick, that accumu-wastes buried within the RWMC include an estimatedlated in a depression in the basalt flow–covered land-18.5 TBq of 14C (McCarthy et al., 2000) and 14C has beenscape. Unsaturated fractured basalt, with intermittentidentified as a significant dose contributor relative toaeolian and alluvial interbeds, underlies the fill materialthe RWMC’s performance objectives as a Low-Leveland extends to the water table of the Snake River PlainWaste (LLW) disposal facility (Case et al., 2000; McCar-aquifer, which lies about 180 m below the surface (An-thy et al., 2000).derson and Lewis, 1989) at the RWMC. Carbon-14 inCarbon-14 within the RWMC (Fig. 1) is located withinthe buried waste is primarily contained in activated met-the Subsurface Disposal Area (SDA), a 39-ha disposalals associated with discarded reactor components, wheresite where solid radioactive waste has been buried in14C was produced by neutron bombardment of 13C andsoil vaults, pits, and trenches, and covered with fine-14N. These activated metals corrode in the subsurface,grained material excavated from a nearby playa. A ra-slowly releasing 14C that is rapidly oxidized to formdiological performance assessment completed in 200014CO2. Understanding 14C transport from the wastes is(McCarthy et al., 2000) conservatively assumed that alltherefore primarily a problem of understanding 14CO2

transport.M.A. Plummer, L.C. Hull, and D.T. Fox, Idaho National EngineeringIn arid and semiarid regions, water content in theand Environmental Laboratory, P.O. Box 1625, MS 2107, Idaho Falls,

ID 83415-2107. Received 10 July 2003. Special Section: Understanding unsaturated zone is generally low, and CO2 transportSubsurface Flow and Transport Processes at the Idaho National Engi-neering & Environmental Laboratory (INEEL) Site. *Corresponding

Abbreviations: DIC, dissolved inorganic carbon; INEEL, Idaho Na-author ([email protected]).tional Engineering and Environmental Laboratory; LLW, Low-LevelWaste; RWMC, Radioactive Waste Management Complex; SDA,Published in Vadose Zone Journal 3:109–121 (2004).

Soil Science Society of America Subsurface Disposal Area; STOMP, Subsurface Transport Over Mul-tiple Phases [model].677 S. Segoe Rd., Madison, WI 53711 USA

109

110 VADOSE ZONE J., VOL. 3, FEBRUARY 2004

Fig. 1. Location of the Subsurface Disposal Area (SDA) of the Radioactive Waste Management Complex (RWMC) at the Idaho NationalEngineering and Environmental Laboratory (INEEL).

occurs primarily by gaseous diffusion, a phenomenon tion gradient of 12CO2 or 13CO2. Differences in the diffu-sion rates of these isotopes are negligible when consider-that has been extensively studied (Sheppard et al., 1994).

Under these conditions, as Thorstenson et al. (1983) ing their independent movement, so 14CO2 transport canbe described as CO2 transport. In partially saturatedhave pointed out, 14CO2 diffuses in response to its own

concentration gradient, independent of the concentra- media, the greater fraction of available CO2 is commonly

www.vadosezonejournal.org 111

the same sediment that is used as cover material at the SDA.present in dissolved form, as bicarbonate ion. CarbonAlso used for studies of C cycling and actinide transport, thedioxide transport models typically account for this parti-column contains an active colony of microbes that consumetioning into the aqueous phase (Thorstenson et al., 1983)natural soil organic matter and O2 and respire CO2, producingas well as the attendant transport of the dissolved inor-an approximately exponential increase in CO2 concentrationganic C (DIC) species (Ross, 1988; Simunek and Suarez,with depth. The column thus provides a relatively large physi-1993). The distribution of C between CO2, DIC, and cal model of vadose-zone flow at the SDA that allows mon-

solid carbonate is typically described using the thermo- itoring of gas- and liquid-phase transport over relatively longdynamic constraints for calcite equilibria (Thorstenson distances and also includes significant variability in the geo-et al., 1983). Dissolved carbonate species may interact chemical conditions that have a major influence on carbon-with the solid matrix in a number of ways that act to ate partitioning.

Before conducting a transport experiment with 14C, we per-retard its movement, including anion sorption and disso-formed a set of experiments in the mesoscale column to exam-lution–precipitation reactions (Simunek and Suarez,ine the movement of water, gas, and conservative solute1993; Striegl and Healy, 1990). Isotopic fractionationthrough the sediment. These experiments included an infiltra-during these exchanges is, again, negligible (Lu andtion test; injections of a highly insoluble gas, sulfur hexafluo-Ross, 1994) compared with the isotope concentrationride, both before and after the transient infiltration period;differences arising from their different origins. Whileand an injection of a conservative liquid tracer, lithium bro-interactions with the solid matrix may be negligible in mide, after reaching a quasi-steady-state flow regime. Datasome conditions (Striegl and Armstrong, 1990), or ne- from these experiments were used to estimate characteristics

glected to provide conservative transport predictions of the unsaturated flow system that would affect transport of(Lu and Ross, 1994), the inability of carbonate equilib- 14C, including average linear seepage velocity, dispersivity, andrium transport models to match the observed retarda- gas- and liquid-phase tortuosity. Combining those parameterstion of downward moving 14CO2 in the unsaturated zone with calculated aqueous–gas and estimated aqueous–solid par-(Thorstenson et al., 1983; Striegl and Healy, 1990) sug- titioning ratios for 14C, we then used the multiphase, multicom-

ponent, flow and transport model, STOMP (Subsurface Trans-gests that these phenomena are important. Indeed, sev-port Over Multiple Phases; White et al., 1995), to design aeral studies of 14CO2 retention have demonstrated that14C injection experiment that would be used to improve ourthe mass retained by the solid phase can be large com-estimates of 14C-specific transport characteristics of the SDApared with other reservoirs for C exchange in the unsat-sediments. The subsequent transport test involved injectionurated zone (Martin, 1991; Striegl and Armstrong, 1990;of a 14C-labeled bicarbonate solution.Garnier, 1985), including the quantity of DIC deter-

mined by carbonate equilibria.Column DescriptionBecause the surficial fill materials represent the first

barrier to gas and liquid transport out of the SDA, one The mesoscale column is a 0.91-m-diam., 2.9-m-tall, stain-of the primary goals of this study was to develop and less-steel cylinder (Fig. 2) sealed at both ends. The columntest parameters that characterize the transport of 14C in contains approximately 1.7 m3 of sediment that extends fromthat material. These include contaminant-specific fac- the bottom of the column to a height of 2.6 m. The headspacetors, such as aqueous–gas and solid–aqueous parti- above the sediment is sealed at the top with a sheet of Plexiglas

and is connected to a gas monitoring and flow control system.tioning parameters, as well as factors that describe moreAmbient air is circulated through the headspace at a rate ofgeneral controls on the advective and diffusive transport7 L min�1. This circulation system prevents accumulation ofof gaseous and liquid contaminants. Although these pa-CO2 and provides a constant concentration boundary condi-rameters could be determined via laboratory-scale ex-tion at the sediment surface for gas-phase components. Theperiments, the attendant size restrictions greatly restrictcolumn is instrumented at 30.5-cm (1-ft) intervals along itsthe number of details of the subsurface environmentlength at vertical positions denoted as Levels 1 through 8.that can be incorporated in a single experiment. Instead, Instrumentation includes four tensiometers, four water con-

we sought to design a system that would allow us to tent reflectometers (Model 615, Campbell Scientific, Logan,examine 14C transport behavior at relatively large spatial UT), 10 sampling lysimeters, eight gas-sampling ports, and sixand temporal scales and at infiltration rates and soil thermocouples (Fig. 2 and 3). The reflectometers measuremoisture conditions at least broadly representative of water content using time-domain measurement methods. Wa-those at the SDA. To capture some of the geochemical ter content is a critical parameter in gas transport modeling,

and field studies have demonstrated that the CS-615 reflec-characteristics of the SDA subsurface, we also soughttometers can provide dependable and accurate data for ata system that could incorporate the primary naturalleast several years (Delin and Herkelrath, 1999). Seyfried andcontrol on the partitioning of carbonate species betweenMurdock (2001) found that the coefficient of variation in watergas and aqueous phases—microbial respiration and thecontent measurements using the CS-615 reflectometers issubsequent redistribution of microbially generated CO2.�0.05. In addition to monitoring hydraulic head and/or mois-Soil gas around waste disposal trenches at the SDAture potential at five different levels in the column, we trackedwhere organic debris has been disposed can be as highcolumn water balance via periodic measurements of the col-as 10% CO2 by volume (Hull and Bishop, 2003), which umn inflow, effluent mass, mass removed for liquid sampling,yields a significantly lower pH than typically exists in and evaporative loss from the column surface. The latter was

arid environments with sediments that contain calcite. calculated from relative humidity measurements of ambientair and column headspace, combined with headspace sweepMATERIALS AND METHODS rate.

The column is fitted with three separate injection arraysTo meet these requirements, we conducted a set of transportexperiments in a 2.9-m-tall, 0.91-m-diam. column filled with capable of injecting gas or liquid. Each of these comprises 24


to the SDA. In the Unified soil classification system, the sedi-ment is a calcareous silty-clay. X-ray diffraction analysis indi-cates that the sediment is 50 to 75% quartz, 10 to 25% plagio-clase and K-feldspar, 10 to 20% clay minerals, �5% olivineand pyroxene, �5% calcite and �5% Fe. The fine grain sizefraction (�75 �m) is generally 40 to 55% quartz, 30 to 45%clay minerals, 5 to 10% plagioclase and K-feldspar, 5 to 10%calcite, and 5% iron oxides, with trace amounts (�5%) ofgypsum and other minerals. Mixed smectite–illite constitutes50 to 70% of the clay minerals, and kaolinite, illite, and Ca-rich smectite comprise the remaining fraction.

The sediment was obtained from a berm near the SDA,mixed using a backhoe, and passed through a 1.25-mm sieveinto 208-L drums for transport to the mesoscale column. Thesieved sediment was transferred to the column using 19-Lplastic buckets; the weight of each bucket was recorded. Thesediment was manually compacted in 15-cm lifts, with bulkdensity determined following placement of each lift. Whenthe desired density was verified, the lift surface was scarifiedto avoid layering or segregation by particle size, and the nextlift was placed. In situ sensors and sampling devices werepositioned on scarified surfaces. At the conclusion of the pack-ing process, the mean bulk density was 1.31 � 0.05 g cm�3,and mean volumetric water content was between 14 and 16%.The column was then allowed to sit open to the atmospherefor about 2 yr before initiation of the experiments describedherein. Calibrated reflectometer measurements made just be-fore the application of water to the column surface indicatedthat the average volumetric water content of the sediment atthe start of this experimental program was approximately 10%.Mass balance monitoring of water content began with the initia-tion of water flow, and the initial water content for those calcula-tions is based on those reflectometer measurements.

Fig. 2. Schematic diagram of the mesoscale unsaturated flow columnExperiments to Determine General Flowshowing vertical placement of sampling and monitoring devices.

and Transport Parametersstainless-steel tubes (1.6-mm i.d.) that can deliver fluid to

Gas diffusion in porous media is highly dependent on waterevenly spaced locations in a horizontal plane within the col-content because of the nonlinear relationship between gas-umn. The uppermost array lies on the sediment surface andfilled pore space and tortuosity. Previous experiments at thesupplies influent water. The two remaining arrays are situatedINEEL (Hull and Hohorst, 2001) with air-dry sediments inwithin the sediment, 1.07 and 1.98 m above the bottom. Solu-small laboratory columns indicated that the reduction in thetions for aqueous-phase transport experiments were injectedfree-air diffusion coefficient due to tortuosity may be slightlyinto the middle array (1.98-m height) to minimize nonuniformgreater in SDA sediments than is described by the commonlyflow effects due to the periodic application of water at theused Millington equation (Millington, 1959). To evaluate thesurface.relationship between water content and gas diffusivity in theA system of 24 syringe-pump-fed tubes applies water to themesoscale column, we conducted four sulfur hexafluoride in-sediment surface at intervals of 40 min. The applied water isjection experiments: before water was applied to the column,synthetic vadose zone water, formulated to approximate theon 16 Mar. and 4 Apr. 2001, and under near steady-state flowcomposition of natural infiltrating soil moisture at the SDAconditions, on 24 Apr. and 7 June 2002. In each test, 10 mL(Table 1). To limit microbial growth, the water is sterilized byof SF6 were injected into the lower injection array, betweenautoclaving and treated with ultraviolet radiation immediatelyLevels 3 and 4. Injections were made with five 10-cm3 dispos-preceding application. Water application began on 11 Julyable syringes connected to five of the 24 injection tubes, se-2001, at a Darcy flux of about 0.6 cm d�1. After the wettinglected to distribute the gas evenly over the injection plane.front reached the effluent lysimeters at the bottom of theConcentrations of SF6 were then monitored at the eight gascolumn, the flux density was reduced to 0.15 (�4%) cm d�1

sampling ports until concentrations approached backgroundand held at that rate for the remainder of the experiment. Tolevels.withdraw the infiltrating water, constant tension was applied

To estimate average seepage velocities under the estab-to four 0.05-MPa (0.5-bar) suction lysimeters located in alished flow regime, we injected a lithium bromide solution athorizontal plane about 15 cm above the bottom. Thus, nearthe upper injection plane (between Levels 6 and 7) on 22 Mar.steady-state hydraulic conditions in the column are maintained2002. The injection consisted of 240 mL of a 9.89 mg L�1

through application of a consistent flux at the top of the col-bromide solution, delivered into the 24 injection tubes viaumn and constant moisture potential in the lysimeters.gravity flow from 24 10-mL syringes. On 12 Aug. 2002, approx-imately 5 mo after the Br� tracer test, we injected 2850 mLSediment Description of a solution containing 60 MBq 14C (as radio-labeled bicar-bonate) at the upper injection plane. To minimize disturbanceThe sediment used in the column is the same as that used

as cover at the SDA—surficial sediments from playas adjacent of the flow field and water chemistry, water for the solution


Fig. 3. Schematic diagram (plan view) of instrumentation at each of the eight sampling and monitoring levels and at Level 0, where water isremoved via suction lysimeters.

was taken from the column itself, withdrawn from lysimeters measured with a Dionex DX-500 ion chromatograph (Dionexdirectly above and below (Levels 6 and 7) the injection plane Corp., Sunnyvale, CA); 14C activity was measured via liquidduring the 10-d period preceding the injection. Using a 125-mL scintillation counting (Beckman LS6000LL, Beckman Coulter,Pyrex syringe to inject 120 g of solution into each of the 24 Fullterton, CA). For experiments involving gas-phase trans-injection tubes, we emplaced the solution over a period of 4.5 h. port, gas-sample chambers and tubing were first purged with

sample gas. Approximately 500 mL of soil gas was then with-Sampling and Analysis drawn from each sampling level (similar tests at different

sampling intervals, and numerical modeling experiments, in-Following each injection, breakthrough curves of the intro-dicated that these withdrawals had a negligible effect on gasduced species were monitored at the downstream ports andmovement). Sulfur hexafluoride concentrations were withdrawnin the column headspace through sampling and analyses spe-using an automated 12-channel switching device and analyzedcific to that species. For steady-state flow, aqueous transportwith an attached INNOVA 1312 gas analyzer (INNOVA, Bal-experiments, soil water samples were collected using 0.05-MPalerup, Denmark). Gas samples for 14C analysis were collected(0.5-bar) semiporous stainless-steel lysimeters. Lysimeter tub-in Tedlar bags containing an aliquot of 0.5 M NaOH solution,ing was flushed before sample collection and 15 to 25 mL ofand the 14C activity of the gas samples was determined viaporewater was removed from each lysimeter, depending on the

number of analyses required. Bromide concentrations were liquid scintillation counting of the NaOH solution after equili-bration.

Table 1. Chemical composition of synthetic soil moisture applied Flux of 14C from the surface of the column was determinedto the mesoscale column. by measuring the cumulative activity of CO2 captured in a

Analyte Value 2 M NaOH trap solution through which a split of the head-gas airflow was directed. Calculated trap efficiency was 96%NaHCO3 10.60 mg L�1

(�1.4%) after a sample collection period of approximatelyMgCO3 3.17 mg L�1

CaCl2·2H2O 0.62 mg L�1 7 d. At the end of each collection period, the trap weight wasCaSO4·2H2O 10.03 mg L�1

recorded, sample aliquots were taken, and the trap solutionKHCO3 1.78 mg L�1

was refreshed. Head-gas 14C flux was reported as averagesTotal alkalinity 36 ppm as CaCO3

pH 7.8 of three aliquot determinations. Aliquot weight and isotopeElectrical conductivity 0.0315 mS cm�1

activity were combined with CO2 trap efficiency, trap weight,


Fig. 4. Water content and moisture potential histories since start of water application, 11 July 2001. (A) Reflectometer data; (B) the moisturepotential record, as measured with in-situ tensiometers; (C) the average water content calculated from the column water balance data. Notethat the reflectometer at Level 1 malfunctioned and was used only as a relative measure of water content. Lettered vertical lines indicate thedate of (a) the Br� injection, (b) soil sampling, (c and d) sulfur hexafluoride gas injections, and (e) the 14C-labeled bicarbonate injection.

and head-gas split ratio to calculate isotope flux for the period exceeding the porosity of the sediment and was gener-of sampling. ally inconsistent with the tensiometer data (Fig. 4B).

We therefore considered it only a relative measure ofsaturation.RESULTS AND DISCUSSION

The combined reflectometer, tensiometer, and massWetting History and Hydraulic Parameters balance data (Fig. 4C) indicated that water content inthe column reached a near steady-state condition ap-Water flow in the column was initiated on 11 July

2001 (Fig. 4), and water content reflectometers at the proximately 100 d after flow was initiated, at a volumet-ric water content of approximately 30%. Although thelowermost (Level 1) sampling plane detected the wet-

ting front approximately 80 d later (Fig. 4A). At that suction applied to the lysimeters at the base of the col-umn was held nearly constant after that time, the totaltime, the application rate at the surface was reduced

to obtain a steady-state flow of about 0.15 cm d�1. A flux from the lysimeters varied slightly through time,presumably due either to changes in the conductivitycorresponding sharp increase and subsequent decline in

water content was recorded at each of the reflectome- of the lysimeters or to subtle changes in the potentialgradient and hydraulic conductivity of the sediment.ters, although instrument response during that time is

to some extent inaccurate due to leaching and advection Despite these variations, the cumulative mass balanceindicated that volumetric water content remained effec-of dissolved salts with the wetting front, which tempo-

rarily raised salt concentrations beyond the reflectome- tively constant (30 � 1%) for the entire period duringwhich the transport experiments were conducted. Thister calibration range. We note that the lowermost reflec-

tometer (Level 1) sometimes recorded water contents was generally consistent with the reflectometer data,


Fig. 5. Comparison of modeled and observed SF6 breakthrough curves for (A) the initial, prewetting, SF6 test and (B) the first test conductedafter reaching near steady-state flow. The modeled curves were computed from an analytical solution (Luikov, 1968) to the diffusion equation.

which, despite several apparent shifts in electrical re- where Cg is the volumetric gas concentration, � is thesponse, indicated that average water content was consis- gas-phase tortuosity, and Dm is the free-air diffusiontently maintained in the range of 27 to 30%. coefficient for the diffusing gas. The gas tracer tests we

As a third check on water content, we removed two conducted are essentially equivalent to the introductionsmall (5-cm-long, 5-cm-diam.) soil cores from Levels 2 of an instantaneous pulse of heat to the interior of anand 8 on 13 Mar. 2002. Moisture content determinations infinite plate, with a no-flux boundary on one side andon these samples were 27 and 24%, respectively, slightly a constant temperature on the other. An infinite-serieslower than indicated by the mass balance and reflectom- type analytical solution for this asymmetric diffusioneter data. Finally, on 29 July 2003, vertical cores (4.5-cm problem is described in Luikov (1968) and we used thatdiam.) were obtained from six locations along a transect solution to analyze the SF6 data.of the column’s surface, to a depth (76 cm) that inter- In the unsaturated zone, diffusion is strongly con-sected the instrument plane at Level 6. Results of water trolled by the fraction of the total pore space availablecontent determinations on subsamples of those cores for gas movement, due to the nonlinear relationshiptaken from the plane of Level 7 yielded an average between fluid tortuosity and fluid saturation (Jury etwater content of 26% with a standard deviation of 1%, al., 1991). We chose to use the well-known Millingtona value that is within the uncertainty of that measured equation (Jury et al., 1991) to describe that relationshipat the Level 7 reflectometer (28 � 2%) just before and used the data from the SF6 gas tracer tests to testsampling. the applicability of that equation to the SDA sediments.

The Millington expression for gas-phase tortuosity isSulfur Hexafluoride

� ��2

T

�mg

[2]Sulfur hexafluoride migrated rapidly through the col-umn following injection, with peak concentrations arriv-ing at the nearest ports (Levels 3 and 4) between 0.5 where �g is the volumetric air content and �T is the totaland 2 h (Fig. 5). An approximately linear concentration porosity. While the commonly used Millington expres-gradient, from the bottom of the column toward the sion for tortuosity employs the value 7/3 (2.33) for thetop, generally developed within about 2 d. Maximum exponent, m, several studies have suggested other valueslate-time SF6 gas concentrations are significantly higher for that parameter (Sallam et al., 1984; Jury et al., 1991).in the second set of tests, under the higher water content In small-column studies using air-dry sediment, Hullassociated with the quasi-steady-state flow conditions. and Hohorst (2001) found that a value of 2.6 best de-This reflects the increased concentration gradient that scribed SF6 diffusion in INEEL SDA sediments. In thisdevelops in response to the resultant reduction in gas study, we used the value recommended by Millingtondiffusivity. (1959) and the analytical solution to Eq. [1] to perform

Sulfur hexafluoride movement within the column oc- a least-squares fit of computed breakthrough curves tocurs primarily by gas diffusion, the transport equations observed breakthrough curves for all of the eight gasfor which are analogous to those for heat transport.ports and for gas injections conducted both before andFor unidirectional transport, in a system with effectivelyafter the application of water. Using saturation as auniform gas-filled porosity, the applicable continuity ex-fitting parameter, this analysis indicated an initial waterpression iscontent of 11% and a steady-state flow water contentof 25%, values that agree relatively well with the other,�Cg

�t� ��1 Dm

�2 Cg

�2 z[1]

more direct, measures of water content. The coefficient


Table 2. Transport parameters determined from CXTFIT2 analy-sis of the Br� tracer test. Inferred water content assumes aconstant flux density equivalent to that applied at the soilsurface (0.15 cm d�1).

Inferred HydrodynamicBreakthrough Average linear volumetric dispersion Correlationcurve location seepage velocity water content coefficient coefficient

cm d�1 cm2 s�1

Level 6 0.46 0.31 4.6E-06 0.940Level 5 0.51 0.28 5.2E-06 0.994Level 4 0.55 0.27 5.2E-06 0.949Level 3 0.55 0.26 5.4E-06 0.927Level 2 0.51 0.27 6.0E-06 0.993Level 1 0.51 0.29 7.9E-06 0.994

ting analytical solutions to the observed breakthroughFig. 6. Bromide concentration data (symbols) from the six sampling curves at each port, resulting R2 values had a mean of

levels below the injection plane and CXTFIT2-fit curves (lines). 0.97 and standard deviation of 0.04 (Table 2). Whileanion exclusion is commonly observed to produce a

of determination for that fit, which includes eight sets two-region type of breakthrough curve in Br� transportof breakthrough curves measured at two different water experiments, we observed no such effect in these data,contents, was 0.86. Comparison of the observed break- and fitting calculations using a nonequilibrium typethrough curves with those computed with the analytical transport solution to the advection–dispersion equationsolution (Fig. 5) demonstrates that the diffusion model, did not provide significantly improved fits. Consistenteven with the assumption of uniform water content, with column design, and despite column size, the Br�

provides a very accurate description of SF6 transport breakthrough curves indicate that flow is effectivelybehavior. Additional least-squares analyses of the data, one-dimensional. Average linear seepage velocities (�)constraining m or water content, or both, did not provide calculated using CXTFIT2 were between 0.46 and 0.55a significantly improved fit to the data, and we con- cm d�1, with the maximum velocity calculated from thecluded that the standard Millington equation adequately Level 3 data and the minimum from the first level belowdescribes gas-phase tortuosity in the SDA sediments. the injection plane (Level 6). Interpreted as differences

The primary difference between calculated and ob- in average water content of the sediment between injec-served SF6 concentrations is that the calculated values tion plane and sampling point, this suggests an averageproduced nearly identical early-time responses at sam- water content of 28 � 3%. This range agrees well withpling levels equidistant from the injection point, while reflectometer data, the mass balance data, and the gravi-the actual test resulted in significantly higher concentra- metric determinations made in March 2002. Althoughtions at the lower port of each pair of equidistant ports. temporal variations in water content could also explainThis phenomenon is evident in data from both the pre-

some of the implied water content variability, the reflec-infiltration and postinfiltration injections but is signifi-tometer and mass balance data indicate that water con-cantly greater in the latter. Because the difference istent was relatively constant throughout our experiments.accentuated under wetter conditions, these results mayDispersion coefficients calculated via CXTFIT2 rangedreflect an increase in gas diffusivity with depth, re-from 0.4 to 0.7 cm2 d�1 (Table 2), with the largest valueflecting decreased water content with proximity to thelittle more than double the tortuosity-corrected molecu-suction lysimeters. Although it could also be related tolar diffusion coefficient for Br�. Calculated dispersivitiesthe effects of gas density on SF6 diffusion, preliminarywere effectively negligible, ranging from 0 to 7 mm. Thisexperiments with a simulator that incorporates gas den-is consistent with the results of Hull and Hohorst (2001),sity-driven diffusion suggested that those effects arewho conducted saturated Br� and tritium transport testsnegligible.with SDA sediment in 31-cm columns and found thatdispersivity, , was on the order of 0.5 mm.Lithium Bromide

Bromide breakthrough curves at all ports below the Carbon-14 Results and Transport Modelinginjection plane are well fit (Fig. 6) by a solution to theadvection–dispersion equation for equilibrium trans- Gas-phase 14C breakthrough curves (Fig. 7) at virtuallyport of a slug of conservative solute, all levels in the column display the same characteristic

shape as the SF6 curves, but lagged, consistent with gas�Cl

�t� D

�2 Cl

�z2�

�Cl

�z[3] diffusion–dominated transport retarded by exchanges

with other phases. Carbon-14 concentrations at the near-est ports, for example, peaked after approximately 1 d,where Cl is the concentration of solute in the liquidas opposed to about an hour for SF6. On the basis ofphase, D is the coefficient of hydrodynamic dispersion,these results, we analyzed data from the 14C experimentand is the advective transport velocity. Usingusing two different methods. Given the similarity to theCXTFIT2 (Toride et al., 1995) to estimate seepage ve-

locity and hydrodynamic dispersion coefficients by fit- SF6 test results, we used the analytical solution to the


Carbon-14 Transport Parameters

Parameters needed for simulations of 14C transportinclude liquid advection rates, aqueous–gas and solid–aqueous partitioning coefficients, and solute diffusivitiesin the gas and aqueous phases. Our conservative tracerexperiments provide good constraints on the aqueoustransport velocity and the dependence of gas diffusiv-ity on volumetric air content. Diffusion coefficients forgaseous CO2 and for the aqueous carbonate species(HCO3

�) prevalent in the system were taken from theliterature (Lide, 2003). While the Br� tracer test indi-cated that seepage velocity varied slightly with depth,we did not attempt to incorporate that level of detailin our transport simulations. Based on both direct andindirect measures of water content, we assumed thatthe average water content was approximately 28% and

Fig. 7. Measured 14C gas-phase breakthrough curves (symbols) and made small adjustments to the van Genuchten–Mualemsimulated responses (lines) at each sampling level. Simulated curvesparameters to provide that value at the injection portwere calculated from an analytical solution to a conceptual modelunder the controlled inflow rate (0.15 cm d�1) underthat considers reactive diffusive transport in the gas phase, but

neglects aqueous transport. Average column water content was set steady-state conditions. We then varied the van Genuch-to 28%; Kd � 0.45 mg L�1 was determined via least squares fit to ten parameters to produce a set of simulations withthe observations.

mean water contents capturing the estimated uncer-tainty of that value to assess the effect of that uncertaintysame diffusion problem to examine the fit of that modelon 14C transport predictions. Final parameters for theto the 14C transport test. To incorporate aqueous trans- 14C transport simulations are summarized in Table 3.port effects and spatially and temporally variable trans-

The primary remaining parameters are the solid–port parameters in a multiphase flow and transportaqueous and aqueous–gas partitioning functions. Sev-model that could be later extended to SDA transporteral studies have demonstrated that adsorption of inor-problems, we also developed a model of the columnganic carbonate species produces a large reservoir ofusing the air–water mode of the numerical simulator,immobile, exchangeable C that retards transport (StrieglSTOMP. The applied version uses linear partitioningand Armstrong, 1990). In this study, we assumed thatfunctions to distribute solute mass between gaseous,sorption of inorganic C is reasonably well describedaqueous, and solid phases, and accounts for advectiveusing a constant distribution coefficient, or Kd. Dickeand diffusive transport of solutes in both the aqueousand Hohorst (1997) measured a mean Kd of 0.8 mLand gas phases. Because we also used the model tog�1 (range � 0.1–2.0 mL g�1) in batch 14C adsorptionexamine the effect of various aspects of the experimen-experiments on sediments from the SDA. Hull and Ho-tal design on the flow and transport regime, such as thehorst (2001) also found Kd � 0.8 � 0.1 mL g�1 in atemporal and spatial distribution of fluid applicationtransport experiment using a small (30.5-cm) columnand withdrawal, we developed the latter model as afilled with water-saturated SDA sediment. Other studiesquadrant of a three-dimensional cylinder, in a cylindricalof 14C sorption on natural materials have produced simi-coordinate system, with 25 nodes in each horizontallar results. Using a variety of natural sediments, Allardplane and 85 nodes along the vertical axis. Boundaryet al. (1981) measured Kd values ranging from 1.1 to 3.0conditions for the model include zero-flux boundariesmL g�1, while Martin (1991) measured values rangingfor gas, liquid, and solute at the bottom plane as wellfrom 3.5 to 4.6 mL g�1 for sediments from the Hanfordas on the vertical walls and cross-sectional planes of thesite. Much higher distribution coefficients and a depen-quadrant. At the top of the column, the 14CO2 concentra-dence of that parameter on contact time have beention is fixed at zero, total gas pressure is fixed at theobserved in several studies. Allard et al. (1981) notedaverage ambient air pressure, and a constant flux ofincreasing adsorption with time in experiments withwater is applied to approximate the spatial distributioncalcite, obtaining a Kd of 83 mL g�1 after 6 mo contactof the actual injectors. Outflow in the model is simulatedtime, and Garnier (1985) observed a positive correlationby a set of specified liquid-pressure node surfaces 15 cmbetween flow rate and 14C retardation. The contact timeabove the bottom, on the cross-sectional planes of thein transport experiments depends largely on the modecylinder at the level of the suction lysimeters. Gas flowof transport. In unsaturated sediment transport experi-at the simulated lysimeter nodes is controlled as a zero-ments, 14C transport should be dominated by 14CO2(g)flux plane, so that solute exits the lysimeters only viadiffusion. We thus expected that Kd values derived fromadvection in the liquid phase. Parameters for the vanbatch experiments on SDA sediments (Dicke and Ho-Genuchten–Mualem equations describing relationshipshorst, 1997) would predict reasonably well the effect ofbetween soil moisture, matric potential, and hydraulicsorption on redistribution of a spike of 14C applied toconductivity were estimated from measurements of hy-one of the injection planes in the column.draulic properties of the SDA sediments conducted for

this study and previous studies (Porro and Keck, 1998). The partitioning of CO2 between gas and aqueous


Table 3. Summary of transport parameters used to model gas and liquid tracers and 14C in the mesoscale column.

Aqueous diffusion Gaseous diffusion Aqueous–gas Aqueous–solidSolute coefficient coefficient partition coefficient partition coefficient

SF6 – 1.1 � 10�1 cm2 s�1 2.6 � 10�4 –Bromide 1.8 � 10�5 cm2 s�1 – – –Carbon-14 (as CO2g or HCO3

�aq) 1.0 � 10�5 cm2 s�1 1.85 � 10�1 cm2 s�1 �4.5 or pH-dependent 0.5 � 0.1 mL g�1

phases is determined from the combined solubility of model that considers reactive diffusive transport in thegas phase but ignores dissolved-phase transport. Thecarbonate species present, which depends on several

geochemical parameters. Microbial respiration occurs one-dimensional conservation equation describing thistransport process,throughout the column and CO2 concentrations increase

approximately exponentially with depth. Measured pHsthus typically range from about 7.4 near the top of the

�Cg

�t�

1R

��1 Dm�2 Cg

�2z[4]

column to 6.9 near the bottom. We calculated the dimen-sionless ratio, Klg, of total DIC per unit volume to gas- is similar to Eq. [1], except that it includes the dimen-eous C per unit volume from the measured pH and sionless retardation factor, R, to account for the effectsstandard carbonate equilibria expressions (Langmuir, of phase partitioning on diffusive transport (Weeks et1997). The calculated ratios were an excellent match al., 1982). The retardation factor is given by(Fig. 8) to those determined from activity measurementsin the aqueous and gas phases during the first few weeks R � 1 �

Klg �l

�g

�Klg Kd

�g

[5]following the 14C injection. Although Klg is relativelyuniform throughout most of the column, significantly where is the soil bulk density, �l is the volumetricmore C is contained in the aqueous phase than in the water content, Klg is the dimensionless ratio of 14C ingas phase near the top of the column, due to the higher the aqueous phase to that in the gas phase, and KdpH there and the nonlinear relationship between pH is the sorption coefficient. The shape of the diffusionand bicarbonate concentration. We incorporated this breakthrough curves is thus dependent on the volumet-variation in the aqueous–gas partitioning ratios in our ric air content, through both the retardation factor andmultiphase transport simulations by assigning an eleva- tortuosity, and on the partitioning coefficients, throughtion-dependent Klg distribution calculated by interpola- the retardation factor. In this case, we consider the aque-tion of the observed distribution. ous–gas partitioning ratio, Klg, well constrained by the

measured pH and carbonate concentrations. Through-Simulated Carbon-14 Transport and Comparison out about 90% of the column, pH is between 6.9 andwith Experimental Data 7.1; higher pH occurs only relatively near the surface.

The average aqueous–gas partitioning ratio is thus ap-Previous studies of CO2 movement in the unsaturatedproximately 4.5 � 0.3, depending on the thickness con-zone (Thorstenson et al., 1983; Lu and Ross, 1994) indi-sidered. The most uncertain parameters are the sorptioncate that 14C transport can be described by a conceptualKd, and the water content, which a variety of methodsindicate is approximately 28 � 2%. Assuming that rangefor the average water content is valid, we can calculatethe corresponding range of Kd by fitting breakthroughcurves calculated from an analytical solution to the dif-fusion equation to the observed 14C breakthroughcurves. Least-squares fitting to the 14C data yields Kd �0.45 � 0.1 mL g�1 for that soil moisture content range,with a coefficient of determination of 0.88 at 28% volu-metric water content. These Kd values are well within therange (0.1–2.0 mL g�1) measured by Dicke and Hohorst(1997) and close to the average value obtained in thatstudy. The agreement of the least-squares fit Kd valueswith those of batch studies on the same sediment andthe excellent match between computed and observedbreakthrough curves (Fig. 7) also demonstrate that, aspredicted, the gas diffusion model provides a good de-scription of 14C transport under the conditions prevalentin the column.

For multiphase transport simulations, we first devel-oped steady-state flow simulations that provided a rea-

Fig. 8. Calculated aqueous/gas partitioning ratio profile (solid line) sonable match to the flux and water content data charac-for the 14C transport experiment, based on pH measurements made teristic of the transport tests. Carbon-14 transportjust before the 14C injection and measured aqueous-gas partitioning

simulations were then conducted using the best-fit Kdratios (symbols) following the 14C injection, based on aqueous- andgas-phase 14C measurements from each sampling level. values calculated from the diffusion model. Predictably,


Fig. 10. Measured 14C aqueous-phase breakthrough curves (symbols)Fig. 9. Measured 14C gas-phase breakthrough curves (symbols) andand simulated responses (lines) at each sampling level. Simulatedsimulated responses (lines) at each sampling level. Simulated curvescurves were calculated using the multiphase flow and transportwere calculated using the multiphase flow and transport modelmodel STOMP, using the nonuniform, pH-based, aqueous/gas par-STOMP, using the nonuniform, pH-based, aqueous/gas parti-titioning ratio profile shown in Fig. 8. Average water content fortioning ratio profile shown in Fig. 8. Average water content forthe flow simulation was about 28%; Kd ≈ 0.5 mg L�1.the flow simulation was 28%; Kd ≈ 0.5 mg L�1.

the results were very similar (Fig. 9), and the only nota- column, through ports at Levels 2 and 3, and obtainedble differences appeared to result from incorporation of six ≈76-cm-long (4.5-cm-diam.) soil cores from the topnonuniform aqueous–gas partitioning in the numerical of the column. Gas and water samples were collectedsimulations. The higher Klg values in the upper part of on the same date from eight levels in the column. Gas-,the column dampened gas peak responses in that region aqueous-, and solid-phase activities determined throughand thereby produced a significant improvement in combined analysis of these gas, water, and soil samplesmodel fit at Level 8. Curiously, although calculated yielded solid–aqueous partitioning ratios of 0.8 to 0.9aqueous/gas ratios at Level 7 were significantly higher mL g�1 near the bottom of the column and between 1.2than at Level 6, the peak gas-phase responses at both and 2.4 mL g�1 near the top. Mass balance (Fig. 11)locations were nearly identical, so that incorporation of calculated from these measurements of: sorbed activity,a nonuniform Klg diminished the quality of the fit at aqueous and gaseous activities from the eight samplingthat location. Conversely, the multiphase simulation dis- levels, total 14C removed via gas and liquid samples,plays a higher peak in the aqueous Level 7 and Level total 14C lysimeters exiting via the lysimeters at the bot-8 curves (Fig. 10) than in the gas-phase curves, due to tom, and 14C venting into the headspace accounts forthe greater partitioning into that phase. That phenome- 93% of the injected 14C activity. After 1 yr, most (66%)non is apparent in the observed breakthrough curves of the injected 14C was removed from the column viafrom those levels but is much more pronounced than is upward gas release, while only 4% was removed throughpredicted by the simulation. In general these simulations downward aqueous transport. Of that remaining in thewith nonuniform aqueous–gas partitioning suggest that column, 82% is sorbed to the solid phase. Consistentsuch effects can be important where pCO2, and therefore with other studies of the effects of adsorption on 14CpH, is variable. It is likely that similar variability in transport in the unsaturated zone (Striegl and Arm-adsorption would explain much of the remaining dis- strong, 1990; Striegl and Healy, 1990), this demonstratescrepancies between observed and simulated 14C break- the large relative magnitude of the sorbed phase andthrough curves. the correspondingly large impact of that reservoir on

14C transport.Direct Measurements of Carbon-14 Sorption Although Kd values measured about 1 yr after the

pulse injection were larger than those calculated fromApproximately 1 yr after the 14C injection, we col-lected small soil samples from near the bottom of the analysis of the breakthrough curves, large-scale in-

Fig. 11. Mass balance on effluent from the column and 14C remaining in the column, from effluent monitoring and samples collected approximately1 yr after injection.


creases in that parameter with time, such as those noted These questions will be the focus of subsequent studiesby Allard et al. (1981), were not observed. On the con- that include monitoring and interpretation of 14C in thetrary, the range of Kd values measured in this study subsurface of the SDA.(0.3–2.4 mL g�1), determined using entirely differentmethods and representing times spanning a year, is en-

CONCLUSIONStirely consistent with the relatively narrow range of val-ues (0.1–2.0 mL g�1) obtained from short-term and Analysis of the mesoscale unsaturated flow and trans-small-scale adsorption studies and column studies of port experiments performed in this study demonstratesSDA sediments. The higher values obtained during the that 14C transport in the column is well described by alatter part of this study may reflect one or more of conceptual model that considers reactive diffusive trans-several factors that cannot be resolved with the present port in the gas phase but neglects aqueous-phase transport.data, including nonlinearity in the sorption isotherm, a Physical transport parameters for the large unsaturatedslight temporal dependence in adsorption, and kinetic flow column were constrained through a combinationpartitioning effects during the initial spreading of the of gas-phase and aqueous-phase tracer tests. These tests14C pulse. demonstrate that the Millington (1959) equation pro-

vides a good description of the relationship betweenImplications for Carbon-14 Transport at the SDA gaseous diffusion and moisture content in the SDA sedi-

ment and confirm results of previous studies (Hull andSimulations of 14C transport in the mesoscale column,Hohorst, 2001) that dispersivity in the sediment is onwith both a simple gas diffusion model and a multiphasethe order of several millimeters even over distances offlow and transport model, provide a good match toup to a meter. Aqueous–gas partitioning values used inobserved 14C redistribution following a pulse injectionour 14C transport simulations were calculated directlyand indicate that 14C transport in the column is domi-from pH, and aqueous/gas concentration ratios mea-nated by gas diffusion. Sediment type and thickness at

the SDA are similar to that in the column, while esti- sured during the transport experiment confirmed thosemated infiltration rates at the SDA are actually lower calculations and demonstrated the strong dependence(Case et al., 2000). Thus, unless saturated flow occurs of that parameter on pCO2.close to 14C sources, 14C migration at the SDA is also Constraints on other transport parameters provideprobably dominated by gas-phase transport. If, for the increased confidence in the Kd values we determinedmoment, we neglect advective transport and consider a by fitting simulated breakthrough curves to observa-simple one-dimensional model of the SDA, an obvious tions. Kd values determined in that manner were approx-difference between that system and our column is that imately 0.5 � 0.1 mL g�1, while values measured fromthe unsaturated zone at the SDA extends approximately soil, water, and gas phase sampling approximately 1 yr180 m beneath the contaminant source, and 14C may later ranged from 0.8 to 2.4 mL g�1. This range is consis-enter the underlying Snake River Plain aquifer by both tent with that obtained from small batch studies andgas and liquid transport. A one-dimensional model of column studies using SDA sediments and demonstratesthe SDA might therefore incorporate zero-concentra- that those laboratory-scale measurements provide reli-tion boundary conditions at both the upper and lower able data for transport modeling even at the large spatialboundaries, as opposed to our column model, which and temporal scale considered in these mesoscale exper-does not permit gas transport through the lower bound- iments. The models and parameters described in thisary. Assuming a steady-state condition is eventually study should provide a reliable basis for developingreached, the one-dimensional diffusion model would transport models of 14CO2 released from activated met-predict that relative release rates to each boundary are als buried in the SDA. As estimated infiltration ratesroughly proportional to the proximity of the source to at the SDA (Case et al., 2000) are lower than that main-the boundary. Given the approximately 50 times greater tained in the column, 14C transport at the SDA is alsodistance to the underlying aquifer, this suggests that probably dominated by gas movement. Whether or notmost of the 14C released from the SDA would ultimately that includes an advective component, the resultant fluxdischarge to the surface. While the considerable depth to the atmosphere would be expected to outweigh theand fractured nature of the system suggest that atmo- flux to the underlying Snake River Plain aquifer. Givenspheric pressure fluctuations generate significant advec- the tendency for reactive gas transport to control move-tive gas movement in the subsurface (Nilson et al., 1991; ment of 14CO2 in the unsaturated zone, this effect shouldHolford et al., 1993), that effect would likely enhance be considered when evaluating risks to atmospheric andupward gas movement at least as much as downward groundwater receptors downgradient of the SDA.movement. Clearly however, a one-dimensional diffu-sion model is an oversimplification of the problem, and a

ACKNOWLEDGMENTSrealistic estimate of upward and downward fluxes wouldhave to consider the effect of the spatial variability of The authors would like to thank two anonymous reviewershydraulic properties, seasonal changes in boundary con- for comments and suggestions that greatly improved thisditions such as freezing and infiltration of snowmelt, manuscript. Funding for this project was provided by the U.S.diffusive gas transport across air–sediment and air– Department of Energy, Office of Environmental Manage-

ment, under contract DE-AC07-99ID13727.water interfaces as well as pressure-driven gas fluxes.


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transport of carbon-14 in a large unsaturated soil … of carbon-14 in a large unsaturated soil...

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