Ninth International Conference on Permafrost ... - IARC Research

Permafrost Degradation Beneath a Heat-Producing Coal Waste Rock Pile,Svalbard (78°N)Jørgen HollesenDepartment of Geography and Geology, University of Copenhagen, Copenhagen, DenmarkUNIS, Longyearbyen, SvalbardBo ElberlingDepartment of Geography and Geology, University of Copenhagen, Copenhagen, DenmarkUNIS, Longyearbyen, SvalbardIntroductionIn Arctic areas, permafrost and low annual air temperaturesare considered to keep the chemical activity within sulphidecontainingwaste rocks low. This has led to a generalacceptance that permafrost environments are well suited forstoring waste rocks. Although temperatures below 0°C reducethe oxidation rate of sulphide minerals, they do not eliminateoxidation (Elberling 2001). Furthermore, the oxidation ofsulphide minerals is strongly exothermic producing 1409KJ of heat for every mole oxidized. Because the oxidationrate of sulphides increases with temperature (Elberling2005), the release of heat can result in a positive feedbackon the oxidation process, causing subsurface temperaturesto become self-increasing (Lefebvre et al. 2001). Dependingon the local meteorological conditions, the sulphide contentof the waste material and the physical design of the wasterock pile, subsurface temperatures can become so high thatweathering processes continue year-round within the wasterock pile (Elberling et al. 2007). This is not only importantto the overall amounts pollutants released but also to thestability of the permafrost beneath the pile which could bedegraded, causing the foundation of the waste rock pile todestabilise.The objective of this study is to investigate the thermalregime within a coal waste rock pile on Svalbard and tosimulate subsurface temperatures due to current weatherconditions, physical properties of the waste rock material,and subsurface heat generation. The simulations will becarried out using the one-dimensional heat and water flowmodel, CoupModel (Jansson & Karlberg 2001), which willbe calibrated and validated based on data from the studyarea. The validated model will be used to investigate howthe permafrost beneath the waste rock pile is influenced bythe oxidation processes.Study SiteIn this High Arctic study, an abandoned coal waste rockpile near Longyearbyen, Svalbard (78°20′N, 15°40′E) isinvestigated. Based on data from 1975–2005, the meanannual air temperature of the area is -5.8 ± 1.3°C and theannual amount of precipitation is 187 ± 44 mm, of whichapproximately 50% falls as snow. Svalbard is located withinthe region of continuous permafrost with an active layerthickness of 1.0 to 1.5 m. The construction of the wasterock pile was initiated in 1986 and completed in 1990 with aheight of 20 m (roughly 200,000 m 3 of waste rock). It standsout in the landscape with steep sides and is highly exposedto winds.The waste rock pile has previously been described byElberling et al. (2007). The waste rock material is verycoarse and heterogeneous with only 16% ± 6% being lessthan 2 mm in diameter and 40% being above 100 mm indiameter. It is dominated by 1–10 mm rocks, but containsrocks more than 0.5 m in diameter. Heat production ratesdue to pyrite oxidation range from 0.5 to 2.5 µW g -1 and asignificant exponential increase in heat production has beennoted with increasing temperatures (R 2 = 0.98, p = 0.001)which is equal to a Q 10of 2.6.MethodsBased on climatic inputs (air temperature, wind speed,wind direction, relative humidity, radiation, pressure, andsnow depth), grain size distribution, porosity and microbialand chemical heat production rates from pyrite oxidation,the CoupModel is used to simulate subsurface temperatureswithin and below the waste rock pile. The model iscalibrated using measurements of subsurface temperaturesfrom 1 October 2004 to 19 August 2005 and validated usingmeasurements from 1 October 2005 to 19 August 2006.ResultsDespite freezing air temperatures 240 days per yearsubsurface temperatures within the investigated pile werestable around 4.3 ± 0.5°C at 7 m depths throughout theyear. Seasonal temperature readings indicate that an outerlayer of less than 1 m remained frozen during the six-monthwinter period. Observations of nearby natural permafrostaffectedtalus slopes confirm that, without subsurface heatgeneration, only a thin near-surface layer of less than 2 mthaw every summer (Fig. 1).The CoupModel setup is successfully calibrated andvalidated. With r 2 values ranging from 0.99 to 0.64 andp values being