PEDOLOGIC AND CLIMATIC CONTROLS ON RN-222 CONCENTRATIONS. IN SOIL GAS, DENVER, COLORADO. Sigrid Asher-Bolinder, Douglass E. Owen, ...
GEOPHYSICALRESEARCHLETTERS, VOL. 17, NO. 6, PAGES 825-828,
PEDOLOGIC
AND CLIMATIC
CONTROLS
MAY 1990
ON RN-222 CONCENTRATIONS
IN SOIL GAS, DENVER, COLORADO
SigridAsher-Bolinder, Douglass E. Owen,andR. RandallSchumann U.S. GeologicalSurvey
Abstract.Soil-gasradonconcentrations are controlledseasonally byfactorsof dimateandpodology.In a swelling soi!of thesemiarid WesternUnitedStates, soil-
vegetated, studysiteonthe FederalCenter,Denver,
Colorado.Measurements weremadenearlyevery weekdaybetween1000and 1400hoursMST to minimizethe
gas radon concentrations at !00cmdepthincrease inwinterandspring dueto increased emanation withhigher
effects of diurnalradon-concentration variations. A permanentlyinsertedstainless steelprobewasusedto samplesoilgas,usingthetechnique andequipment described byReimer[1989].An estimate of soilpermeabilityat 100cmwasmadeby notingthetimeit tookto extractsoilgasfromthe probe. Radonconcentratiom in the soilgasweremeasured usinganalphascintillometer, andthecounts wereconverted to picocuries perliter (pCi/L) usinga calibration equation.Dailytotalsolar radiationmeasurements weretakenlessthan3 kmaway
soilmoistureandthe cappingeffectof surfacewateror ice. Increasedsoil moisture resultsfrom a combinationof
higher winterandspring precipitation anddecreased insolation in fall andwinter,loweringsoiltemperatures so thatwaterinfiltratesdeeperandevaporates moreslowly. Radonconcentrations in soildropmarkedlythroughthe summer andfall. The increasedinsolationof springand summer warmsand driesthe soil,limitingthe amountof waterthat reaches!00 cm. As the soil dries, radon ema-
fromthestudysite.Soiltemperatures at 60cmdepth wererecordedat thetimeof Rn measurement. Precipitationdatafroma weatherstation0.4kmawaywere collected hourly.The presence, width,andapproximate
nationdecreases, anddeepsoilcracksdevelop.These cracks aid convective transportof soilgas,increase redoh's flux into the atmosphere,andlower its concentrationin soilgas. Probablecontrolson the distribution
depthof soilcracks,andthe presenceor absence of standingwater, snow,or ice on the soirssurfacewere also noted. Soil-gasRn concentrations, insolation, soiltem-
of uranium within the soil column include its downward
leaching, its precipitationor adsorptionontoB-horizon clays, concretions, or cement,and the uraniumcontent andmineralogyof the soirsgraMticand gneissic
perature,soil-crackdevelopment, andprecipitation data are shownfor March1987throughMarch1988(Figure 1). Maximumandminimuminsolation precedes correspondingsoil-temperature maximumandminimumby a monthto month-and-a-half.Degreeof soilcracking echoesthe soil-temperature curve.Precipitation showed varyingdegreesof efficacyin eliminating surface soil
precursors.
Introduction
Investigators havenotedthatsoil-gas radon-222 (Rn)concentrations varyin magnitude byseason andthat theseason of highestconcentration variesfromsitet-o site.Seasonal Rn changes havebeenascribed to differences in soilmoistureaffectingRn emanation fromsoil particles [Lindmark andRosen,1985];sealing of upper soilporesbywateror ice,preventing Rn exhalation into theatmosphere [Kovach,1945andAsher-Bolinder et el., 1989]; andgaseous/aqueous Rn partitioning influenced bysoiltemperature [Washington andRose,thisissue].
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40'
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Moststudies ofseasonal variation insoil-gas Rnconcentrations havebeenperformed in humidor subhumid climates. Thepurpose ofthisstudy wasto determine the major climatic andpedologic controls onseasonal changes in soil-gas Rn concentration in a semiarid climate. Results indicate thatsoil-gas Rn concentrations arecontrolled byclimaticandpedologic factorsdifferent
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