CJSS_2012_48

In situ measurement of snowmelt infiltration under varioustopsoil cap thicknesses on a reclaimed site

Andre F. Christensen1, Hailong He1, Miles F. Dyck1,3, E. Lenore Turner1, David S.Chanasyk1, M. Anne Naeth1, and Connie Nichol2

1Department of Renewable Resources, 751 General Service Building, University of Alberta, Edmonton,Alberta, Canada T6G 2H1; and 2Agrium, Inc., 1751 River Rd, Fort Saskatchewan, Alberta, Canada T8L 4J1.

Received 1 May 2012, accepted 27 May 2013.

Christensen, A. F., He, H., Dyck, M. F., Turner, L., Chanasyk, D. S., Naeth, M. A. and Nichol, C. 2013. In situmeasurement of snowmelt infiltration under various topsoil cap thicknesses on a reclaimed site. Can. J. Soil Sci. 93: 497510.Understanding the soil and climatic conditions affecting the partitioning of snowmelt to runoff and infiltration duringspring snow ablation is a requisite for water resources management and environmental risk assessment in cold semi-aridregions. Soil freezing and thawing processes, snowmelt runoff or infiltration into seasonally frozen soils have beendocumented for natural, agricultural or forested systems but rarely studied in severely disturbed systems such as reclaimedlands. The objective of this study was to quantify the snowmelt infiltration/runoff on phosphogypsum (PG) tailings pilescapped with varying thicknesses of topsoil (0.15, 0.3, and 0.46 m) at a phosphate fertilizer production facility in Alberta.There are currently no environmental regulations specifying topsoil capping thickness or characteristics for these types oftailings piles. Generally, the function of the topsoil cap is to facilitate plant growth and minimize the amount of drainageinto the underlying PG. Experimental plots were established in 2006 to better understand the vegetation and waterdynamics in this reconstructed soil. In 2011, time domain reflectometry (TDR) probes and temperature sensors wereinstalled at various depths for continuous, simultaneous, and automated measurement of composite dielectric permittivity(oeff) and soil temperature, respectively. An on-site meteorological station was used to record routine weather data. Liquidwater and ice content were calculated with TDR-measured effective permittivity (oeff) and a composite dielectric mixingmodel. Spatial and temporal change of total water content (ice and liquid) revealed that snowmelt infiltration into thetopsoil cap increased with increasing topsoil depth and net soil water flux from the topsoil cap into the PG material waspositive during the snowmelt period in the spring of 2011. Given the objective of the capping soil is to reduce drainage ofwater into the PG material it is recognized that a capping soil with a higher water-holding capacity could reduce theamount of meteoric water entering the tailings.

Key words: Seasonally frozen soils, ground thermal regime, water and heat dynamics, phosphogypsum tailing,Fort Saskatchewan, snowmelt inltration

Christensen, A. F., He, H., Dyck, M. F., Turner, L., Chanasyk, D. S., Naeth, M. A. et Nichol, C. 2013. Quantification insitu des infiltrations de leau de fonte sous diverses epaisseurs de sol dans un site restaure. Can. J. Soil Sci. 93: 497510.Comprendre les conditions du sol et du climat qui influent sur le partage de leau entre le ruissellement et linfiltration a` lafonte des neiges printanie`re est indispensable a` la gestion des ressources hydriques et a` levaluation des risquesenvironnementaux dans les regions froides semi-arides. Les mecanismes du gel et du degel du sol ainsi que le ruissellementdes eaux de fonte ou leur infiltration dans le sol gele de facon saisonnie`re sont bien documentes pour les syste`mes naturels,agricoles et forestiers, mais on les a rarement etudies dans les syste`mes tre`s perturbes comme les terres restaurees. Letudeque voici devait quantifier linfiltration/le ruissellement de leau de fonte dans les tas de residus de phosphogypsesurmontes dune couche variable de sol de surface (capuchon de 0,15, 0,3 et 0,46 m depaisseur), a` une usine de fabricationdengrais phosphates de lAlberta. A` lheure actuelle, aucun re`glement sur lenvironnement ne regit lepaisseur ducapuchon de sol ni les caracteristiques des tas de residus de cette nature. En general, le capuchon de sol a pour but defaciliter la croissance des plantes et de minimiser le drainage vers la couche de phosphogypse sous-jacente. En 2006, on aamenage des parcelles experimentales pour mieux comprendre la dynamique de la vegetation et de leau sur un sol ainsireconstitue. Cinq ans plus tard, on a installe des reflectome`tres temporels et des thermome`tres a` diverses profondeurs afinde mesurer de facon continue, simultanee et automatique la permittivite dielectrique des materiaux composites (oeff) et latemperature du sol, respectivement. Une station meteorologique a ete installee sur les lieux pour enregistrer les conditionsclimatiques. La quantite deau a` letat liquide et de glace a ete calculee a` partir de la permittivite oeff mesuree avec lereflectome`tre et dun mode`le de melange des fluides servant a` determiner les caracteristiques dielectriques des composites.La variation de la concentration totale deau (etats liquide et solide) dans lespace et le temps indique que linfiltration deleau de fonte dans le capuchon de sol augmente avec lepaisseur de la couche superficielle et que le flux net deau venant de

3Corresponding author (e-mail: [email protected]).

Abbreviations: PG, phosphogypsum; TDR, time domainreflectometry

Can. J. Soil Sci. (2013) 93: 497510 doi:10.4141/CJSS2012-048 497

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la surface et atteignant le phosphogypse avait ete positif durant la fonte, au printemps 2011. Puisque le capuchon de sol apour but de reduire le drainage de leau dans le phosphogypse, on convient quune couche retenant mieux leau pourraitreduire la quantite deau entranant la meteorisation qui pene`tre dans la strate de residus.

Mots cles: Sols geles saisonnie`rement, regime thermique du sol, dynamique de leau et de la chaleur, residus de phosphogypse,Fort Saskatchewan, inltration de leau de fonte

Snowmelt in cold, semi-arid areas may contribute tothe recharge of soil water reservoirs and groundwaterbecause of low evapotranspirational demand duringspring snow ablation. Snowmelt, however, may becomerunoff if soil infiltration capacity is inhibited by icelenses, ice-filled pores or a basal ice layer built up on thesoil surface at the base of the snowpack (Cary et al.1978; Kane 1980; Miller 1980). Snowmelt runoff canlead to an increased probability of erosion of fertilesurface soils, migration of pesticides and other agricul-tural chemicals, and the potential for spring flooding(Janowicz et al. 2002; Hall et al. 2012). Partitioning ofthe snowmelt into soil water and runoff has importantimplications for water resource management strategiesand the development of mitigation strategies to reduceenvironmental risks associated with dissolved or sus-pended contaminants.Prediction of infiltration into seasonally frozen soils

is much more complicated than that of infiltration intounfrozen soils since (1) water flow is strongly coupledwith heat transport, hence both hydraulic and thermalproperties of the soil affect the spatial and temporalvariability of the processes; (2) phase changes betweenice and infiltration water may occur; and (3) hydraulicproperties are considerably influenced by the amountof ice and its spatial and temporal distribution. Theinfiltration capacity of partially frozen soils is governedby many factors such as soil-atmosphere energy ex-change, soil thermal regime, quantity and rate of snowwater release from the snowpack, heat content of theinfiltrating water, thermal and hydraulic propertiesof the soil, soil structure, total soil water content priorto ground freezing or ice content at the time of snowmelt, the number of freezethaw cycles that change totalwater and ice content during the winter time, and theircomplex interactions (Kane 1980; Stein and Kane1983a; Granger et al. 1984).Methods developed for unfrozen soils are commonly

applied to frozen soils for measurement of infiltrationrate into frozen soils. These methods include timedomain reflectometry (TDR) (Stein and Kane 1983a;Iwata et al. 2008), double/single-ring infiltrometers(Kane and Stein 1983), neutron scattering (Grangeret al. 1984), lysimeter (Kane 1978; Stahli and Lundin1999), and dye tracing methods (Stahli et al. 2004).Total and liquid soil water content, soil temperature,matric potential, snow pack characteristics (i.e., snowdepth and snow water equivalent), depth to ground-water table, and meteorological data (i.e., air tempera-ture, precipitation, and albedo) are usually monitored

concurrently to quantify snowmelt infiltration. Empiri-cal expressions consisting of snow water equivalent andsoil water content at melt time (Granger et al. 1984;Zhao and Gray 1997) and numerical models (Gray andGranger 1985; Stadler et al. 1997; Stahli and Lundin1999; Gray et al. 2001; Zhao et al. 2002) have beendeveloped to predict snow infiltration into frozen soils.Of these above-mentioned methods, TDR can be

easily multiplexed and automated making it widelyused for both field and laboratory measurement ofsoil water content (Spaans and Baker 1995). The TDRtechnique is based on the measurement of the travel timeof an electromagnetic wave pulse generated by a TDRcable tester through a wave guide (also called probe)inserted into a porous medium such as soil, unfrozen orfrozen. The travel time of the wave through the probeis a function of the effective dielectric permittivity ofsoil, oeff, which in turn, is a function of permittivity ofthe individual constituents in the soil (air, water, solidsand ice), their volumetric fractions, and geometricarrangements. Recent research (He and Dyck 2013)showed composite dielectric mixing models can be usedto estimate unfrozen water content and ice content infrozen soils. The ability to monitor unfrozen water andice content in the field may contribute to improving ourunderstanding of snowmelt infiltration into partiallyfrozen soils.Extensive investigations of snowmelt infiltration be-

havior of frozen agricultural land (Zhao et al. 2002;Iwata et al. 2008), forested land (Stein and Kane 1983b;Stadler et al. 1997), prairie (Granger et al. 1984; Grayand Granger 1985), or alpine soil in the field as well as incold chambers (Stadler et al. 2000) have been reported.No study was found by the authors for markedly dis-turbed systems such as reclaimed sites. Therefore thepurpose of this research is to investigate snowmelt in-filtration on reclaimed systems, and specifically, in atopsoil cap overlying phosphogypsum (PG) tailings.Phosphogypsum is a by-product of the production of

phosphoric acid, a necessary component in the produc-tion of phosphate fertilizers. The production of phos-phoric acid involves the reaction of sulfuric acid andphosphate rock. The resulting solution is a combinationof gypsum (CaSO4), hydrogen fluoride and phosphoricacid (Rutherford et al. 1994). Once the PG materialhas been filtered from the solution, it is mixed withwater to form a slurry which can be pumped into settlingponds (wet stacking). These settling ponds rise as the PGsettles out of suspension, forming large stacks, referredto as PG stacks (Wissa 2002). The stacks can grow to

498 CANADIAN JOURNAL OF SOIL SCIENCE

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immense scales depending on the engineered capacityand can occupy vast amounts of land.There may be numerous impurities in the phosphate

source material including radium, uranium, arsenic,barium, cadmium, chromium, lead, mercury, seleniumand phytotoxic fluoride (Rutherford et al. 1995). Thesetrace elements may accumulate in the PG by-productand in the water held within its pore space. Studieshave demonstrated that concentration and mobility ofmost trace elements within the PG pore water decreaseswith time as the stack drains (Rutherford et al. 1995).However, the introduction of precipitation into the PGstack may facilitate the mobility of these trace elementsas well as dissolve the PG material (SENES 1987).Therefore, the leading concerns regarding PG stacksinclude contamination of the environment by multi-integrated vectors such as PG leachate to groundwater,wind and water dispersion to surrounding land andradionuclide emissions from the stack (SENES 1987;Thorne 1990) and stack stability.The main objective of this work was to investigate the

temporal and spatial changes of soil ice (ui) and liquidwater (ul) content under various snow depths andtopsoil cap thicknesses to understand the influences offreezethaw cycles on snowmelt infiltration into thetopsoil caps and subsequent drainage (if any) into theunderlying PG over the course of the spring thaw periodin 2011 and the following 2011 freezing period and partof the spring, 2012 thaw. The relationship betweensnowmelt infiltration, snow depth, topsoil cap thicknessand the soil freezing characteristic will be discussedthroughout the paper.

MATERIAL AND METHODS

Site DescriptionThe study was conducted at the Agrium NitrogenOperations facility, located in Fort Saskatchewan,Alberta, Canada (lat. 53844?07N, long. 113811?28W,624 m above the sea level). The area is described as cold,semi-arid having a mean annual temperature of 2.98Cand a mean annual precipitation of 460 mm, with 355mm in the form of rainfall mainly occurring betweenMay and August and 105 mm as snow. The average temp-erature from December through February is 11.78Cand 15.88C from June through August (EnvironmentCanada 2011). The growing season typically starts mid-to late April as marked by a mean daily temperature of18C for a period of 5 consecutive days. The growingseasons for the area typically last through to earlySeptember, when the maximum duration of sunshine,calculated from latitude and Julian day, decreases below13.5 h (Hayashi et al. 2010).The field site is permanently located on top of a

decommissioned PG stack that was operational for 8 yrbefore its closure in 1991. The PG stack is approxi-mately 15 m in height and occupies a base area of 9.3 ha.Eighteen (50 m10 m) plots were constructed atop the

decommissioned PG stack in a complete randomizeddesign during the fall of 2006 (Jackson 2009). The plotswere arranged into three replicates each containing sixtopsoil capping depth treatments including 8, 15, 30, 46,and 91 cm, as well as a 0-cm control; however, for thepurposes of this research only replicate 2 was usedfor sampling due to the logistical constraints thatwere imposed with respect to site instrumentation. Thegeneral topography on which the topsoil was placed wasrelatively flat with a slope of B1%. Each topsoil depthtreatment was further subdivided into five subplots(10 m10 m) that contained five different plant speciestreatments including Agropyron trachycaulum (Link)Malte ex H.F. Lewis (slender wheatgrass), Agrostisstolonifera L. (redtop), Deschampsia caespitosa (L.) P.Beauv. (tufted hairgrass), Festuca ovina L. (sheep fescue)and a mixture of 54% redtop, 2% slender wheatgrass,28% tufted hairgrass, 8% sheep fescue and 8% alsikeclover (Jackson 2009).

Topsoil and PG CharacterizationThe substrate that was used as the capping soil was asandy loam to loamy sand textured Black Cherozemicsoil excavated from a nearby alfalfa field. The capp-ing soil had an average bulk density of 1.5 g cm3 andan average porosity of approximately 0.44 cm3 cm3.The saturated hydraulic conductivity of the topsoil capwas 7.4103 cm s197.9105 (Jackson et al.2011). Water retention curves for the topsoil indicatedthat the air entry potential for the topsoil materialwas approximately 28 cm and that the residual watercontent at 15 000 cm was 0.11 cm3 cm3 (Fig. 1).The PG material matrix is dominated by silt sized

particles that have a diameter ranging from 0.25 to 0.045mm (Rutherford et al. 1994). Bulk density and porosity

Matric Potential (-cm)100 101 102 103 104

0v (c

m3cm

-3 )

0.0

0.1

0.2

0.3

0.4

0.5TopsoilPG

Fig. 1. van Genuchten (1980) model tted to the mois-ture retention curves for the topsoil and PG material.van Genuchten parameters are a0.036 cm1, n1.57,ur0.11 cm3 cm3, us0.46 cm3 cm3 for topsoil, anda0.0054 cm1, n1.67, ur0.027 cm3 cm3, us0.44 cm3cm3 for PG.

CHRISTENSEN ET AL. * SNOWMELT INFILTRATION ON A RECLAIMED SITE 499

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estimates for the PG material found on site averagedapproximately 1.4 g cm3 and 0.45 cm3 cm3, respec-tively. The saturated hydraulic conductivity of PGmaterial was 3.7104 cm s196.6106 (Jacksonet al. 2011). The water retention curve for the PGmaterial indicated that the predicted air entry potentialwas approximately 186 cm and that the residual watercontent at 15 000 cm was 0.03 cm3 cm3; however,it is worth noting that the air entry potential is basedon empirical parameters used in the van Genuchtenmodel (Fig. 1). Some discrepancies regarding the fit ofthe van Genuchten model to the measured data are duein part to the finer-textured PG material and its narrowpore size distribution; therefore, the air entry potentialwould most likely be lower than the predicted value.

Experimental DesignThe 15-, 30- and 46-cm treatments of replicate 2 wereselected for soil water investigation based on theintended final reclamation depth, that would fall withinthe range of 1546 cm as suggested by previous studies,and for the close proximity of each plot to one anotherthat was necessary given the limitation of cable lengthfor the TDR sensors (Jackson 2009; Hallin 2009).All experimental measurements were conducted withinthe Agropyron trachycaulum subplots of the three soiltreatments in order to establish a controlled plantcommunity that would be representative of the finalreclamation goals.Within each subplot, TDR probes were installed

along with dielectric water potential sensors (MPS-1,Decagon Devices Inc., Pullman, WA) and soil tempera-ture sensors (TMC50-HD, Onset Computer Corpora-tion, Bourne, MA) (Table 1). The TDR probes andMPS-1s were horizontally installed at the soil/PG inter-face as well as 7.5 cm above and below the interface.Additional TDR probes were horizontally installedat 15-cm increments from the soil/PG interface as wellas a single vertical probe installed into the topsoil cap

equivalent to the soil treatment depth (15, 30 and 46cm). The horizontal TDR probes were installed intoa 60-cm50-cm trench that was dug to a depth ofapproximately 1 m using a backhoe in November of2010. Once the TDR probes were installed the substratematerial was repacked in the order in which it wasremoved in order to avoid mixing. The soil temperatureprobes were installed vertically into a narrow hole thatwas created within 50 cm of the TDR probes in Aprilof 2011 due to winter conditions inhibiting earlierinstallation. Soil temperature sensors were installed atdepths corresponding to the horizontal TDR probesfor each subplot and subsequently routed to HOBO U12external data loggers and logged on an hourly basis. TheMPS-1 probes were routed to a CR 1000 data loggerand monitored every 2 h [Campbell Scientific (Canada)Corp. 2007].

TDR MeasurementsThe TDR probes that were used during the experimentwere constructed using two stainless steel rods, eachwith a diameter of 0.5 cm with 4-cm inter-rod spacingattached directly to a 10-m-long RG 58/U coaxial cable.Vertically installed TDR probes varied in length corre-sponding to topsoil treatment depth (15, 30 and 46 cm),while the horizontally installed TDR probes were astandardized length of 30 cm. The TDR probes thatwere installed were routed to 3 SDMX 50 multiplexerscorresponding to the three subplots under investigation.The 3-SDMX 50 multiplexers were subsequently routedto a central multiplexer, which was connected directly toa TDR100 and CR 800 data logger [Campbell Scientific(Canada) Corp. 2007]. In order to reduce signal lossalong the length of the cable RG 8/U coaxial cables wereused to link the SDMX 50s to the central SDMX 50multiplexer.During spring melt and fall freeze up, electrical

conductivity and the dielectric permittivity (oeff) werelogged every 2 h, while the individual wave formswere logged every 6 h. During the mid-summer monthsthe electrical conductivity and oeff were logged every 4 hand waveforms were logged every 12 h.

Meteorological MeasurementsAn on-site weather station was used to measure andrecord routine meteorological data on an hourly basisusing a CR 10X data logger [Campbell Scientific(Canada) Corp. 2007]. Precipitation and solar radiationwere measured using a TE525WS Texas Electronic0.254 mm tipping bucket rain gauge and a Kipp &Zonen silicon pyranometer (SP-Lite), respectively.Wind speed and direction were measured using a05103-10 RM Young wind monitor. Finally, air tem-perature and relative humidity were measured using aHMP45C Vaisala probe housed in a radiation shield[Campbell Scientific (Canada) Corp. 2007]. Snow-fall data were measured at the Fort Saskatchewan

Table 1. Soil sensors and placement depth below soil surface

Soiltreatment Sensor Depth

15 cm TDR probe 7.5, 0z, 7.5, 15, 30, 45 cm, and15 cm vertical

Temp. probeMPS-1 probe

7.5, 0z, 7.5, 15, 30 and 45 cm7.5, 0z, and 7.5 cm

30 cm TDR probe 15, 7.5, 0z, 7.5, 15, 30, 45 cm,and 30 cm vertical


15, 7.5, 0z, 7.5, 15, 30, and 45cm7.5, 0z, and 7.5 cm

46 cm TDR probe 30, 15, 7.5, 0z, 7.5, 15, 30 cm, and46 cm vertical


30, 15, 7.5, 0z, 7.5, 15, and 30 cm7.5, 0z, and 7.5 cm

zSoil/phosphogypsum interface.


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meteorological station, located 1.79 km south of thefield site (Environment Canada 2012).

Snow SurveySnow depth and density were measured on 2011Feb 26. The Fort Saskatchewan meteorological station,located 1.79 km away, was used after 2011 Feb. 26 toobtain further snow fall records. A snow survey wasnot conducted in 2012 due to the lack of cumulativesnow pack; however, snowfall data were obtained fromthe nearby meteorological station in Fort Saskatch-ewan to estimate winter precipitation. For each 50 m10 m plot, 12 locations were selected for snow depthmeasurements (D, cm) using a standard ruler. Withineach Agropyron trachycaulum subplot (10 m10 m) asingle snow core was collected using a clear polyvinylchloride tube (2.38-cm radius). For soil treatments withTDR probes installed the snow cores were collectedwithin 50 cm of probe location. The snow cores weretransferred to plastic sampling bags and weighed. Thedensity of snow was used to compute snow waterequivalent (mm) measurements for the plots underinvestigation.

Estimation of Unfrozen Water and Ice Contentwith TDR-measured oeffFreezing of soils affects the dielectric permittivitysimilarly to the drying of unfrozen soils but the dryingprocess in an unfrozen soil involves the replacement ofwater with air whereas the freezing process involves thedisplacement of water by ice. The dielectric permittivityof ice is about 3 times greater than air (3.2 comparedwith 1, respectively). Therefore, the oeff of a frozen soilwill be greater than the oeff of an unfrozen soil withthe same liquid water content if there is ice present. Theamount of ice present is proportional to the watercontent prior to freezing and any water moving into thesoil volume sampled by TDR probe.He and Dyck (2013) showed that dielectric models

could be used to calibrate TDR for the measurement ofunfrozen water and ice content in frozen soils. Further-more, He and Dyck (2013) presented evidence to showthat the mixing models could be parameterized usingunfrozen soil at a variety of water contents and thenextended to frozen soils. However, the main limitationof this calibration method is that it only remains valid ifthe total water content (sum of ice and unfrozen water)within the measurement volume of the TDR proberemains constant while the soil is frozen. This conditionis easy to satisfy in laboratory conditions, but maynot always be satisfied in the field due to translocationand percolation of water within the soil profile. Howthis assumption affects the estimation of unfrozen waterand ice content for the field measurements presented inthis paper and their interpretation will be discussed inthe results and discussion section.To estimate the liquid water and ice content, the

confocal ellispsoid model initially developed by Sihvola

and Lindell (1990) and further modified by He andDyck (2013) was calibrated using unfrozen soil samples.Air-dry samples of the topsoil and PG were wetted withknown quantities of deionized water to obtain varioussaturation levels, from air-dry to near saturation atincrements of around 0.05 kg H2O kg

1 soil and thenequilibrated for at least 24 h at room temperature.The mixed soil samples at each of the prescribedmoisture contents were then uniformly packed to adepth of 15 cm in a copper cylinder of 5.08-cm internaldiameter in a range of bulk densities to test its influenceon parameters of a specific soil. A TDR probe consistingof triple parallel stainless steel needles of 1.6 mm indiameter, 10 cm in length with 1-cm inter-rod spacingwas inserted in the center of the cylinder, and connectedto a TDR cable tester for measurement of oeff. Using thisexperimental data, the parameters for the mixing modelwere optimized using MathCad software such thatthe mean squared difference between TDR-measuredoeff and the modeled oeff were minimized (ParametricTechnology Corporation 2010).Once a mixing model is parameterized, it can be used

to estimate the unfrozen water content (ul) and the icecontent (ui) given the measured oeff. However, the oeff ina frozen soil at a given temperature is dependent on theamount of ice present. The amount of ice present duringa freezing process is proportional to the liquid watercontent prior to freezing and the temperature of the soil.To estimate the measured ul and ui, ul(oeff) and ui(oeff)calibration curves were simulated using the parameter-ized confocal ellipsoid model for the field-measuredwater content just prior to the onset of soil freezing (i.e.,the initial water content prior to freezing) (Table 2).To simulate the calibration curves for each initial watercontent, oeff was calculated with the confocal ellipsoidmodel for a range of pairs of ul and ui, such that the totalwater content was always equal to the initial watercontent:

uluiri

rwul;init (1)

where ul is the liquid, unfrozen water content(cm3 cm3), ui is the ice content (cm3 cm3), uinit isthe initial unfrozen water content prior to freezing, rw isthe density of water (assumed to be 1 g cm3) and riis the density of ice (assumed to be 0.9167 g cm3).The ul(oeff) and ui(oeff) for each are then constructed withlinear interpolation between oeff and ul, and oeff and ui.The calibration curves are presented in Fig. 2.The data presented in this paper were measured

between 2011 Mar. 07 and 2012 Apr. 17. Soil tempera-ture was not logged until 2011 Apr. 05 because coldconditions arrived earlier than expected in the previousfall and their installation was delayed. The TDR probeswere installed prior to soil freezing in the fall of 2010,but data logging did not commence until March 2011.The initial water contents required for calculating


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unfrozen water and ice content in the frozen soil weremeasured gravimetrically on soil samples taken in thefall of 2010 (Turner 2013), and with the TDR data forthe winter of 2011 and 2012. It is assumed that soilfreezes at temperatures 508C based on field tests thatindicate that supercooling of soil water rarely occursin the field (Golubev 1997). Ice contents, therefore, wereonly estimated when the soil temperature was 508C.(Miller 1980; Spaans 1996). Soil water content hereafteris referred to as liquid water content and the total watercontent is the sum of liquid water content and icecontent. During the 2011 field season the snowmeltoccurred between Mar. 29 and Apr. 15, while duringthe 2012 field season the snowmelt occurred betweenMar. 12 and Apr. 05. The growing season for 2011started on Apr. 23 and ended Sep. 01, the 2012 dataanalyzed in this paper ends prior to total soil thawing.

RESULTS AND DISCUSSION

Soil Freezing Processes in Different Thicknessof Topsoil TreatmentsFor the 15-cm topsoil treatment, the time series of airtemperature and precipitation, liquid water, ice, and soiltemperature at six depths below the ground are pre-sented in Fig. 3. Soil froze to a depth greater than 45 cm

(Fig. 3G) below the soil/PG interface in both wintersand remained frozen well into the spring followingsnowmelt. The PG at 45 cm below soil/PG interfacehad completely thawed by 2011 May 17 and had notthawed as of 2012 Apr. 17 (Fig. 3G). An increasedtemperature lag was observed with increasing depth.During the 2011 snowmelt event, the topsoil reachednear saturated conditions for a period of about 2 wkas a result of the reduced hydraulic conductivity of theunderlying frozen PG material (Fig. 3D and E).The PG material directly below the interface beganthawing on 2011 Apr. 21 marked by PG temperaturethat was 08C and an increase in liquid water content.The remaining frozen PG material at greater depthsunderwent similar phase changes for the following 26 d,after which point, the liquid water content stabilized at0.35 cm3 cm3 for the remainder of the growing season.With respect to the topsoil, fluctuations in the watercontent above the soil/PG interface between May andNovember 2011 are due to periodic rainfall eventsfollowed by periods of evapotranspiration, potentialdrainage, and subsurface lateral flow. The summerrainfall events between mid-June and November 2011negligibly affected the soil water content at depthsgreater than 7.5 cm below soil/PG interface, which

Fig. 2. TDR calibration curves for the estimation of liquid unfrozen water content (ul) and ice content (ui) for the topsoil cap (A & B)and the phosphogypsum (C & D).


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indicated that limited amounts of water percolatedbelow the soil/PG interface and the water losses fromthe topsoil following precipitation events were fromevapotranspiration or subsurface lateral flow. Thehydraulic gradient across the soil/PG interface estimatedwith the matric potential sensors, however, suggests thatthere are discrete periods following precipitation eventsthat could contribute to downward percolation whichwill be explored later in this paper.Winter 20112012 was relatively warm compared

with the previous 20102011 winter. The topsoil layer(Fig. 3B) was dry because of the low precipitation in thefall 2011 and as a result of limited winter precipitation

total soil water content remained relatively constantduring the winter. Total soil water content at the soil/PG interface and below (Fig. 3CG) was much higher atthe onset of soil freezing (0.3 cm3 cm3) in the fallof 2011 and a significant drop in liquid water contentwas observed as soil froze. As the PG material froze,the estimated liquid water content decreased and theestimated ice content increased.The soil water content at 7.5 cm above the soil/PG

interface (Fig. 3B) increased concurrently with thesnowmelt events and exceeded the initial water contentindicating infiltration of snowmelt. Figure 3C shows asimilar pattern to Fig. 3B in that percolation appearedto be restricted by the presence of ice in the underlyingPG layer, which was a result of the high water contentin the fall of 2010 (Fig. 3D). Although, a few thawingfreezing cycles occurred during the winter of 2012,only the refreezing event of 2012 Mar. 27 resulted inrefreezing of infiltrating water.Figures 4 and 5 illustrate the soil water content

and temperature measurements for 30- and 46-cmtopsoil cap treatments, respectively. Figure 3A, Fig.4A and Fig. 5A are the meteorological conditions atthe surface, but are repeated to show the correlationbetween surface boundary conditions and soil waterdynamics.Figures 4 and 5 are similar to Fig. 3 (15-cm treatment)

in many respects. Steep increases in water content in thetopsoil cap and PG during the snowmelt periods wereobserved. The date at which the snowmelt reached eachdepth increased with depth especially in the PG becausethe PG took longer to thaw. Liquid water content in thetopsoil responded very rapidly to summer rainfallthroughout the whole topsoil cap as indicated by theliquid water content response measured by the TDRprobes at the soil/PG interface (Fig. 3C, Fig. 4D andFig. 5E). The shallower the topsoil depth, the greater themagnitude of the response of the water content changeto the rainfall events (i.e., Fig. 5BFig. 5CFig. 5DFig. 5E) but the PG water content did not change verymuch. The topsoil was relatively dry after August 2011due to the small amounts of autumn rainfall. Totalwater content in the PG layers (Fig. 3DG, Fig. 4EGand Fig. 5FG) remained relatively constant at approxi-mately 0.35 cm3 cm3 during the periods after springsnowmelt event and before winter 2011. Despite the lackof snow in the winter of 20112012, snowmelt infiltra-tion into the topsoil was very efficient because it was sodry from the previous fall. There is evidence of refreez-ing of snowmelt in the topsoil cap in March 2012, butthis refreezing only resulted in a small amount of iceformation due to the initially dry soil conditions and asa result, infiltration was not inhibited. The observedwater content at the interface of the three treatmentswas higher than the overlying topsoil cap, but lowerthan the underlying PG. The interface is generally theboundary between differing materials which may disruptvertical water movement as discussed in Dyck and

Mar-11 May-11 Jul-11 Sep-11 Nov-11 Jan-12 Mar-12 May-12

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Kachanoski (2009). The water content at the interface ofthe 15-cm plot treatment (Fig. 3C) was consistentlyhigher than that of the 30-cm treatment (Fig. 4D) and46-cm treatment (Fig. 5D), which could be due in part togreater evapotranspirational demand from the 30- and46-cm plots as evident by their slightly more robustplant growth during the growing season (not shownhere). The variability among total soil water contentsbetween the various plot interfaces may be explained bymultiple factors including: (1) topographical variabilityof soil/PG gradients resulting in convergence or diver-gence of lateral flow; and (2) misrepresentative propor-tion of soil versus PG material within the sample volumeof the horizontally installed TDR probes along the soil/

PG interface resulting in measurements more reflectiveof PG than of soil. Generally, the interface between thetwo layers is characterized by the very transient andvariable moisture regime of the topsoil cap and verysteady moisture regime in the PG.The lack of fluctuation of water contents observed

in the PG during the growing season is likely a result ofthe nature of the hydraulic properties between thesoil and PG as well as the evapotranspirational demandof the active vegetation. However, increases in watercontent of the PG material at deeper depths wereobserved for the 30-cm plot following high magnitudeprecipitation events (Fig. 4G), which could be attributedto preferential flow paths. The decrease in water storage



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within the topsoil cap could also be attributed to planttranspiration and potential subsurface lateral flow.Utilization of pore water from the PG material by theplants was minimal because of the lack of rootswithin the PG material. Field observations indicatedthat the majority of root material was located within thetopsoil cap and concentrated on the soil/PG interface(L. Turner, 2013). The only time when the water contentin the PG appeared to increase significantly was duringsoil thawing, suggesting deeper movement of snowmeltwhen the vegetation was still dormant.

Estimation of Cumulative Snowmelt Infiltrationinto Topsoil and Net Vertical Flux within PGTo better assess the performance of the various cappingdepths, the net water flux within the topsoil cap and theunderlying PG material was quantified using the one-dimensional soil water continuity equation:

@u(z; t)

@t

@q(z; t)

@z(2)

Where u is the total water content (cm3 cm3), t is time(s), q is the soil water flux (cm3 cm2 s1) and z is depth(m). Integration of both sides of Eq. 2 between twodepths (z1 and z2) results in:

@

@t gz2

z1

u(z; t)dzgz2

z1

@q(z; t)

@zdz (3a)

which may be expressed as:

@Wz1;z2(t)

@tq(z1; t)q(z2; t) (3b)

where Wz1,z2(t) is the soil water storage between depthsz1 and z2 as a function of time, and q(z1,t) and q(z2,t)are the net vertical soil water fluxes at depths z1 and z2,respectively. In other words, the change in soil waterstorage between depths z1 and z2 over a period of time isequal to the net soil water flux between those two depthsover the same time period. For discrete time periods thatare small enough such that q(z1,t) and q(z2,t) do notchange significantly with time, Eq. 3b may be simplifiedto:

DWz1;z2jDt [q(z1)q(z2)]Dt (4)

Which means the change in soil water storage betweentwo depths measured at two discrete points in time is aresult of the cumulative, net vertical flux occurring in thesoil volume bounded by depths z1 and z2.The cumulative, net vertical flux integrates fluxes

affecting the change in total soil water storage betweentwo depths such as infiltration, drainage, evapo-transpiration and lateral flow over the time period inquestion. Positive changes in storage indicate that thecumulative flux coming into the soil volume (i.e.,infiltration evapotranspiration) bounded by z1 andz2 was greater than the net cumulative flux leaving that

volume (i.e., percolation and other losses such as lateralflow), whereas decreases in soil water storage indicatethe opposite. No change in soil water storage indicatesa scenario where net water coming into the volumebounded by z1 and z2 was equal to the net water goingout and those fluxes may or may not be zero.Equation 4 was applied to two separate soil volumes,

topsoil cap and PG, during periods of time during thesnowmelt period and growing season. Estimates for thetopsoil caps were calculated using the measurementsfrom the vertically installed TDR probes and estimatesfor the PG material was calculated using the measure-ments from the horizontally installed probes at 7.5and 15 cm for discrete time intervals during thesnowmelt period of 2011. Setting z0 at the topsoil/PGinterface with z increasing upward (Table 1) results inz1depth of topsoil, z20 cm for the topsoil cap,and z10 cm, z27.5 cm, z315 cm for the PG.The vertical probes spanned the entire depth of thetopsoil cap. The vertical probes therefore, measured theaverage soil water content over the entire topsoil cap,which can then be used to estimate soil water storageby multiplying by the length of the TDR probe. Forthe horizontal probes in the PG, each TDR probe isassumed to represent the 3.75-cm of PG material aboveand below its location.Time series of the vertical TDR probe measurements

for the 2011 snowmelt period and growing season areshown in Fig. 6. Snowmelt began on 2011 Mar. 29as marked on Fig. 6 when the average air tempera-ture exceeded 08C (solid grey vertical line in Fig. 6A).The start of infiltration was noted by the initial increasein volumetric water content (solid black vertical lines inFig. 6BD). Field observations noted that all snow hadmelted and infiltrated before 2011 Apr. 27.During the spring snowmelt, the underlying PG

material remained frozen limiting the amount of waterable to drain from the topsoil cap. As a result of thiscondition the topsoil within the 15-cm plot approachednear saturation during the early spring, while the 30-and 46-cm plots did not as a result of their increasedstorage capacity. Restrictions in flow between the soiland PG material can be attributed to the hydraulicnature of the underlying PG material, which has asaturated hydraulic conductivity that is an order ofmagnitude lower than that of the capping soil. However,the hydraulic conductivity is further lowered by thepresence of ice-filled pore space within the PG materialthat had already thawed within the topsoil cap earlier inthe spring. Water content in the topsoil caps began todecrease only when the underlying PG material had atemperature of 08C, indicating that the presence of icewas restricting percolation of water from the topsoil intothe PG. On 2011 Apr. 23, the 15-cm plot started to losewater and 2011 Apr. 26 so did the 30- and 46-cm plots(dashed dark grey vertical lines in Fig. 6BD). Beforethese dates the PG material directly underlying thecapping soil had an average temperature B08C and can


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be assumed to have ice within the pore space restrictingflow (Fig. 6BD; Figs. 35).Snowmelt infiltration into the topsoil cap was esti-

mated between the beginning of snowmelt (2011 Mar.29) and the beginning of the active growing season (2011Apr. 23). Between these dates the potential evapotran-spiration is negligible due to the limited photosyntheticability of the plants during this part of the year wherethe average air temperature is B18C. The amount of

infiltration into the topsoil cap as it pertains to availablewater from spring snowmelt (2011 Mar. 29 to Apr. 23)follows a general linear relationship with capping depth(Table 3). The estimates for infiltration for the 15-,30- and 46-cm treatment are 38, 77 and 97 mm,respectively, indicating that an increase in soil cappingdepth results in an increase in infiltration of spring meltwater into the capping soil. Conversely, this wouldsuggest that the potential for runoff and/or subsurfacelateral flow is highest in treatments where the topsoilis limited (i.e., 5 15 cm), due to the insufficient capacityof the topsoil to store the added water. Infiltrationestimates for the 15- and 30-cm treatment would suggestthat potential runoff and subsurface lateral flow couldbe occurring during snowmelt periods; however, the46-cm treatment exhibits an excess amount of waterwithin the topsoil cap than what was available fromsnowmelt and spring precipitation (Table 3). In thisregard, more water has been observed to enter the 46-cmcapping soil than is available from the environment.This phenomenon could be explained by localizedvariability in relief underlying the 46-cm plot that couldcontribute to the convergence of subsurface water flowas well as runoff water from the adjacent 15-cm plot.In general, the relationship between the amount ofinfiltration and capping soil thickness was linearlyrelated, whereby the thicker the topsoil cap the greaterthe snowmelt infiltration capacity.Estimates of net vertical flux can be measured from

the amount of total water entering and leaving the PGas measured by the horizontally installed TDR probedirectly below the soil/PG interface. Integrating thechanges in total water content below the soil/PG inter-face with depth, from 2011 Apr. 23 to Sep. 01, givesan estimation of the net vertical flux below the interfaceduring the entire growing season. This estimation isconservative with regards to the potential drainagebecause it does not take into account the potential per-colation that could have occurred immediately followingthe snowmelt infiltration as represented by the humpin the water content distributions curves (Fig. 3D,

Table 2. Parameters of confocal ellipsoid models for estimating unfrozenwater and ice content

Parameters

Soil type

Self-

consistency Aspect RMSEz Bulk density

Topsoily 0.649 1 0.809 1.3Gypsumx 0.537 1 0.918 1.2

zRMSE1Ni1Noioi.yTopsoil is Mundare loamy sand.xGypsum is phosphogypsum tailings.

Table 3. Estimates of cumulative spring inltration into the topsoil capand net storage and vertical ux within PG material over the 2011growing season

Soil treatment

Available H2O(SWEz

Precip.y) (mm)

Springx cumulativeinfiltration topsoil (mm)

Net cumulativevertical fluxvw

PG (mm)

15 cm 88 38 2030 cm 90 77 2846 cm 82 97 32

zSnow water equivalent.yCumulative precipitation on site from 2011 Feb. 26 to Apr. 23.x2011 Mar. 29 through to Apr. 23.w15-cm-thick layer of phosphogypsum (PG) directly below soil/PGinterface.v2011 Apr. 23 to Sep. 01.

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Fig. 4E, Fig. 5F). As a result of this simplification thenet vertical flux estimates are reflective of the overallwater balance within the PG material directly below thesoil/PG interface, showcasing the net gains and/or lossesduring the entire 2011 growing season.The cumulative, net vertical flux estimates (Eq. 4)

over the entire growing season (2011 Apr. 23 to Sep. 01)for the PG layer directly below the interface were 20,28 and 32 mm for the 15-, 30- and 46-cm treatments,respectively. The increase in the estimated cumulative,net vertical flux of the PG material with increasingtopsoil depth is primarily due to the increased avail-ability of infiltrating spring snowmelt water that ispresent within each treatment. The more water that isavailable within the topsoil, from infiltration, the greaterthe potential will be for percolation and potentialdrainage. Although this result is counterintuitive withrespect to conventional hypotheses regarding topsoilcapping depth and drainage estimates, it can be ex-plained by the limited ability of the loamy sand materialto retain water (Fig. 1) in combination with the in-creased amount of available water that is permitted toinfiltrate into the topsoil. For example, the amount ofwater that infiltrated into the topsoil cap of the 15-cmtreatment was 38 mm, which accounted for 43% of theavailable water during spring snowmelt, compared withthat of the 30-cm treatment that allowed 86% ofavailable water to infiltrate. The increased infiltrationcapacity of the thicker capping soils allows more waterto enter into the system; however, the inability of theloamy sand material to retain the water allows forexpedited redistribution of water within the reclaimedsystem resulting in increased estimates for net verticalflux with increased capping soil depth.

Direction of Net Vertical FluxChanges in soil water storage, or net vertical flux,within the PG material, between the months of Apriland September 2011, are positive indicating that ingeneral the amount of soil water entering into the PGfrom the topsoil cap is greater than the amount of soilwater leaving the PG material; however, it can only beassumed that the water is percolating from the topsoilinto the PG. To estimate the direction of flow totalhydraulic head was quantified using the matric potentialsensors installed at 7.5 and 7.5 cm from the soil/PG interface. The difference in total hydraulic headbetween 7.5 and 7.5 cm was used to obtain thehydraulic gradient across the interface.During the snowmelt of 2011, the hydraulic gradients

for the 15-, 30- and 46-cm treatment were all positive,indicating that the overall water flux was in the negativedirection (downward) and that drainage/percolationwas occurring. For the 15-, 30- and 46-cm treatmentsthe gradients remained positive for 32, 47 and 39 d,respectively, from early April to mid-May, 2011 (Fig. 7).During this significant period of time soil water losseswithin the topsoil caps, following snowmelt infiltration,

is most likely primarily a function of drainage and canbe minimally attributed to active evapotranspirationthat was limited during the early spring. However, frommid-May through to September 2011 evapotrans-pirational demand increased, resulting in periods ofnegative hydraulic gradient across the interface, thusindicating upward water flux from the moist PGmaterial to the drier loamy sand topsoil. This generalsummer trend was interrupted by several high magni-tude rainfall events on 2011 Jun. 16, Jul. 09 and Jul. 22,which produced 45, 27 and 66 mm of precipita-tion, respectively (Fig. 6A). During these rainfall events(25 mm) positive hydraulic gradients were observedacross the soil/PG interface for each of the topsoiltreatments, indicating that drainage/percolation wasoccurring. The total amount of days from the start ofsnowmelt to the end of the growing season (2011 Mar.29 to Sep. 01) that were observed to have drainage/percolation events present were 49, 78 and 57 for the15-, 30- and 46-cm treatments, respectively. No trendappears to be present with regards to capping soiltreatment and duration of drainage/percolation events;however, the 30-cm capping soil treatment was observedto have the longest periods of drainage/percolation,

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accounting for 44% of the growing season, compared tothat of the 15- and 46-cm treatments that have drainage/percolation estimates that account for 28 and 33% ofthe growing season (2011 Apr. 23 to Sep. 01).

Uncertainties in Soil Water, Infiltration andNet Flux EstimatesSources of errors in the snowmelt infiltration estimatescan be traced back to the distribution of water contentmeasurements that were used to compute storage.As a result of increasing depth computation in derivingstorage terms, the errors will linearly increase withincreasing capping depth; however, variability betweenthe various water content measurements during thesteady state observations was very small (B0.01 cm3

cm3). Other sources of error with regards to infiltra-tion estimates include the possibility of steady state flowwithin the PG material during snowmelt periods. Forexample, the topsoil cap of the 15-cm treatment hada very high water content that remained stable duringsnowmelt infiltration, we assumed the lack of watermovement was due to the limited ability of the frozenPG material to transmit the water; however, understeady state flow dynamics water content distributionwithin the topsoil cap would not change. The result ofthis assumption could have been an underestimation ofthe potential amount of available water infiltratinginto the 15-cm plot; however, the evidence for limitedflow dynamics below the soil/PG interface during theearly spring snowmelt event indicated that water move-ment below the interface was limited as evident by theminimal increase in water content within the PGmaterial prior to PG thaw. Furthermore, the hydraulicgradient across the soil/PG interface within the 15-cmplot remained negative until 2011 Apr. 09, indicatinga positive flux direction; however, after 2011 Apr. 09the interface temperature exceed 08C and the hydraulicgradient reversed its direction, indicating drainage(Fig. 7). This does not entirely rule out the possibilityof drainage during periods where there is no observablechange in soil water storage in the topsoil cap, but it islikely very small if it does occur.As mentioned above, TDR estimates of unfrozen

water and ice content during periods when the soil isfrozen are based on the assumption that the total watercontent within the sampling volume of the TDR probedoes not change. Unfortunately, it is during times ofsnowmelt infiltration into frozen soils that this assump-tion is most likely to be violated. Because soil thawingand snowmelt infiltration both increase the liquidwater content of the soil, there is some uncertaintyassociated with the unfrozen water and ice contentestimates presented, especially in the more responsivetopsoil cap. However, because of the very dry fallsoil water conditions in the topsoil, these uncertaintiesare not expected to cause a misinterpretation of theprocesses occurring in the field. Furthermore, the

estimates of net cumulative vertical flux are based onsoil water measurements in unfrozen soil where theconstant total water content assumption is no longerrequired.

CONCLUSIONTDR-estimated liquid water and ice contents in frozensoils were used to aid in the interpretation of hydro-logical processes in frozen soils at a reclaimed site.Results indicate that increases in capping soil thicknessresulted in increases in infiltration of available snow-melt water into the capping soil and that lateral runoffestimates increased with decreasing capping depth. Thisresult is primarily a response to the increase in thecapacity of the deeper capping soils to infiltrate water;however, the limited ability of the loamy sand topsoilto retain water allows for the expedited redistribu-tion of water within the reclaimed system. Therefore,greater capping soil depths appear to be associatedwith greater potential for percolation below the cap-ping soil. Soil water balance estimates for the PGmaterial directly underlying the treatment plots indicatethat an increase in capping soil depth increased thenet vertical flux within the first 15 cm of PG, whichreflects the limited water retention characteristics of thetopsoil. The estimated hydraulic gradient across thesoil/PG interface indicates that during the early springand severe precipitation events (25 mm) drainage/percolation was occurring throughout all treatmentdepths; however, no clear trend was present in relationto treatment depth and duration of drainage/percola-tion events.This research also confirms the importance of

establishing very clear reclamation goals to aid in theselection of capping materials for mine tailings. At theminimum, a reclaimed tailing stack or pile should beable to support the establishment and maintenance ofvegetation. It should limit exposure of environmentalreceptors to potential contaminants within the tailings.A soil with good infiltration and water-holding capa-city should be able to aid in achieving these two goals.However, as shown in this paper, the thickness of thetopsoil cap in combination with its infiltrability andwater-holding capacity is a key consideration as well.Under the conditions reported in this paper a thickertopsoil cap resulted in greater water movement throughthe underlying PG tailings, which may or may not bedesirable depending on the reclamation goals. Further-more, moisture dynamics during freezethaw periodsneed to be taken into consideration when assessingreclamation goals and successful site reclamation.

ACKNOWLEDGMENTSFunding for this research was provided by the NaturalSciences and Engineering Research Council of Canada,Agrium, Inc. and the University of Alberta. Special


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thanks also go to Dick Puurveen and the researchstaff at the Ellerslie Research Station for technicalsupport.

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