Introduction

More than half of the communities on Baffin Islanddraw their drinking water from earthwork reservoirs ordammed lakes (Smith et al.,1984). Construction andmaintenance of these facilities is expensive and may becomplicated by the presence of permafrost. In particu-lar, thermal disturbances associated with constructionactivities and heat released from water bodies maydestabilize earthwork structures. Although groundthermal regimes are well known from several arcticreservoirs (e.g., Dufour and Holubec, 1988), there arevery few accounts of ground temperatures around

water storage facilities on Baffin Island (Harris et al.,1988; Hyatt, 1993). Moreover, the scant temperaturedata available for Baffin Island are often not publiclyavailable, and rely heavily on single point temperaturemeasurements taken at irregular intervals over a shortperiod of time. In contrast, this study examines a 3-yearground temperature record collected at 12 hour to bi-monthly intervals to depths of 15 m at 10 sites near alarge earthwork reservoir located 1 km fromPangnirtung on southeastern Baffin Island (Figure 1).

Ground temperature cables installed in the summer of1987 were monitored on a regular basis between

Abstract

Seasonal and mean annual ground temperature (MAGT) trends (1988-1991) are described for undisturbed,disturbed, and embankment sites to depths of 15 m at a large earthwork reservoir on southeastern BaffinIsland. At sites away from the reservoir, undisturbed and disturbed MAGT are similar, follow trends in air tem-perature, and indicate that climate has a greater influence on ground temperature than does thermal distur-bance caused by land clearing. MAGT near the reservoir are warmest close to the high water line, and are cool-er beneath a heated pumphouse, and under the crest of the embankment. Despite having the warmest absolutetemperatures, water-side probes show the greatest and most rapid rates of cooling. This is attributed to thecombination of climatically driven cooling trends and declining water levels in the reservoir. The validity ofthis hypothesis is confirmed by analytical models of temperature next to the water body.

James A. Hyatt 479

GROUND THERMAL REGIMES AT A LARGE EARTHWORK RESERVOIR ONBAFFIN ISLAND, NUNAVUT, CANADA

James A. Hyatt

Department of Physics, Astronomy and Geosciences, Valdosta State University, Valdosta, GA., USA. 31698-0055, e-mail: jhyatt@valdosta.edu

Figure 1. Location of (inset) Pangnirtung on Baffin Island, (a) the reservoir and airport weather station, and (b) the positions of temperature probes installed in1987.

August 1988 and July 1991. The purposes of this paperare to (1) summarize instrumentation and site condi-tions, (2) describe seasonal and mean annual groundtemperature trends, and (3) relate these trends to ther-mal disturbances caused by climatic fluctuations, con-struction activities, and changing water levels.

Site description, intrumentation anddata analysis

Pangnirtung is located within the zone of continuouspermafrost, experiences a mean annual air temperatureof -10¡C, and receives nearly 400 mm of precipitationannually, 60% as snow (Maxwell, 1980). Ground iceoccurs as pore ice, thin ice lenses, and small ice wedgeswithin the coarse-grained sand-gravel diamict andraised deltaic sediments that underlie the reservoir site(Hyatt, 1990).

The reservoir is a fully lined, 10 m deep earthen exca-vation with a storage capacity of 10.9 x 104 m3 (Smyth,1986). Excavation began in the winter of 1985 with exca-

vated silty sands used to construct a large down-sloperetaining embankment. The embankment was leftexposed for two thaw seasons prior to installing cover-ing sands, an impermeable liner, and filling the reser-voir in September 1987 (Smith et al., 1989). Direct rain-fall is the only natural input, making it necessary to fillthe reservoir each autumn from the Duval River (Figure 1).

Ground temperatures were measured using thermis-tor cables installed at sites away from, and near thereservoir (Figure 1b). Measurement error for individualsensors varies between ±0.1¡C and ±0.3¡C (Hyatt, 1993).Probe specifications, sampling intervals and periods ofmissing data longer than 60 days are summarized inTable 1. Analysis of temperature records followedRiseborough (1988) and involved editing raw data filesto remove spurious readings, generating equally-spaced 5-day interval data sets using cubic spline inter-polation, and calculating running mean annual (RMA)temperatures. Where necessary, interpolated valueswere extracted from isotherm plots to obtain tempera-

The 7th International Permafrost Conference480

Table 1. Configuration, sampling period , missing data for thermistor probes

ture data for similar depths at different sites. Thesetechniques smooth the data somewhat, but are consi-dered appropriate for identifying general temperaturetrends (Burgess and Riseborough, 1989).

Temperature probes away from the reservoir (Figure 1b) are located in undisturbed terrain (probe 1)and at several disturbed sites (probes 2-5) where sur-face vegetation and top soil were removed during reser-voir construction in 1985. The undisturbed site is cha-racterized by hummocky terrain with micro-relief of upto 0.5 m and several large boulders on the surface. Soilsare poorly drained and are covered by 0.2-0.5 m ofmoss, grass and woody plants. The disturbed sites arelocated on level to hummocky terrain between thereservoir and an upslope drainage cutoff ditch.Vegetation removed from these sites in 1985 has rege-nerated on the well-drained sandy soils forming a thinand discontinuous cover of grass and moss. Probes nearthe reservoir are located under a small insulated pump-house (probe 6), within the main embankment under itscrestline (probes 7, 8) and near the maximum fill waterline (probes 9, 10). These sites are unvegetated, areeither flat (probes 6-8) or gently sloping (probes 9, 10),and are covered respectively by well-drained sand andgravel fill and by cobble-sized protective rip rap alongthe shoreline.

The subsurface stratigraphy for all sites is summa-rized in Figure 2. Most temperature probes away fromthe reservoir were installed through large surface boul-ders frozen into permafrost. These probes pass through1 to 1.5 m of rock (boulder), 2 to 3 m of ice-rich siltysand diamict, and greater than 15 m of very dense ice-poor sands and gravels (Smith et al., 1989; Hyatt, 1990).Subsurface conditions for sites near the reservoir con-sist of varying thicknesses of dry sand and gravel fillover pre-existing ice-poor sandy permafrost.Embankment probes 9 and 10 near the high-water line

are located below the lined portion of the reservoir andare overlain by 0.9 m of layered bedding sands, cove-ring sands, and protective cobbles.

Results

AIR AND GROUND SURFACE TEMPERATURES

RMA air temperature and RMA ground surface tem-perature trends are compared in Figure 3. AlthoughRMA ground surface temperatures are 1 to 3¡C warmerthan RMA air temperatures, both data sets show warm-ing trends between May 1989 and May 1990, and coo-ling from May to August 1990. The warmest RMA tem-peratures for both records occur in late winter (April) of1990. Slight differences in air and ground surface tem-peratures near the end of the record likely reflect micro-climatic differences between the reservoir and theweather station (Figure 1a), or the influence of driftingsnow.

James A. Hyatt 481

Figure 2. Configuration and subsurface stratigraphy for temperature probes installed at the Pangnirtung reservoir.

Figure 3. RMA air temperature from the Pangnirtung airport weather sta-tion and RMA ground surface temperature from temperature probes at thereservoir site.

The 7th International Permafrost Conference482

Figu

re 4

. Com

pari

son

of s

easo

nal g

roun

d te

mpe

ratu

res

at d

epth

s of

1, 5

, and

9 m

for

site

s lo

cate

d (a

-e) a

way

from

the

rese

rvoi

r, an

d (f

-i) s

ites

nea

r th

e re

serv

oir.

SEASONAL AND MEAN ANNUAL GROUND TEMPERATURE

Seasonal ground temperatures at depths of 1, 5, and 9m at all sites are illustrated in Figure 4. Temperaturerecords away from the reservoir, except for probe 5, aresimilar and show the following trends: (1) summer tem-peratures at shallow depths warm progressively after1989, (2) winter temperatures at shallow depths arewarmest in February 1990, and (3) both summer andwinter temperatures at depths greater than 6 m coolprogressively toward the end of the record. Seasonaltemperature trends differ for sites near the reservoir.The insulated pumphouse building clearly dampensseasonal temperature variations by as much as 8¡C inthe winter, and 3¡C in the summer (Figure 4f). Seasonaltemperatures under the crest of the embankment(probes 7 and 8) are broadly similar to sites away fromthe reservoir (compare Figures 4g and 4h with Figures 4a-4d). However, temperatures next to thewater line (probes 9, 10) display progressively coolerwinter temperatures and more varied thermal regimesthan any other site (Figure 4i).

Mean annual isotherm plots (Figure 5) display moregradual temperature change. All probes away from thereservoir display steep near-surface temperature gradi-ents with respect to depth (e.g., probe 4 in Figure 5a).Warm RMA air temperatures in the winter of 1990(Figure 3) disrupt isotherms to depths of 6 m (Fi-gure 5a). However, at greater depths, temperatures coolcontinuously through time, causing isotherms to stee-pen. Mean annual temperatures are less regular nearthe reservoir. Although temperatures beneath theembankment crest are nearly isothermal between 7 and11 m, they cool at greater depths indicating a down-ward flux of heat out of the embankment into under-lying undisturbed permafrost (Figure 5b). Near thewater-line (probes 9, 10) mean annual temperaturetrends at all depths display persistent cooling and aremoderated only slightly by the warm winter pulse in1989/90 (Figure 5c).

LONG-TERM GROUND TEMPERATURES

Long-term temperature profiles are obtained by aver-aging mean annual temperatures for the entire record(1988-1991). The resulting temperature profiles providea direct comparison of "average" temperature condi-tions for all sites (Figure 6a). As expected, ground tem-peratures are warmest beside the water body; tempera-tures at probe 10, are 2 to 3¡C warmer than at other sitesnear the reservoir and are as much as 5¡C warmer thanat sites away from the reservoir. Temperatures underthe heated pumphouse, although cooler than at probe10, are warmer than temperatures under the crest of themain embankment. Temperature profiles within themain embankment display irregular perturbations, butgenerally increase to 8 m, remain nearly constant to 11m, and decrease at greater depths reflecting continued

thermal disequilibrium within the embankment.Temperatures away from the reservoir (at depthsgreater than 1 m) are consistently cooler than tempera-tures at similar depths near the reservoir. The shapes oftemperature profiles are similar for both the undis-turbed site (probe 1) and the disturbed sites (probes 2-

James A. Hyatt 483

Figure 5. Mean annual isotherm plots for (a) probe 4 away from the reser-voir, and for probes near the reservoir including (b) probe 7 under the crest ofthe embankment, and (c) probe 10 near the high water line.

5), although temperatures vary by as much as 1.5¡C.Thermal offsets (cf. Burn and Smith, 1988) for probesaway from the reservoir vary from 0.1 to 2¡C, but arenot observed near the reservoir, although this may bedue to the absence of shallow temperature data atprobes 7 - 10.

Temperature changes through time are compared forall sites by plotting the difference between long-termaverage temperature and RMA ground temperature(Figure 6b). The influence of warming air temperaturesbetween May 1989 and May 1990 (Figure 3) is evidentat shallow depths for all sites (Figure 6b). In addition,temperature changes for disturbed and undisturbedsites away from the reservoir are similar, whereas tem-perature changes near the reservoir are more variablereflecting thermal disequilibrium within the embank-ment. The slopes of curves in Figure 6b indicate relativerates of cooling for different sites. Probe 10, despite ha-ving the warmest absolute temperatures (Figure 6a),

displays the greatest and most rapid change in temper-ature. Thus, temperatures beside the water body (9, 10)have cooled more and more rapidly than all other sites.

Discussion

The most important ground temperature trendsdescribed above are: (1) mean annual ground tempera-tures at all sites are warmest in late winter 1990 andcool toward the end of the record, (2) both disturbedand undisturbed sites away from the reservoir displaysimilar ground temperature trends which are similar toRMA air temperature trends, and (3) embankment tem-peratures near the high-water line (probes 9, 10) arewarmest but have cooled faster than all other sites.These trends are explained qualitatively by consideringthe influence of climate, construction activities, andfluctuating water levels during the period of study.

Ground temperatures are most directly linked to airtemperature at probes away from the reservoir thatwere installed through large surface boulders. Theseboulders provide a highly conductive link between theatmosphere and the ground (cf. Smith andRiseborough, 1996) and are not influenced by phasechange latent heat effects to the same extent as probesinstalled in porous soils. As a result, temperaturerecords at probes 1 through 4 do not display dampenedseasonal temperatures or zero curtain effects like thoseat probe 5 which was installed directly into the sandysoil (Figure 5e). The general similarity between RMA airtemperature and RMA ground surface temperatures forthese sites, particularly in late winter of 1990 (Figure 3)suggests that ground temperatures away from thereservoir are responding primarily to climatic fluctua-tions. Furthermore, the similarity between disturbedand undisturbed sites (Figure 6a) indicates that landclearing in 1985 has not had a lasting influence onground thermal regimes.

Temperatures in the embankment beside the waterbody (probes 9, 10), although warmer than other sites,have cooled rapidly. This likely reflects both climaticcooling trends, evident at sites away from the reservoir,and declining water levels which effectively move thewater body heat source further away from probes 9 and10. Detailed water level records are not available,although surveys place the water line at or very nearprobes 9 and 10 each summer between 1988 and 1990.However, the likelihood that declining water levelshave driven observed cooling trends at probes 9 and 10may be tested by modeling thermal disturbances nextto the reservoir.

The theoretical basis for analyzing steady-state ther-mal disturbances caused by water bodies is well estab-lished (Lachenbruch, 1957) and subsequent discussion

The 7th International Permafrost Conference484

Figure 6. (a) Period-of-record average temperatures for all temperatureprobes, and (b) temperature envelopes showing the difference between long-term average temperatures and running mean annual temperatures for allsites. Values greater than 0¡C indicate temperatures warmer than the aver-age condition.

follows Smith (1976). This approach assumes that theseverity of thermal disturbance at any given location iscontrolled by the geometry of the lake, temperature dif-ferences between disturbed and undisturbed areas,geothermal heat, and the duration of the disturbance.Latent heat effects are not considered directly. However,by calculating an "apparent" thermal diffusivity fromground temperature data, a slowed rate of heat transferis achieved. Smith (1976) suggests that this is the reflec-tion of latent heat effects within the active layer. For thePangnirtung site, an average apparent thermal diffusiv-ity of 0.0065 cm2s-1 was calculated using equation 9from Smith (1976) and temperature data from probes 4and 7. Other inputs include a mean annual ground sur-face temperature of -8¡C, a measured mean annualwater temperature of 3.8¡C, and a geothermal gradientof 0.017¡C m-1 obtained from a linear regression fit tothe nearest available deep borehole temperature data(Judge et al., 1981).

The thermal model described above is used to assessthe potential influence of small water level fluctuationson temperature in the vicinity of probes 9 and 10. Twomodeling runs are presented (Figure 7). First, meanannual ground temperatures are predicted for a typicaldesign life period (25 years) for the reservoir. This simu-lation assumes fixed water levels equal to the elevationof probe 10. Results are presented for a width-wisetransect across the reservoir half space (Figure 7a).Maximum thaw to 13 m occurs under the center of thereservoir, decreasing toward the side of the water body.Temperatures change rapidly near the locations ofprobes 9 and 10 suggesting that small changes in theposition of the waterline may significantly alter groundtemperatures in this region.

The second modeling run predicts temperatures nextto the water body 2 years after the reservoir was filled,with water levels set equal to the elevation of probe 10,and 0.5 m below the elevation of probe 10. A two-yeartime frame was selected because it represents the mid-point between the installation of temperature probes

and filling of the reservoir in 1987 and the end of tem-perature measurements in 1991. Thermal disturbancesare applied to long-term average temperatures underthe crest of the embankment (probe 7) which arethought to more closely represent thermal conditions inthe embankment before the reservoir was filled.Predicted temperatures define an envelope that enclo-ses the observed long-term average temperatures forprobe 10. The implication here is that a lowering ofwater levels by as little as 0.5 m is capable of reducingthermal disturbances significantly, resulting in coolertemperatures similar to the range observed next to thereservoir.

Summary and conclusions

Temperature data from several sites at a large earth-work reservoir near Pangnirtung provide the mostdetailed account of near-surface ground thermalregimes available to date for Baffin Island. Warmingand cooling trends in air temperature are most clearlyreflected in ground thermal regimes at sites away fromthe reservoir where temperature probes were installedthrough large boulders frozen into permafrost.Temperatures do not differ substantially betweenundisturbed and disturbed sites away from the reser-voir, suggesting that land clearing has had less of aninfluence on ground thermal regimes than has varia-tions in air temperature. Temperatures near the reser-voir are more variable and show dampening effectscaused by an insulated pumphouse, thermal disequilib-rium within the main retaining embankment, and high-ly dynamic surface thermal regimes adjacent to thewater bodies. Although warmer than other sites, tem-peratures beside the reservoirs show substantial coolingtrends. The magnitude and rate of these trends are con-sistent with predicted temperature changes associatedwith small fluctuations in water levels at the reservoir.

James A. Hyatt 485

Figure 7. Thermal modeling results used to estimate temperatures under the reservoir (a) after 25 years, and (b) after 2 years for different water levels.

Acknowledgments

This paper draws from the authorÕs doctoral disserta-tion, and was conducted with grants to Robert Gilbertfrom NSERC, Indian and Northern Affairs NorthernScientific Training Grant Program, Tower ArcticConstruction Ltd., and contracts with the Governmentof the Northwest Territories. Additional support wasprovided by Queen's University, NSERC, RCGS, andthe ACUNS Trust Fund. Field assistance by R. Cambell,

M. Campbell, B. Crawford, R. Francouer, T.Papakyriakou, and A. Wolfe is appreciated. Conferencetravel support was provided by Valdosta StateUniversity Faculty Development andInternationalization funds. Critical reviews by D.Riseborough and an anonymous referee have improvedthis paper.

The 7th International Permafrost Conference486

References

Burgess, M. M. and Riseborough, D. W. (1989). Measurementfrequency requirements for permafrost ground tempera-ture monitoring: analysis of Norman Wells pipeline data,Northwest Territories and Alberta. In Current research, PartD, Geological Survey of Canada, Paper 891D, pp. 6975.

Burn, C. R. and Smith, C. A. S. (1988). Observations of the`thermal offset' in near-surface mean annual ground tem-peratures at several sites near Mayo, Yukon Territory,Canada. Arctic, 41, 99-104.

Dufour, S. and Holubec, I. (1988). Performance of two earth-fill dams at Lupin, N.W.T. In Proceedings Vth InternationalConference on Permafrost, Trondheim, Norway. Tapir,Trondheim, pp. 1217-1222.

Harris, M. C., Smith, L. B. and Graham J. P. (1988). Water sup-ply lake upgrading Pond Inlet, Northwest Territories, report 4thermal analysis of dikes . Thurber Consultants Ltd.Geotechnical Engineers Report, File 17357. (21 pp).

Hyatt, J. A. (1990). Reconstruction of Holocene periglacialenvironments in the Pangnirtung area based on ice wedgecharacteristics. Fifth Canadian Permafrost Conference.Nordicana no. 54, Laval University, Quebec City, pp. 17-21.

Hyatt, J. A. (1993). Permafrost conditions near two water storagefacilities on Baffin Island, Northwest Territories. Ph.D. thesis,Department of Geography, Queen's University, Canada.(264 pp).

Judge, A. S., Taylor, A. E., Burgess, M. and Allen, V. S.(1981). Canadian geothermal data collection Northernwells 197880. Energy, Mines and Resources Canada,Geothermal Series, Number 12. Ottawa, Canada.

Lachenbruch, A. H. (1957). Thermal Effects of the ocean onpermafrost. Bulletin of the Geological Society of America, 68,15151530.

Maxwell, J. B. (1980). The climate of the Canadian Arctic islandsand adjacent waters Volume 1. Atmospheric EnvironmentService, Canada. (532 pp).

Riseborough, D. W. (1988). On the reliability of temperaturesestimated using cubic spline interpolation. Final report forGeological Survey of Canada, Energy Mines and Resources,Addendum to DSS Contract No 2323370115/01ST.

Smith, L. B., Shevkenek, A. and Milburn, R. (1984). Designand performance of earthworks water reservoirs in theNorthwest Territories. In Smith, D.W. (ed.), Proceedings,Third International Specialty Conference on Cold RegionEngineering. Canadian Society for Civil Engineering,Edmonton, Alberta. pp. 323342.

Smith, L. B., Notenboom, W. G., Campbell, M. and Cheema,S. (1989). Pangnirtung water reservoir: geotechnicalaspects. Canadian Geotechnical Journal, 26, 335347.

Smith, M. W. (1976). Permafrost in the Mackenzie Delta,Northwest Territories. Geological Survey of Canada, Paper7528. (34 pp).

Smith, M. W. and Riseborough, D. W. (1996). Permafrostmonitoring and detection of climate change. Permafrost andPeriglacial Processes, 7, 301-309.

Smyth, T. (1986). Dynamic drawdown analysis water storagefacilities in cold regions. Second International Cold RegionEnvironmental Engineering Conference. Edmonton, Alberta,pp. 7894.