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Surface Water–Groundwater Connection at the Los Alamos Canyon Weir Site:Part 2. Modeling of Tracer Test Results

Philip H. Stauffer* and William J. Stone

ABSTRACT to highly vesicular, to highly fractured. Although thisunit is widespread, transport behavior in the Cerros delField observations of bromide transport in the unsaturated zoneRio basalt is currently poorly understood, and betterare used to constrain simulations that provide estimates of bulk poros-

ity and permeability for the Cerros del Rio. The Cerros del Rio basalt constraints on physical properties are vital to under-is of particular interest because it underlies many of the potential standing movement of water and chemicals beneathwaste sites at the Los Alamos National Laboratory. A highly simplified large areas of the Pajarito Plateau (Birdsell et al., 2000,model is able to capture the general behavior of the breakthrough 2005; Vesselinov et al., 2002; Stauffer et al., 2000). Thisdata. The simplifying assumption is that the basalt can be modeled thick and highly fractured unit has the potential to allowas a homogeneous continuum with high permeability and low porosity. rapid transport from the surface toward the regionalWe estimate that the permeability of the bulk rock is 10�11 to 10�12 m2,

water table.whereas the porosity is estimated to lie between 0.001 and 0.01. TheTo address concerns about vertical movement of waterporosity estimates from this study are particularly useful for kilometer-

and associated contaminants from the weir pond to thescale simulations that include flow and transport through the Cerroswater table, LANL installed three monitoring boreholesdel Rio basalt because estimates based on other methods, such as core

testing, are highly scale dependent and should not be extrapolated (Fig. 1). Details concerning construction and location ofto larger scales. Although this model does not include the complex the boreholes can be found in Stone and Newell (2002)physics of flow in the fractured basalt, it is useful for simulations on and Levitt et al. (2005). Researchers at LANL have takenthe kilometer scale that require averaging of rock properties and op- advantage of the weir site to establish a tracer experi-timization of computational speed. The porosity and permeability values ment with the objective of learning more about flow andobtained from this analysis will help to weight probability distributions

transport through the basalt under transient unsaturatedused in the kilometer-scale simulations of contaminant transport.conditions (Levitt et al., 2005). The bromide tracer testat the Los Alamos Canyon Low-Head Weir provides anexcellent opportunity to study the transport propertiesThis paper is the second in a two part series. Part 1of the Cerros del Rio basalt.(Levitt et al., 2005) includes the details involved in

In this study we used bromide tracer data from thesetting up and executing an unsaturated tracer test inlow-head weir tracer test to reduce uncertainties in boththe highly fractured Cerros del Rio basalt. Part 1 alsoporosity and permeability in the Cerros del Rio basaltincludes all of the data collected as part of this experi-using a highly simplified single continuum model. Al-ment. These data are referenced, but replicated in Partthough these simulations do not include the complex2. In Part 2 we briefly summarize the material from Partphysics of flow in the fractured basalt, the simplified1, then use the results of the tracer test in Part 1 tomodel is useful for simulations on the kilometer scaleperform a numerical analysis of the data with the goalwhere rock properties must be generalized due to lackof providing estimates for porosity and permeability ofof data and limits on computational size and speed.the Cerros del Rio basalt.The porosity and permeability values obtained fromIn the summer of 2000, the Army Core of Engineersthis analysis will help to weight distributions of theseconstructed a low-head weir to trap sediment near theparameters used in the kilometer-scale simulations thatLaboratory’s eastern boundary in Los Alamos Canyon.are designed to predict future risk associated with trans-During construction, the underlying bedrock, the Cerrosport of contaminants found within the Los Alamos Na-del Rio basalt, was exposed. The Cerros del Rio basalttional Laboratory.is found in many of the regional boreholes that have

been drilled around the laboratory and can be seen inoutcrops throughout Los Alamos County (Broxton and Previous ModelingVaniman, 2005; Woldegabriel et al., 2001). Turin (1995) Birdsell et al. (2000) conducted a numerical modelingcharacterized rock samples from the Los Alamos area study of flow and transport in the vadose zone beneath aand found that the Cerros del Rio basalt was highly waste disposal area at LANL using FEHM, a multiphasevariable, ranging from dense with no apparent porosity, porous simulation code (Zyvoloski et al., 1997). There

were very little hydrologic data available at this time,P.H. Stauffer and W.J. Stone, Los Alamos National Laboratory Earth and the basalts were modeled as an equivalent contin-and Environmental Science Division Hydrology, Geochemistry, and uum that incorporated fracture and matrix properties.Geology EES-6, Los Alamos, NM 87545. Received 12 Aug. 2004.

The hydrologic properties were based on equivalent*Corresponding author (stauffer@lanl.gov).continuum basalt properties for Idaho basalt published

Published in Vadose Zone Journal 4:718–728 (2005). in Bishop (1991). As a conservative assumption, BirdsellSpecial Section: Los Alamos National Laboratory et al. (2000) assumed that the porosity of the basaltdoi:10.2136/vzj2004.0121

matrix was very low.© Soil Science Society of America677 S. Segoe Rd., Madison, WI 53711 USA Vesselinov et al. (2002) included the Cerros del Rio

718

Published online August 16, 2005

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www.vadosezonejournal.org 719

pond and the vertical Borehole LAWS-01. Importantassumptions implicit in this simplification were that thepond is long relative to its width and that infiltrationduring ponding events is constant along the length ofthe pond. Additionally, the pond and bank are approxi-mately symmetric about the vertical plane running alongthe center of the pond in the direction of stream flowand we have made the center line of the weir pond asymmetry (reflecting) boundary. Because the data fromLAWS-01 are located well below the pond floor, thedomain can be truncated at the top by the horizontalplane representing the pond floor. The lateral boundaryof the domain (100 m from the pond center line) liesfar enough from the pond so infiltration in the pond doesnot propagate to this distance. The bottom boundary issituated below the area of interest (the deepest port inLAWS-01) and conceptualized as a drain in this study.

Background InfiltrationLong-term average infiltration on the Pajarito Plateau

is a complex function of precipitation distribution, run-off patterns, stream channel location, and bedrock expo-sure. Estimated values range from near zero on the mesatops to tens of centimeters per year within stream chan-nels that have good connectivity to high permeabilitybedrock, such as the Cerros del Rio basalt. The concep-

Fig. 1. Schematic of the experimental setup. This figure portrays a tual model of infiltration used in this study is based onvertical slice perpendicular to the weir at the location of the moni- Rogers et al. (1996) and assumes that infiltration occurstoring boreholes. Monitoring Ports 1 through 4 are shown as red

primarily in existing stream channels while the sur-circles on LAWS-01 (from Levitt et al., 2005).rounding stream banks and mesas allow very little waterto infiltrate. We used an intermediate value of 0.1 m3/basalt in simulations of drawdown associated with pump-(m2 yr) for infiltration in the stream channel (Rogersing in the Los Alamos area. The drawdown simulationset al., 1996; Kwicklis, 2005), while the rest of the topare sensitive to basalt permeability; however, the poros-boundary was conceptualized as being quite dry withity of this unit is not constrained in these simulations.effectively zero net long-term infiltration as suggestedThis model forms a part of the baseline groundwaterby Birdsell et al. (2000). Although background infiltra-pathway assessment being developed at LANL, and po-tion is important in establishing an initial saturationrosity in the basalt will be important for constrainingprofile in the subsurface for the bromide tracer simula-predictions of contaminant travel times and pathwaystions, infiltration during the ponding events is manythrough the subsurface.orders of magnitude greater and should dominate tracermovement at this site.MODEL DEVELOPMENT

To create a numerical representation of the bromide Tracer Application and Ponding Eventstracer experiment, the physical system must be simpli- The amount of bromide per square meter applied tofied from the highly complex conditions found at the the pond floor in the cross section was calculated as thefield site. Specific physical complexities at the site that total mass of applied bromide (45 kg KBr) divided bymust be addressed include complex three-dimensional the total area of the pond floor (1821 m2) as describedtopography, heterogeneous subsurface rocks, hetero- in Levitt et al. (2005). Thus, 16.8 g m�2 bromide wasgeneous distribution of water in the unsaturated zone, assumed to be the maximum amount of bromide avail-and a discrete fracture network. Furthermore, the time able for transport into the subsurface. Our conceptualhistory of tracer application and movement into the model of transport at the site allows some portion ofsubsurface must be simplified to allow efficient simula- the applied bromide to be swept downstream and out oftion of the experiment. The simulations also include the area of interest. Mechanisms for loss of the appliedsimplifications of preexperimental conditions and both bromide include windblown dust removed before thelateral and vertical boundaries. first ponding event or mixing of tracer in the standing

pond water, part of which then flows through the weirDomain Geometry and out of the system. Very high concentrations of bro-mide will increase the density of the infiltrating waterThe simulations idealize the tracer experiment as a

two-dimensional cross section. The cross section used during the initial wetting of the pond floor; however,this effect on gravity driven flow was ignored in thein the analysis runs parallel to the weir through the

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720 VADOSE ZONE J., VOL. 4, AUGUST 2005

Fig. 4. Pond depths recorded behind the weir for the four events.Because the last two events were closely spaced in time, they werecombined as a single event for the conceptual and numericalmodeling.

experiment. Although head data are available for theponding events (Fig. 4), the total flux through the systemis poorly constrained. Simultaneous inflow and outflowmeasurements were not recorded with sufficient detailto allow calculation of infiltration beneath the pond.Therefore, the amount of infiltration occurring duringeach ponding event was allowed to vary. Allowing theinfiltration from each ponding event to vary is supportedby the Idaho Box Canyon tracer test, where measured

Fig. 2. Bromide concentration vs. time from LAWS-01. Time zero infiltration vs. time was difficult to simulate and presim-corresponds to 22 June 2002. Missing data from Ports 1 and 2 at ulation predictions were not accurate (Doughty, 2000;late times is caused by the ports going dry.

Faybishenko et al., 2000).

simulations. We assumed that mixing quickly reduces Rock Propertiesconcentrations and that gravity driven flow is not af-fected by increased density due to dissolved bromide. Well logs from the boreholes drilled at the site showed

Four ponding events occurred within the time period that the basalt varies from vesicular to massive; how-of interest (330 d starting on 21 June 2002) and are ever, how these qualities affect the hydrologic propertiesshown on Fig. 2. These events were described in more is unknown. The degree of fracturing was observed todetail in Levitt et al. (2005). The last two events (at 125 vary in the borehole videos, but whether these observa-and 137 d) were closely spaced in time, and these events tions are representative of the stratigraphy as a wholewere lumped together in the simulations. Figure 3 shows or are a random picture through the stratigraphy isa schematic of the geometry designed to represent the unknown. Significant variations in permeability and po-

rosity are found within the Cerros del Rio basalt. Asummary of Cerros del Rio basalt material propertyvalues determined through field pumping tests and labo-ratory measurements of rock samples are listed in Table1 and Table 2. As a first-order approximation of thesystem, we envision that the bulk of flow and transportoccurs through a network of fractures and rubble zones,with some interaction with the matrix porosity. Rubblezones and vesicular lenses are not fractures in the tradi-tional sense; however, these zones will create effective,nonmatrix porosity in the rock mass well above esti-

Table 1. Field measured Cerros del Rio basalt hydraulic conduc-tivity (Broxton, 2001; Broxton et al., 2002; Vaniman et al.,2002).

Well Permeability

m2

R-9 1.12E-15R-9i 1.33E-11R-9i 2.84E-13R-31 1.44E-12-2.52e-12R-22 9.71E-14Fig. 3. Schematic of model domain with boundary conditions.

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Table 2. Cerros del Rio basalt properties measured on cores (Springer, 2005, personal communication).

Permeability Porosity van Genuchten � Residual saturation van Genuchten n

m2 m�1

1.15E-18 0.09 1.60 0 1.254

mates based solely on fracture spacing and aperture data from Port 4. To capture the effects of saturated(Doughty, 2000). We assumed that the highly vesicular conditions in the perched region, this region was as-lenses and rubble zones fill and drain rapidly and can signed unsaturated parameters that maintain saturationbe added into the total transport and flow porosity. The above 95%. Capillary forces were also set to be negligi-effect of the matrix is grossly captured by the assigned ble (�0.01 MPa). This created a fixed region of nearlywater retention properties; however, we did not attempt saturated conditions through which water from the pond-to simulate dual porosity effects such as diffusion into ing events must move to reach the deepest samplinga separate matrix porosity (Liu et al., 2003). This simpli- port. Because of the high saturations and increased liq-fication allowed us to simulate the basalt as a homoge- uid phase continuity, dispersivity in the perched regionnous unit and constrain the bulk transport properties of was increased relative to dispersivity in the unsaturatedpermeability and porosity within this complex lithologic region above.unit. We note that some communication between matrixand fractures is likely occurring, and address this further

Data Matchingin Results and Discussion.Clay layers within the basalt have been found at many The data from a vertical borehole, LAWS-01 with

locations around the laboratory, but the connectivity four screened intervals, provide a high resolution timeand extent of individual clay layers is not well under- history of bromide breakthrough and form the basis forstood. If the clay layer found at 41 m in LAWS-01 was the analysis presented here. Two other boreholes drilledcontinuous and extensive, it would be quite difficult to at the site, LAWS-02 and LAWS-03, were angled holesmove tracer through this low permeability material to that both encountered serious problems. LAWS-02 pro-Ports 2 through 4 in the required time. Therefore, we vided very limited bromide data and some supportingassumed that the clay layer is not continuous and did not

moisture data while LAWS-03 provided no useful data.include it in our simulations. The thin alluvium shown inThese holes were discussed in detail in Levitt et al.Fig. 1 is also not explicitly included in the simulations,(2005).but the effects of this material are indirectly included

The data from LAWS-01 are shown in Fig. 2 and spanthrough their ability to reduce the amount of water that330 d from late May 2002 to early May 2003, with timeinfiltrates during the ponding events.0 d set to coincide with the first ponding event of 21June 2002. Bromide was seen at low concentrations afterPerched Waterformation of the first pond from time 0 to 60 d in Ports

The water level data for this site are presented in 2 and 3. The bulk of the bromide breakthrough occurredLevitt et al. (2005), and in this section we give a brief after the second ponding event at 67 d, when a coherentsummary of how the water level data relate to the mod- trend was observed. Integration of the area under theeling. For LAWS-01 Port 1, perching is not an issue breakthrough curves can provide a rough quantificationbecause virtually no perched water was found. For Port of the size of the small breakthrough seen in the first2, the perched water is generally 0 to 2 m except for

60 d to the breakthrough seen after 60 d. The ratiotwo increases to approximately 5 m in response to: (i)of early/late integrated breakthrough showed that thethe ponding at 67 d and (ii) the combined signal of theearliest breakthrough (0–60 d) represents �10% of theponding events at 125 and 137 d. Increased water levelstotal time-integrated mass seen in the sampling ports.in Port 2 dissipate quickly and imply that this perchedFor this reason, this study addressed breakthrough oc-zone allows water to move through the system rapidlycurring only after the second ponding event at 67 d.(15–30 d). Port 3 exhibits similar behavior to Port 2 withThe justification for separating the early time data froman average water level of 1 to 3 m with the same twothe later time data is based on our conceptual modelperturbations rising to approximately 10 m and drainingof the fractured system. We propose that a bimodalback toward original water levels in the same timeframe.distribution of fractures could lead to a small percentageThe final perched horizon extends from Port 4 upwardof the bromide quickly reaching depth while the bulkapproximately 22 m and maintains a relatively constantof the transport occurs through a slower fracture net-head during the timeframe of the analysis. The deep

perched water is included in this study to better fit work with higher effective porosity.

Table 3. Cerros del Rio basalt properties used in other modeling studies.

Permeability Porosity van Genuchten � Residual saturation van Genuchten n Model

m2 m�1

3.02E-13 0.3 0.10 0.066 2.000 Vesselinov et al. (2002)9.71E-14 1e-4 3.84 6.6e-6 1.474 Birdsell et al. (2000) matrix9.71E-07 1e-4 3.84 3e-6 1.474 Birdsell et al. (2000) fracture

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722 VADOSE ZONE J., VOL. 4, AUGUST 2005

system is isothermal (20�C) with constant air pressure(0.101 MPa) at the land surface. At the specified pres-sure and temperature, water density is approximately1000 kg m�3. Water pressure varies only in response tochanges in saturation due to coupling in the unsaturatedflow equations described below.

Model DomainThe model domain consists of a 100 by 100 m square

and spans the region shown schematically in Fig. 3. Thefinite volume grid is rectangular with horizontal spacingof 1.0 m and vertical spacing of 0.5 m. This arrangementleads to a grid consisting of 20 000 elements and 20 301nodes.

Boundary ConditionsFig. 5. Simulated water pressure vs. saturation.

The lateral boundaries are both no-flow. The left-most 14 m on the top of the grid are assigned infiltrationMODEL APPLICATION corresponding to the stream channel for the initial back-

Simulation Logic ground and the ponding events for the tracer simula-tions. The remaining top boundary is assigned no-flow.Because the goal of this paper was to determineThe bottom boundary is assigned a slightly lower satura-bounds on the bulk permeability and porosity of thetion than the steady-state background values for thebasalt in a simplified single continuum, we limited thenodes immediately above, causing the bottom of theparameters that are varied in a given simulation. Poros-numerical grid to act as a drain for water. The bottomity was chosen as the primary variable to be variedis far enough below Port 4 so that this numerical conve-systematically in our simulations. Saturated permeabil-nience in mimicking a free drainage condition shouldity, bromide input signal, and pond infiltration werenot impact flow or transport within the region of inter-then varied to achieve an approximate fit to the generalest. Flux out the bottom was checked to ensure that thecharacter of the data for each value of porosity explored.fixed saturation bottom boundary is acting as a sourcefor water, and at all nodes along the bottom of the

General Simulation Details model water is indeed exiting the domain.The simulations were run with FEHM (Zyvoloski et

al., 1997), a multidimensional finite-volume heat- and Initial Conditionsmass-transfer computer code capable of simulating non- The initial conditions for the simulations of bromideisothermal multiphase, multicomponent flow in porous transport were generated by running a backgroundmedia. The governing equations in FEHM are derived streambed infiltration of 10 cm yr�1 to steady state. Thefrom the principles of conservation of mass and energy. measured background concentration of bromide in theDarcy’s Law was assumed valid for the flow of the air region was on the order of 0 to 1 mg Br� kg�1 water, andand water phases. Unsaturated flow driven by the com- we chose to run the simulations with an initial bromidebined forces of gravity and capillary suction is included concentration of zero everywhere in the domain. Be-in FEHM. The retention properties of the porous media cause the tracer breakthrough reaches values of morecan be represented by several different theoretical forms than 20 mg kg�1, this slight shift in the initial condition(van Genuchten, 1980; Corey, 1954) in FEHM. Solute does not affect the broad conclusions of the simulations.transport in FEHM is governed by the advection–dis-persion equation (Fetter, 1999). For a discussion of the Pond Timingassumptions regarding solute transport incorporatedinto the numerical model see Zyvoloski et al. (1997). For all simulations, there were three ponding events

representing (i) the June 2002 event at 0 d simulationValues of porosity, permeability, and van Genuchtenparameters used in previous numerical modeling studies time, (ii) the August 2002 event at 67 d simulation time,

and (iii) the combined October and November 2002are listed in Table 3. Figure 5 shows the water pressureas a function of saturation for the van Genuchten prop- events at 125 d simulation time. During the first two

events, pond infiltration was applied evenly in a 3-derties that were used in the simulations. Temperatureand pressure variations at the land surface were assumed period. For the last event, the duration of infiltration

was varied from 6 d for the lowest infiltration simulationto have very minor impacts on the flow and transportcaused by the ponding events. Vapor transport and (0.25 m3 m�2) to 15 d for the simulation with the highest

infiltration (1.25 m3 m�2). The pond infiltration wasbarometric pumping in the subsurface were also notaddressed because these mechanisms produce relatively stretched over 3 d to allow the simulation to run more

quickly. We note that the highest rates calculated bysmall driving forces with respect to the large infiltrationevents that drive the bromide transport. The simulated applying the total infiltration (0.25 m) by the actual

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Table 4. Simulation independent variables. results of the simulations. Table 4 summarizes the modelparameters that were fixed for all simulations.Variable Value

van Genuchten � (unperched section), m�1 3.84van Genuchten n (unperched section) 1.474 RESULTS AND DISCUSSIONResidual saturation (unperched section) 0.01Bromide diffusion coefficient, m2 s�1 3.0 � 10�10

Porosity was chosen as the primary variable to beLongitudinal and transverse dispersivity in the 0.5unperched section, m varied systematically in our simulations. Saturated per-

Longitudinal and transverse dispersivity in the 2.0 meability, bromide input signal, and pond infiltrationperched section, mwere then varied until the simulated bromide concentra-Background infiltration in the stream bed, m yr�1 0.1

Background bromide concentration, mol kg�1 0.0 tions at Ports 1 through 4 were brought within the rangeof the observed data (0–25 mg kg�1). We reiterate thatthe simulations are meant to provide guidance for bulklength of time of ponding (≈0.4 d) were on the orderrock properties and not to recreate complex fractureof 0.6 m d�1 and imply very high conductivity in theflow physics as in models such as Doughty (2000). Tonear surface. The actual calculated rates were muchsummarize the results of this study, we chose four simu-greater than the 0.01 to 0.12 m d�1 reported by Doughtylations (S-1, S-2, S-3, and S-4) that span the range in(2000); however, the pond floor reported in their studyporosity that is able to capture the general behavior ofhad significant soil in the near surface and was notthe late-time (�60 d) arrival data. The simulations spanfreshly scraped.one order of magnitude of porosity from 0.001 (0.1%) toFluid flow and tracer time-step size during infiltration0.01 (1%). Values for the variable parameters (porosity,were kept small (0.2 and 0.02 d, respectively) to accu-saturated permeability, and pond infiltration) used inrately capture the transient pulses. For the intervalsthe four representative simulations are listed in Tablebetween the ponding events, the time-step size is in-5. Figure 6 shows the simulated time breakthrough re-creased to 2.0 d for fluid flow and 1.0 d for transport.sults for bromide at the ports in LAWS-01. These imagesinclude bromide data from Ports 1 through 4 in BoreholeBromide InputLAWS-01 for comparison.

To maintain a consistent approach between simula- When porosity is reduced in the simulations, the satu-tions for the movement of bromide from the pond floor rated permeability also must be reduced because theto the subsurface, bromide concentrations were fixed true velocity of the water is directly proportional toin the initial pulse of infiltrating water such that the permeability and inversely proportional to porosity. Inentire simulated bromide mass was applied in the first the Darcy experiment, as porosity drops by a factor of0.01 d for all simulations. Subsequent pulses of water 10, the saturated permeability must also drop by a factorwere input with zero concentration. of 10 to keep the travel time constant. Similarly, our

results show that Simulation S-1 requires a permeabilityDispersion and Diffusion one-tenth that of Simulation S-4.As porosity was reduced in the sensitivity analysis,Dispersivity of 0.5 m in both the longitudinal and

transverse directions was fixed in the unsaturated sec- bromide mass loading needed to be reduced. Bromidemass loading for Simulation S-1 is a factor of 5 less thantion of the model for all the simulations presented in this

paper, and was approximately equal to the numerical for S-4. S-1, with a porosity of 0.1%, requires that 80%of the applied bromide be swept downstream and outdispersivity inherent in the grid (1/2 grid spacing). In

the perched region, a value of 2.0 m in both longitudinal of the system. Simulations using lower porosity wouldrequire that an even smaller fraction of the appliedand transverse directions was fixed for the simulations

presented. Because the signal in Port 4 is not very strong bromide enter the subsurface. We do not know whatfraction of the applied bromide was swept downstream.relative to the upper three ports, we chose to simply

report the dispersivity that gave the best fit in the prelim- However, the fact that the bromide was dissolved inliquid that was sprayed onto the dry floor of the weirinary sensitivity study. Similar results could have been

achieved by either increasing the horizontal/vertical per- pond suggests that the applied bromide was not merelylying on the surface but most likely would have beenmeability ratio or decreasing the porosity in this region.

Molecular diffusion in all cases was fixed at 3 � 10�10 m2 pulled a few centimeters into the ground by capillarysuction. Therefore, the S-1 is presented as a lower likelys�1; however, because of the rapid advective transport in

this experiment and the lack of simulated matrix storage, bound on the bulk porosity of the basalt. At the highend of the porosity explored, 1% in S-4, the simulationchanges in the diffusion coefficient did not affect the

Table 5. Variable parameters used in Simulations S-1 through S4.

Volume infiltratedPond 3 Bromide

Simulation Porosity Permeability Pond 1 Pond 2 Pond 3 duration mass loading

m2 m3 m�2 d g m�2

S-1 0.001 1.E-12 0.0025 0.025 0.25 6. 3.2S-2 0.005 1.E-11 0.025 0.125 1.0 12. 9.0S-3 0.0075 1.E-11 0.025 0.25 1.25 15. 16.8S-4 0.01 1.E-11 0.125 0.25 0.75 9. 16.8

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724 VADOSE ZONE J., VOL. 4, AUGUST 2005

Fig. 6. Comparison of bromide data and simulated bromide concentrations at the sampling ports in Simulations S-1, S-2, S-3, and S-4. Simulationspecific parameters are shown in Table 5.

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www.vadosezonejournal.org 725

Fig. 7. Saturation in the model domain as a function of time for Simulation S-3. In this figure, the simulated Borehole LAWS-01 lies at x � 25m, while Ports 1 through 4 are located at elevations of z � 76.5, 54, 44.5, and 22 m, respectively. The approximate locations of the ports aremarked on the images as white circles.

does not generate the sharp spike seen at approximately and Simulation S-4 is presented as a likely upper boundon the porosity of the basalt.120 d (Fig. 6). This is due to the larger volume available

for transport at higher porosity. The increased volume The four simulations lead to similar breakthroughcurves, but the magnitudes of the ponding events re-acts as storage and limits the magnitude of spikes caused

by increased flow. Higher porosity simulations resulted quired to fit the data were quite different (Table 5). Asthe porosity of the rock was reduced, the total amountin progressively lower peak concentrations in all ports

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726 VADOSE ZONE J., VOL. 4, AUGUST 2005

Fig. 8. Bromide concentration in the model domain as a function of time. Results are for Simulation S-3. Maximum initial concentrations in thesimulations are more than 10 000 mg kg�1; however, the color bar has been truncated at 40 mg kg�1 to more clearly show the growth of theplume. In this figure, the simulated borehole LAWS-01 lies at x � 25 m, while Ports 1 through 4 are located at elevations of z � 76.5, 54,44.5, and 22 m, respectively. The approximate locations of the ports are marked on the images as black circles.

of water necessary to carry the bromide to depth was In all cases the first ponding event requires significantlyless infiltration while the third event requires the highestalso reduced. In the case of the lowest porosity simula-

tion, S-1, the total water allowed to enter the pond floor amount of infiltration. This result is somewhat puzzlinggiven that the pond depth during the first event was theduring the first event was 50 times lower than for the

high porosity case (S-4). Similarly, the second event for highest, but it is consistent with the pressure transducerdata presented in Levitt et al. (2005). The pressure trans-S-1 is best fit with 10 times less infiltration than for S-4.

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ducer data show that the first event led to little change CONCLUSIONSin pressure at the Port 2 and 3 transducers, while the This study shows that representing the Cerros del Riothird event produced the largest signal, implying that basalt as single continuum may be a reasonable ap-this event had the greatest infiltration. proach for large-scale simulations of flow and transport

Figures 7 and 8 show a time series of saturation and within the Pajarito Plateau. Simulations of a bromidebromide concentration in the simulation domain for transport experiment through the Cerros del Rio basaltSimulation S-3. Time zero corresponds to the simulated suggest that this unit has bulk porosity in the range ofbackground conditions. These images highlight how wa- 0.001 to 0.01 and a saturated permeability in the rangeter and tracer move through the subsurface. In Fig. 7, of 10�11 to 10�12 m2. The properties developed in thisinfiltration resulting from the three simulated ponding analysis can be used as conservative approximations forevents can be seen at 3, 71, and 142 d. The bulk of the bulk flow and transport through the Cerros del Rioinfiltrating water moves nearly vertically downward basalt for use in kilometer-scale calculations of humanfrom the pond. Lateral perturbations to the background health and environmental impacts.saturation profile propagate �40 m from the center ofthe pond (x � 0.0), which can be seen by following the

ACKNOWLEDGMENTS20% saturation contour for the duration of the simula-This work was funded through emergency funds providedtion. The most interesting conclusion drawn from Fig.

to the DOE and LANL to remediate damage and address8 is that the location of Borehole LAWS-01 was welldemonstrated vulnerabilities associated with the Cerro Grandechosen to monitor breakthrough during the experiment.fire. This work has been conducted by LANL through thePlaced laterally 11 m from the edge of the ponding area, Rehabilitation Project to address near- and long-term activi-

ports in LAWS-01 are in the region where bromide is ties required for the Laboratory to fully recover from thepushed by the ponding events and left behind at mea- Cerro Grande fire. This work was carried out as part of thesurable concentrations. This bromide fringe, especially Well-Head Protection Task of the Rehabilitation Project.

Helpful reviews were provided by Kay Birdsell and Danielapparent from 127 d onward, is much less susceptible toLevitt, and Monty Vesselinov, and one anonymous reviewer.flushing by later ponding events. The simulations sug-

gest that a borehole drilled directly in the axis of theweir pond (x � 0.0) would have needed much higher fre- REFERENCESquency sampling to capture the bromide breakthrough. Birdsell, K.H., B.D. Newman, D.E. Broxton, and B.A. Robinson.Additionally, the modeling shows that a borehole at x � 2005. Conceptual model of vadose zone flow and transport beneath

the Pajarito Plateau, Los Alamos New Mexico. Available at www.0.0 would have been flushed rapidly, providing less in-vadosezonejournal.org. Vadose Zone J. 4:620–636 (this issue).formation about the overall system.

Birdsell, K.H., A.V. Wolfsberg, D. Hollis, T.A. Cherry, and K.M.Better matches to the data may be possible using Bower. 2000. Groundwater flow and radionuclide transport calcu-more complex models, but the goal of this study was to lations for a performance assessment of a low-level waste site.fit the data with a single continuum approximation and J. Contam. Hydrol. 46:99–129.

Bishop, C.W. 1991. Hydrologic properties of vesicular basalt. M.S.gain insight into the bulk permeability and porositythesis. University of Arizona, Tucson.within the Cerros del Rio basalt. The more complex

Broxton, D.E. 2001. Characterization Well R-9 completion report.dual porosity or discrete fracture models that have been Rep. LA-13742-MS. Los Alamos Natl. Lab., Los Alamos, NM.developed in the literature require more parameters, Broxton, D.E., and D.T. Vaniman. 2005. Geologic framework of a

groundwater system on the margin of a rift basin, Pajarito Plateau,which cannot realistically be measured at the kilometerNorth-Central New Mexico. Available at www.vadosezonejournal.scale. Additionally, more complex fracture matrix inter-org. Vadose Zone J. 4:522–550 (this issue).actions would increase travel times through the basalt Broxton, D.E., D.T. Vaniman, W.J. Stone, S.G. McLin, M.C. Everett,

as tracer or contaminants are moved into the low perme- and A.J. Crowder. 2002. Characterization Well R-9i completionreport. Rep. LA-13821-MS. Los Alamos Natl. Lab., Los Alamos, NM.ability matrix. In contrast, the single continuum approxi-

Corey, A.T. 1954. The interrelation between gas and oil relative per-mation minimizes travel time. This makes our model andmeabilities. Producers Monthly 19:38–41.predictions of timing of impacts on local water supply Doughty, C. 2000. Numerical model of water flow in a fractured basalt

wells conservative with respect to human health calcula- vadose zone: Box Canyon site, Idaho. Water Resour. Res. 36:3521–tions. Because the basalt spans many square kilometers 3534.

Faybishenko, B., C. Doughty, M. Steiger, J.C.S. Long, T.R. Wood,in site-wide simulations of transport beneath Los Alamos,J.S. Jacobsen, J. Lore, and P.T. Zawislanski. 2000. Conceptualthe simplified single continuum approach is particularlymodel of the geometry and physics of water flow in a fractured

attractive because there will be fewer uncertain parame- basalt vadose zone. Water Resour. Res. 36:3499–3520.ters that must be characterized and given probability dis- Fetter, C.W. 1999. Contaminant hydrogeology. Prentice Hall, Engle-

wood Cliffs, NJ.tributions.Kwicklis, E.M., M. Witkowski, K.H. Birdsell, B. Newman, and D.Finally, our results show that the simulated porosity

Walther. 2005. Development of an Infiltration Map for the Los Ala-and permeability are highly sensitive to the surface flux mos Area, New Mexico. Available at www.vadosezonejournal.org.of both water and the fraction of bromide swept down- Vadose Zone J. 4:672–693 (this issue).

Levitt, D.G., W.J. Stone, D.L. Newell, and D.S. Wykoff. 2005. Surfacestream. With better experimental constraints to measurewater–groundwater connection at the Los Alamos Canyon Weirvolume flux and near surface concentration that infil-Site. Part 1: Installation, monitoring and tracer test. Available attrates, we envision that a future tracer test and addi- www.vadosezonejournal.org. Vadose Zone J. 4:708–717 (this issue).

tional simulations could be used to reduce uncertainty Liu, H.-H., C.B. Haukwa, C.F. Ahlers, G.S. Bodvarsson, A.L. Flint,and W.B. Gurtal. 2003. Modeling flow and transport in unsaturatedin the porosity and permeability estimates.

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urna

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by S

oil S

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eric

a. A

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.

728 VADOSE ZONE J., VOL. 4, AUGUST 2005

fractured rock: An evaluation of the continuum approach. J. Con- Vaniman, D., J. Marin, W. Stone, B. Newman, P. Longmeir, N. Clay-tam. Hydrol. 62–63:173–188. ton, R. Lewis, R. Koch, S. McLin, G. WoldeGabriel, D. Counce,

Rogers, D.B., B.M. Gallaher, and E.L. Vold. 1996. Vadose zone infil- D. Rogers, R. Warren, E. Kluk, S. Chipera, D. Larssen, and W.tration beneath the Pajarito Plateau at Los Alamos National Labo- Kopp. 2002. Characterization Well R-31 completion report. Rep.ratory. New Mexico Geological Society Guidebook, 47th Field Con- LA-13910-MS. Los Alamos Natl. Lab., Los Alamos, NM.ference, Jemez Mountains Region. NM Geol. Soc., Las Cruces, NM. Vesselinov, V.V., E.H. Keating, and G.A. Zyvoloski. 2002. Analysis

Stauffer, P., B. Robinson, and K. Birdsell. 2000. Modeling transport in of model sensitivity and predictive uncertainty of capture zones inLos Alamos Canyon: Effects of hypothetical increased infiltration the Espanola Basin regional aquifer, Northern New Mexico. LA-after the Cerro Grande Fire. Rep. LA-UR-00-5923. Los Alamos UR-02. Los Alamos Natl. Lab., Los Alamos, NM.Natl. Lab., Los Alamos, NM. Woldegabriel, G., R.G. Warren, D.E. Broxton, D.T. Vaniman, M.

Stone, W.J., and D.L. Newell. 2002. Installation of the monitoring Heizler, E.C. Kluk, and L. Peters. 2001. Episodic volcanism, petrol-site at the Los Alamos Canyon low-head weir. Rep. LA-13970.ogy, and lithostratigraphy of the Pajarito Plateau and adjacent areasLos Alamos Natl. Lab., Los Alamos, NM.of the Espanola Basin and the Jemez Mountains. Rep. LA-UR-Turin, H.J. 1995. Subsurface transport beneath MDA G: A conceptual01-2616. Los Alamos Natl. Lab., Los Alamos, NM.model. Rep. LA-UR 95-1663. Los Alamos Natl. Lab., Los Ala-

Zyvoloski, G.A., B.A. Robinson, Z.V. Dash, and L.L. Trease. 1997.mos, NM.Summary of the models and methods for the FEHM application-van Genuchten, M.Th. 1980. A closed-form equation for predictinga finite element heat- and mass-transfer code. Manuscript LA-the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am.

J. 44:892–898. 13307-MS. Los Alamos Natl. Lab., Los Alamos, NM.