1 INTRODUCTION

Fort Wainwright is located adjacent to Fairbanks incentral Alaska within the Tanana and Chena Riverwatersheds (Fig. 1). Discontinuous permafrost occursthroughout the region, which experiences large rangesin seasonal temperature (between �40 and �32°C)and small amounts of precipitation (under 0.3 m/yr).Petroleum hydrocarbon contamination released fromformer fuel storage tanks, abandoned pipes andpumps have introduced non-aqueous-phase liquids(NAPLs) at the water table, requiring site characteri-zation and analysis to identify contaminant migrationpathways for remediation.

Beneath Fort Wainwright, irregularly shaped, imper-meable bodies of discontinuous permafrost occur inalluvial sediments and fractured bedrock and deter-mine aquifer shapes and distribution. The complexity

of the aquifer distribution has prevented simple estima-tions of the directions and rates of contaminant migra-tion that are essential to ongoing remedial efforts atmultiple sites across the main cantonment area. Factorsincluding the unknown distribution and geometry ofthe permafrost, intricate alluvial stratigraphy, pres-ence of a fractured and faulted bedrock aquifer, andlarge fluctuations in the stages of the Chena and TananaRivers make it difficult to define each site’s hydroge-ology. Traditional methods were therefore not suffi-cient to develop geological or ground water models.

Thus, the ground water system cannot be suffi-ciently understood without first determining wherepermafrost is located. In order to address this prob-lem, we developed geophysical methods to investigatethe subsurface geology. The vertical and lateral extentof permafrost were predicted based on these geophys-ical data, and in conjunction with information acquiredfrom ground water measurements and field mapping,three-dimensional model simulations of the per-mafrost and aquifer distribution allowed us to definesite-specific ground water flow directions and ratesand contaminant migration pathways.

2 GEOLOGIC SETTING

2.1 Surficial geology

The stratigraphy of the Fort Wainwright area consistsof the Chena Alluvium, a complex sequence of fluvialdeposits that overlie heavily weathered metamorphicbedrock referred to as the Birch Hill Schist (Péwé1958). The fluvial materials include layers of sand androunded gravels inter-bedded with numerous silt lenses,

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Predicting the extent of discontinuous permafrost and affiliated ground water aquifers

S.E. Kopczynski*, D.E. Lawson†, S. A. Arcone* & C.F. Snyder‡

*Cold Regions Research and Engineering Laboratory, Hanover, NH, USA†Fort Richardson, AK, USA‡YEC Consulting, New York, NY, USA

ABSTRACT: Hydrocarbon spills at a former tank farm on Fort Wainwright, near Fairbanks Alaska, required adelineation of the contaminant pathways in aquifers within discontinuous permafrost. Ground water flow is com-plicated by large, irregularly shaped and impermeable bodies of frozen alluvial sediment that create aquifers thatare of irregular dimension and hydraulically interconnected above and below the permafrost. Both natural andanthropogenic disturbances form isolated or interconnected unfrozen zones, further complicating the potentialpatterns of ground water flow and contaminant migration. Geophysical investigations coupled with borehole geo-logic information provide the basic data on the configuration and dimensions of the active layer, suprapermafrostand subpermafrost aquifers, and the perennially frozen materials. These data were synthesized in a three-dimensionalmodel of the permafrost and aquifer distributions using the software “EarthVision”. This model is the geologicframework for on-going site characterization and contaminant remediation.

Permafrost, Phillips, Springman & Arenson (eds)© 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7

Fairbanks

Anchorage

Fort WainwrightFairbanks

Figure 1. Location of Fort Wainwright.

deposited by historical meandering of river channels.The alluvium thickness ranges from tens of metersnear the base of Birch Hill to greater than 120 m southof the Chena River (Fig. 2). A thin (approx. 1 m) layerof silt is present just below the ground surface abovethe floodplain deposits in many areas (Péwé 1958). Asmuch as 10 m of eolian silt has been observed on thehillside, and may be thicker in some areas (Lawson et al. 1996).

2.2 Permafrost

Permafrost occurs discontinuously throughout thealluvium and underlying bedrock. Its impermeablenature significantly impacts the local gradient, andground water flow, causing it to vary in direction andrate from that of the prevailing regional gradient.Several factors have influenced its distribution includ-ing present and past climatic conditions, terrain char-acteristics, vegetation cover, historic use and surfacedisturbances (Hopkins et al. 1955, Williams 1970).Permafrost tends to be absent or sporadic on south-facing slopes, such as Birch Hill (Fig. 2). Deep thawof the upper permafrost surface has resulted fromanthropogenic activities, such as construction of theTruck Fill Stand and Canol Road (Fig. 2). Permafrostis also typically absent beneath and adjacent to waterbodies such as the Chena River and along formersloughs (Lawson & Snyder 1998).

2.3 Bedrock

The bedrock beneath the study area is fractured,faulted and foliated phyllitic schist with a weatheredhorizon up to 30 meters thick (Peapples et al. 2000,Myse et al. 2002). Its permeability is thought to beless than the alluvium but greater than competent

bedrock. Primary permeability in the weatheredbedrock apparently exists along quartz veins that lieparallel to or crosscut the foliation (Peapples et al.2000). In the competent bedrock, it apparently occurswithin fracture zones (Lawson et al. 1996).

3 METHODS

3.1 Geophysical methods

We developed ground-penetrating radar (GPR) equip-ment and techniques to profile the permafrost inter-faces, ground water beneath permafrost and underlyingbedrock surface (Lawson et al. 1996, Arcone et al.1998, Lawson & Snyder 1998). We used the Geophys-ical Survey Systems, Inc. (GSSI) Models 4800 and10a� control units to set radar parameters and col-lect data with 100- and 50-MHz antennae. We towedantennas along cleared trails behind a tracked vehicle.Profiles were recorded continuously at 20 to 30 tracesper second with densities of 512 or 1024 8- or 16-bitsamples per trace.

We later applied DC-resistivity to supplement theGPR data in specific areas of the site where we couldnot use GPR for logistical reasons (Astley et al. 1999).An Iris SYSCAL R1 Plus instrument was used tomeasure DC resistivity. Electrodes were arranged in aWenner array in which the inner electrodes are equi-spaced between two outer electrodes through whichthe current was induced. Resistivity is simply theinduced potential between the two inner electrodes.We interpreted the resistivity profiles for the presenceor absence of permafrost and depth to bedrock. Thismethod was mainly employed in heavily woodedareas covered by brush where additional data wererequired but towing radar antennae was not feasible.

3.2 Ground water flow

We used site-specific ground water flow data, includ-ing seepage rate and direction, to infer local geome-tries of the aquifer and permafrost interface whereother subsurface data were unavailable. The CRRELGround Water Flow System provides a measure ofwater flow through monitoring wells screened in thesuprapermafrost and subpermafrost aquifers, as wellas in sediments unfrozen to bedrock (Lawson et al.1996). This automated system measures seepage rateand direction up to four times daily using a heat pulsetechnique called thermal tag and trace. A down-holesensor with four pairs of platinum Resistive Tempera-ture Devices (RTDs) is embedded in glass beads,placed at the base of the well screen, around a centralpoint source heater. Ground water surrounding the

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Figure 2. Location of sites on Fort Wainwright.

sensor is heated for 1 hour at which time the resistiv-ity is measured, and then re-measured after a speci-fied time interval. Resistivity is then converted toactual temperature. The difference in the temperaturesof opposing pairs of RTDs above the no-flow condi-tion results from convective heat transfer by theground water flowing through the glass beads andthus provides a measure of seepage rate and direction.

3.3 Modeling

We used Dynamic Graphics software “EarthVision”to represent permafrost and aquifer distributions inthree dimensions. “EarthVision” uses a simple mini-mum tension griding algorithm to model surfaces andvolumes in two and three dimensions by applyinginverse distance weighted averages of the grid nodes,followed by the application of a spline function tosmooth the surface. This griding approach developsoutput surfaces that closely conform to the input data.

Initially, field data from the various studies at FortWainwright were synthesized to develop the parame-ters for the subsurface geologic model.

We created two-dimensional maps of permafrostdistribution based on GPR data from over 150 tran-sects located north of the Chena River and borehole logsfrom over 600 drill sites (Lawson et al. 1996, Larson &Snyder 1998). These maps provided the basis forfuture field studies and analyses and development of aconceptual model of the three-dimensional distribu-tion of permafrost and alluvial aquifers.

The second phase used these data to develop a firstapproximation of the subsurface geology in threedimensions (Snyder et al. 2000). The model then wasrefined using new data on the permafrost and aquifersfrom additional geophysical studies, vegetation map-ping, and ground water flow measurements. We alsomapped vegetation on aerial photographs from 1939and 1997 and supplemented it with field measure-ments of active layer thickness. Conceptual informa-tion based on geological knowledge of the site and itshistory of disturbance are currently being used torefine the modeled distribution and configuration ofpermafrost and aquifers so as to represent what isknown about the subsurface geology better.

4 RESULTS

4.1 Geophysical results

Geophysical techniques were successful at this sitebecause the dielectric constants of the materials beingmeasured were sufficiently distinct for adequatedelineation of the subsurface geologic horizons. Thedielectric constant of permanently frozen sediment

ranged from 4.8 to 5.6, unfrozen bedrock averagedabout 11, and the active layer varied from 24 to 36.The usual value of water of about 87 allowed us toidentify the water table, but also enhanced interfacesbetween the frozen alluvium and unfrozen, aquifer-bearing sediments.

Thus all primary interfaces in the materials includedthawed sediment above permafrost, partial thaw inactive layers, deep unfrozen zones above permafrost,permafrost in contact with ground water aquifers andpermafrost within the bedrock. Permafrost is usuallyin contact with ground water and thus the dielectriccontrast clearly identified the upper and lower inter-faces of frozen and unfrozen materials, which if thiswere not the case, the change in dielectric constantwould not be sufficient to identify these horizons(Arcone et al. 1998).

The GPR interpretations were limited in some loca-tions by geometrically uneven interfaces and thosepassing through wet media, which absorb radio waveenergy. This effect is diminished at lower radio wavefrequencies and we achieved the greatest penetrationat the 50 MHz frequency. Lower frequencies than thiswere not possible because the antenna would not fitalong the wooded trails. The 400 MHz antenna hadvery high resolution in the shallow active layer becauseof its high dielectric constant and shallow depth. The100 MHz antenna is usually a good compromise thatcan typically show the bottom of the active layerwhere it is 1 meter or greater in depth, the layer struc-ture in unfrozen materials, and the bottom of the per-mafrost where it is 25 meters or less in depth.

Vertical resolution varies with frequency fromabout 30 cm at 400 MHz, 1.2 meters at 100 MHz and2.4 meters at 50 MHz. These values are based on theactual center frequency of the transmitted wavelet (it isalways lower than 400, 100 or 50 MHz cited), which is about 1.5 cycles long.

Lateral resolution of the GPR is equal to the beamwidth of the antenna. In permafrost, the beam spreadsout plus or minus 30 degrees from the antenna. So at a depth of 10 meters, the beam sensitivity is spreadover a width of about 5.8 meters on either side of thecenter beam. However, if the antenna interface is flatand then ends abruptly, there is almost a one to onecorrespondence between radar image and actual inter-face extent.

We were able to interpret the depth to the top ofpermafrost from DC resistivity profiles because of itshigh apparent resistivity value compared to that of theoverlying active layer. However, the resistivity sound-ings were not able to delineate the bottom of the per-mafrost accurately due to a lack of contrast. Depth tothe basal contact could only be estimated to �5 metersof actual depth. Typical resistivity values are 2000 to10,000 ohm-m in the unfrozen alluvium, 10 ohm-m in

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the bedrock (it is slightly graphic and thus conductive)and 40 ohm-m in the active layer.

4.2 Permafrost distribution

A comparison of a preliminary model based only onborehole data (Fig. 3a) and a second generation modelthat incorporated geophysical data (Fig. 3b) showsthat the first version overestimated the distribution ofpermafrost due to the limited spatial distribution ofthe borehole data. The later model shows the discon-tinuous nature of the permafrost, more accurately rep-resents the relief as exists in the surfaces of thepermafrost, and identifies localized taliks and local-ized unfrozen zones bounded by permafrost. The sec-ond version of the model simulates permafrost andaquifers in three dimensions as independent volumes(Fig. 4), while delineating surfaces such as that of theunderlying bedrock.

The current permafrost model shows that per-mafrost is sporadically distributed at the base of BirchHill and typically frozen into the underlying bedrock.Several isolated bodies of permafrost extend southfrom the base of Birch Hill to the Truck Fill Stand area(Figs 2, 4). Permafrost is highly discontinuous furthersouth of the Truck Fill Stand. Unfrozen zones boundedby permafrost create a very complex configuration toground water aquifers, with the suprapermafrost andsubpermafrost aquifers interconnected with theseunfrozen zones. Permafrost extends into bedrock inmuch of the area to the east of Canol Road, and eastand west of the Truck Fill Stand, thus isolating groundwater flow within the main part of the site (Figs 2, 4).The subsurface model helps define the probable dis-tribution of permafrost where measured data do not

exist and with this information, analyze probableground water flow patterns.

4.3 Aquifer distribution

The vertical extent of the subpermafrost aquifer variesfrom about 3 to 30 m in the Birch Hill area. Its thick-ness increases to the south, where just north of theChena River it exceeds several tens of meters or more.Thaw channels here create semi-confined conditionsat depth, with several abrupt changes from a thin to athick aquifer evident. This in turn creates a complexflow regime in which ground water moves from envi-ronments of extreme constrained flow below the per-mafrost to open zones where flow is not constrainedby bedrock or permafrost. These complex flow pat-terns have been observed and quantified with theCRREL ground water flow systems. Conceptualizedflow patterns such as this are shown on Figure 4.

The suprapermafrost aquifer tends to be thin in theTruck Fill Stand area. The thickness of the unfrozensediments above the permafrost generally ranges from1 to 7 m, but ranges from 10 to 12 m beneath the TruckFill Stand. The thickest zones are due to disturbanceby roads, fill areas, buildings and other human activi-ties. There are several large, undisturbed areas wherethe permafrost is present within less than 1 m of theground surface. While this thin active layer is gener-ally not a significant physical component of the suprap-ermafrost aquifer, it significantly controls the patternsof ground water flow. Deeper active layers can alsofreeze to depths of 4 m or more during the winter,decreasing the vertical extent of the suprapermafrostaquifer and creating a new geometry of impedimentsto ground water movement. Our CRREL flow system

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Figure 3. Permafrost models viewed at the upper surfaceof the permafrost. (a) first approximation based on onlyborehole data (b) model based on borehole and geophysicaldata. Conceptual refinements are not yet represented (Figuremodified after Astley et al. 1999).

Figure 4. Conceptualization of permafrost (lighter lenses)and aquifer distribution (darker material) upon bedrockwith arrows indicating direction of ground water flowbased on in situ flow measurements (lengths not propor-tional to magnitude).

data show cases where near surface flow rates increaseand directions change significantly during the winter,compared with other seasons, apparently due to thisseasonal freezing of the active layer. Thaw channelsmake the suprapermafrost aquifer geometrically com-plex and can create semi-confined conditions wherechannels are hydraulically connected to the subper-mafrost aquifer. The presence of permafrost can explainotherwise enigmatic ground water flow paths in andaround the site (Fig. 4). Understanding where per-mafrost is frozen to bedrock is also important becauseit impedes flow within the alluvial aquifer, forcingany ground water movement to occur beneath the per-mafrost (Lawson & Snyder 1998).

5 CONCLUSIONS

The ever-expanding military, commercial and privateland uses in cold regions such as Alaska necessitate anability to map the presence or absence of permafrostwhere it is discontinuous effectively. Assessing groundwater flow or contaminant migration in areas of dis-continuous permafrost is challenging because of theuncertain effects of permanently frozen ground onmigration. This uncertainty is primarily due to a lackof available methods to define the extent of permafrostand ground water aquifers in three dimensions andthen model it effectively for use in predicting groundwater flow and contaminant migration pathways.

We have developed an integrated approach to delin-eating the vertical and lateral extent of permafrost thatuses geophysical field investigations, surficial geologicand vegetation mapping, in situ hydrological meas-urements and geospatial modeling of the subsurfaceaquifer stratigraphy. The subsurface geologic modelprovides us with the tool to interpret the distributionof permafrost where field data are lacking, and todelineate probable ground water flow patterns throughwhich contaminants may migrate. We are currentlyrefining the model further to include conceptual databased on geologic knowledge and interpretations inareas where geospatial data do not exist. This will allowus to model the permafrost and associated aquifer dis-tributions better, and thereby account for observedground water movement and predict contaminantmigration patterns.

ACKNOWLEDGEMENTS

We thank Christopher Williams, James Morse andTommie Hall of CRREL for their assistance in ground

water flow system measurements, Beth Astley, PatriciaWeyrick and Allan Delaney in conducting field inves-tigations, and Colby Snyder for developing the initialversion of the model. This work was supported bycontracts with US Army Alaska, Department of PublicWorks.

REFERENCES

Arcone, S.A., Lawson, D.E., Delaney, A.J., Strasser, J.C. &Strasser, J.D. 1998. Ground-penetrating radar reflec-tion profiling of groundwater and bedrock in an area of discontinuous permafrost. Geophysics 65(5):1573–1584.

Astley, B., Snyder, C., Peapples, P., Lawson, D., Arcone, S. &Delaney, A. 1999. A Summary of current hydrogeologicinvestigations of the Birch Hill Tank Farm and TruckFill Stand, OU3, Fort Wainwright, Alaska. SummaryReport. Hanover NH: US Army Cold Regions Researchand Engineering Laboratory.

Hopkins, D.M., Karlstrom, T.N.V. & Black, R.F. et al. 1955.Permafrost and ground water in Alaska. U.S. GeologicalSurvey Professional Paper 264-F. 113–146.

Lawson, D.E. & Snyder, C.P. 1998. Preliminary Model ofPermafrost and Aquifer Distribution in Part of OU3,Fort Wainwright, Alaska. Preliminary Draft Report.Hanover NH: US Army Cold Regions Research andEngineering Laboratory.

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Peapples, P., Astley, B., Lawson, D., Delaney, A., Arcone, S., Bigl, S., Staples, A., Wilkinson, J., Epps, S.,Snyder, C. & Sauceda, J. 2000. Bedrock and structurecharacterization: Birch Hill Tank Farm and Truck Fill Stand, Fort Wainwright, Alaska. CRREL ContractReport. Hanover NH: US Army Cold Regions Researchand Engineering Laboratory.

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Snyder, C.F., Astley, B.N., Lawson, D.E., Arcone, S.A. &Delaney, A.J. 2000. Understanding groundwater flowin a permafrost environment. Abstracts with programs:Geologic Society of America. Reno, NV.

Williams, J.R. 1970. Ground water in permafrost regions of Alaska. U.S. Geological Survey Professional Paper, 696.

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