University of Nebraska - Lincoln
DigitalCommons@University of Nebraska - Lincoln
USGS Staff -- Published Research
US Geological Survey
2016
Deep subsurface drip irrigation using coal-bed
sodic water: Part II. Geochemistry
Carleton R. Bern
Denver Federal Center
, [email protected]
George N. Breit
Denver Federal Center
Richard W. Healy
Denver Federal Center
John W. Zupancic
BeneTerra LLC, 1415 N. Main St., Sheridan, WY
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Bern, Carleton R.; Breit, George N.; Healy, Richard W.; and Zupancic, John W., "Deep subsurface drip irrigation using coal-bed sodic
water: Part II. Geochemistry" (2016).
USGS Staff -- Published Research
. 912.
ContentslistsavailableatSciVerseScienceDirect
Agricultural
Water
Management
jo u r n al h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / a g w a t
Deep
subsurface
drip
irrigation
using
coal-bed
sodic
water:
Part
II.
Geochemistry
Carleton
R.
Bern
a,∗, George
N.
Breit
a, Richard
W.
Healy
b, John
W.
Zupancic
caCrustalGeophysicsandGeochemistryScienceCenter,U.S.GeologicalSurvey,DenverFederalCenter,Denver,CO80225,USA bNationalResearchProgram,U.S.GeologicalSurvey,DenverFederalCenter,Denver,CO80225,USA
cBeneTerraLLC,1415N.MainSt.,Sheridan,WY82801,USA
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:Received14March2012 Accepted16November2012
Available online 25 December 2012
Keywords:
Gypsum PHREEQC
PowderRiverBasin,Wyoming Sodiumadsorptionratio Sulfuricacid
a
b
s
t
r
a
c
t
Waterswithlowsalinityandhighsodiumadsorptionratios(SARs)presentachallengetoirrigation becausetheydegradesoilstructureandinfiltrationcapacity.InthePowderRiverBasinofWyoming, suchlowsalinity(electricalconductivity,EC2.1mScm−1)andhigh-SAR(54)watersareco-produced
withcoal-bedmethaneandsomeareusedforsubsurfacedripirrigation(SDI).TheSDIsystemstudied mixessulfuricacidwithirrigationwaterandapplieswateryear-roundviadriptubingburied92cm deep.Aftersixyearsofirrigation,SARvaluesbetween0and30cmdepth(0.5–1.2)areonlyslightly increasedovernon-irrigatedsoils(0.1–0.5).Only8–15%ofaddedNahasaccumulatedabovethedrip tubing.Sodicityhasincreasedinsoilsurroundingthedriptubing,andgeochemicalsimulationsshow thattwopathwayscangeneratesodicconditions.Insoilbetween45-cmdepthandthedriptubing, Nafromtheirrigationwateraccumulatesasevapotranspirationconcentratessolutes.SARvalues>12, measuredby1:1water–soilextracts,arecausedbyconcentrationofsolutesbyfactorsupto13.Low-EC (<0.7mScm−1)iscausedbyrainandsnowmeltflushingthesoilanddisplacingionsinsoilsolution.Soil
belowthedriptubingexperienceslowersoluteconcentrationfactors(1–1.65)duetoexcessirrigation waterandalsocontainsrelativelyabundantnativegypsum(2.4±1.7wt.%).Geochemicalsimulations showgypsumdissolutiondecreasessoil-waterSARto<7andincreasestheECtoaround4.1mScm−1,thus
limitingnegativeimpactsfromsodicity.Withsustainedirrigation,however,downwardflowofexcess irrigationwaterdepletesgypsum,increasingsoil-waterSARto>14anddecreasingECinsoilwaterto 3.2mScm−1.Increasedsodicityinthesubsurface,ratherthanthesurface,indicatesthatdeepSDIcanbe
aviablemeansofirrigatingwithsodicwaters.
Published by Elsevier B.V.
1. Introduction
Anirrigationmethodhasbeendevelopedtodisposeofaportion of the largevolumes of sodic waters co-produced by coal-bed methaneenergy(CBM)developmentinthesemi-aridPowderRiver Basin,whilealsoputtingthewaterstobeneficialuse(Zupancic etal.,2008).Themethodcombinesyear-roundirrigation, acidifica-tionoftheCBMwaterbyadditionofsulfuricacid(H2SO4),anddeep placementofdriptubingforsubsurfacedripirrigation(SDI). Year-roundirrigationisrequiredtokeep pacewithwaterproduction fromgaswells.Aswillbeshownhere,acidificationhelpsto miti-gatetheimpactofthesodicwatersonsoil.Deepplacement(92cm) ofthedriptubingpreventsfrostdamagetothesystemduring win-teroperationandhelpspreventNafromtheirrigationwaterfrom risingtothesoilsurface.Between2000and2010,over9.8×108m3 ofCBMwater wereproduced intheWyoming portionof Pow-derRiverBasin(WyomingOilandGasConservationCommission,
∗Correspondingauthor.Tel.:+13032361024;fax:+13032363200.
E-mailaddress:[email protected](C.R.Bern).
2011).Currently,SDIsystemsliketheonedescribedutilizeabout 5%ofannualCBMwaterproductioninthePowderRiverBasin(Don Fischer,WyomingDepartmentofEnvironmentalQuality,personal communication,3/3/2011).
The CBM water in the Powder River Basin is
attrac-tive to agriculture because some of it is low-salinity
(ranges: EC=0.3–4.1mScm−1, total dissolved solids, TDS=200–4000mgL−1) (Bartos and Ogle, 2002; Jackson and
Reddy,2007;Riceetal.,2002).Incontrast,watersco-produced fromtraditionaloilandgaswellsaremuchmoresaline(Wyoming median TDS=6500mgL−1) (Breit, 2011). Sulfate reduction, cation exchange, microbial processes and precipitation of car-bonate minerals in thesource rockscause groundwater in the methane-producing coal bedsto evolve to a characteristic Na-HCO3 composition (Brinck et al., 2008; Van Voast, 2003). The sodiumadsorptionratio(SAR)isacommonmeasureofsodicity andiscalculatedas
SAR= [Na+]
([Ca2+]+[Mg2+])1/2
(1)
0378-3774/$–seefrontmatter.Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.agwat.2012.11.013
This document is a U.S. government work and is not subject to copyright in the United States.
whereconcentrations are millimolesperliter. Unitsof SAR are mmol1/2L−1/2,butcommonconventionistoomitunitsinreporting thevalues.SurveysofwatercompositionsfromCBMwellsinthe PowderRiverBasinfoundSARvaluesrangingfrom5to69(Bartos andOgle,2002;JacksonandReddy,2007;Riceetal.,2002).Such SARvaluesindicatethat someCBMwatersinthePowderRiver Basinposenohazardtosoilifusedforirrigation,whileotherspose aseverehazardtosoilstructure(AyersandWestcot,1985). Increas-ingtheproportionofNa+ontheexchangesitesofexpansiveclays resultsingreaterhydration(swelling)whichdisruptssoilstructure throughdisaggregationanddispersionofclays(Quirk,1986).The swellingisincreasedwithgreateractivityofwater(lower salin-ity),aswellashigherpHwhichfavorsrepulsionofclayparticles (RengasamyandSumner,1998).Soilsexhibitingphysicaleffects fromhighsodicityandlowsalinitycansufferfromcrusting,erosion, pooraerationandlackofinfiltration(OsterandJayawardne,1998). Bicarbonatealkalinityposesanadditionalproblemforirrigation,as hasbeenseeninothersettings(Bajwaetal.,1992).Uponcontact withsoil,thebicarbonate(HCO3−)cancombinewithavailablesoil solutionCa2+toprecipitateascalcite(CaCO
3),drivingEClowerand SARhigher.
SurfaceirrigationwithCBMwaterinthePowderRiverBasin increasestheSARandECofsaturatedpasteextractsandcausesNa toaccumulateinsoils(Ganjegunteetal.,2005).Whileirrigation canincreasebiomassintheshort-term,overtimeitcanresultin accumulationofsaltsanddegradationofsoilstructure(Ganjegunte etal.,2008;Vanceetal.,2008).Evenasingleyearofsodic-water irrigationcanbesufficienttodecreaseinfiltrationrates(Johnston etal.,2011).AstudytestedPowderRiverBasinsoilsundercycles ofirrigationwithamixtureofCBMwaterandlowsodicitywater (mixture: EC 3.1mScm−1, SAR 13) followed by infiltration of rainfall-qualitywater(Bauderetal.,2008).Thecombination gen-eratedthehigh-SARandlow-ECconditionswhichcancontribute substantiallytoswellinganddispersionofclayparticlesandresult inlossofsoilpermeability(Bauderetal.,2008;Suarezetal.,2006). Soilandwatertreatmentsformitigatingthesoileffectscaused bysurfaceirrigationwithCBMwaterhavebeentested.Gypsum (CaSO4·2H2O)isacommonsoilamendmenttoameliorate sodic-ityproblemsbecauseitlowersSARbyprovidingCaandraisesEC throughitshighsolubility(Gobranetal.,1982;Oster,1982). Gyp-sumhasbeenusedwithsomesuccessinthePowderRiverBasin (BrinckandFrost,2009;Johnstonetal.,2008).Betterresultshave beenobtainedbyaltering thechemistryofCBMwaterbyusing sulfurburnersandgypsuminjectioncombinedwithapplicationof gypsumandelementalsulfurtosoil(Johnstonetal.,2008).Despite additionsofacidityandCa,toirrigatedfields,solublesalts period-icallymustbeleachedwithexcessirrigationwatertoremoveNa accumulationsinsoilssurface-irrigatedwithCBMwater(Johnston etal.,2008).Theamountofwaterprovidedbyprecipitationinthe PRBmaybeinsufficientforleachingofexcesssalt,asindicatedbya studyconductedtwoyearsafterasingleyearofCBMsurfacewater irrigation(31cmCBMwater;SAR=24)(Johnstonetal.,2011).
ThedeepSDImethoddescribedhereisexaminedfromthe per-spectiveofwaterbalanceandsolutemovementinthecompanion tothispaper(Bernetal.,2012).Thestudyoftwositesfoundthat theirrigationwaterisdrawnupwardintosoilabovethedrip tub-ing,whereevapotranspirationconcentratestheassociatedsolutes (Fig.1).ExcesswaterbelowthedriptubingkeepsSDIsolutesfrom concentratingthereandleachesbothSDIandnativesolutestoward thewatertable.Hereweexaminethegeochemicalpatternsthatsix yearsofirrigationhaveimprinteduponanalfalfafieldandagrass fieldatasingleareabycomparingthemtonon-irrigatedsoilfrom adjacentfields.Computersimulationswereusedtodescribethe geochemicalinteractions betweenirrigationwater andsoil.The goalistoshowhowconditionsandprocessesaredifferentinsoil aboveandbelowthedriptubing,yetbothdeveloprelativelyhigh
Fig.1.SchematicdepictingSDIsystemandgeneralizedsoilzoneswheredifferent patternsofwaterandsolutemovementdominate.Intheupperzone,solutesfrom theSDIsystemmixwithdiluteprecipitationwaterbutconcentrationby evapo-transpirationdominates.Inthelowerzone,excesswatermaintainsmoredilute concentrationsandleachessolutesdownward.Whitearrowsdepictrelativewater movementfromthedriptubing.Theboundarybetweenzonesisdepictedasa distinctblackline,butislikelytobediffuseanddifficulttoidentifyinsoil.
sodicityandlowsalinity.Previousstudieshaveusedcoupled geo-chemicalandunsaturatedflowsimulationstoassessdevelopment ofsoilsodicity,butthecalibrationandassessmentdatatypically comefromcolumnexperiments(Jalalietal.,2008;Suarezetal., 2006).Hereweusedatafromnon-irrigated,controlsoilsadjacent toSDIfieldstosimulatepre-irrigationsoilconditions.The acidifica-tionofCBMwateranditsequilibrationwithsoilisthensimulated, alongwithevapotranspirationconcentrationofsolutesand(below thedriptubing)transportofwater andsolutesdownward with excessirrigationwater.Resultsfromthesimulationsarecompared togeochemicalpropertiesofsoilinthetwofieldsirrigatedforsix yearsbythedescribedSDImethod.Understandingsoil geochem-icaleffectsofthisSDImethodwillimprovetheassessmentofits impactsonsoilsofthePowderRiverBasin(Engleetal.,2011),and willalsoprovideinsightonthebroaderissueofusingsodicwaters forcropirrigation(QadirandOster,2004).
2. Materialsandmethods
2.1. SitedescriptionandSDIoperation
The Platmak SDI site is located ∼14km northeast of Sheri-dan,Wyomingatanelevationof∼1120m.Theregionalclimateis semi-aridwithmeanannualprecipitationof∼370mmandamean annualtemperatureof∼7◦C(NationalClimaticDataCenter).Oneof theSDIfieldsstudiedwasplantedwithalfalfa(Medicagosativa)and thesoilismappedasaZigweid–Kishona–Cambriacomplex(Soil SurveyStaff).Theotherfieldisplantedwithgrasses(Dactylis
glom-erataL.,Festucaarundinacea,Bromusbiebersteinii)andismappedas
Platmakloam,asarenearbynon-irrigated,controlsoils(SoilSurvey Staff).Thealfalfafieldstudiedis2.9hainsizeandthegrassfieldis 3.7hainsize.TheextentofSDIirrigationinthestudyareais84ha. ThesitewasnotirrigatedpriortoinstallationoftheSDIsystem. DailyirrigationviatheSDIsystembeganinJanuary2005andhad
beencontinuousthroughsixgrowingseasonsatthetimeofthis study.CBMwaterisproducedbygaswellsyear-round,and year-roundirrigationisrequiredtokeeppace.TheCBMwateris deliv-eredtoaholdingpondattheSDIsiteviapipeline.DegassingofCO2 andothervolatilecompoundsoccursatthepond.Subsequently,the waterisacidifiedtoapproximatelypH6.0byautomatedin-line additionofsulfuricacid.ThetargetpHis5.5,butthereare tech-nicalchallengestoin-linemixingoftheviscousacidwithwater. Continuousacidificationhasbeenaccomplishedsinceabout2007. Discfiltersremovedparticulatematterfromthewater,whichwas pumpedtodriptubingbeneaththefields.Depthandspacingof thedriptubingwereselectedbytheoperatoronthebasisofsoil permeability,frostline,andcropcharacteristics(Zupancicetal., 2008).Polyethylenedriptubingisinstalled92cmbelowthesurface andspaced152cmapart.Pressure-compensatingemitterswitha flowrateof1Lmin−1arespaced92cmapartalongthetubing.A totalof8410and6600mmofwaterwereappliedtothestudied alfalfaand grassfields, respectively,betweenJanuary2005and August2010.Becauseofthelimitedlife-spanofboththeproduced waterboomandthenormaldegradationofSDIinfrastructure,the planneddurationofirrigationforthesefieldsisaboutadecade.
2.2. Samplingandanalyticalmethods
2.2.1. Watersamplingandanalysis
Watersampleswerecollectedtodeterminethecompositionof CBMwatersuppliedtothesite,holding-pondwater,andacidified irrigationwaterpumpedtothefields.Quarterlysamplingfound lit-tlevariationinthecompositionofCBMwateroverthecourseofsix yearsofirrigation,apatternthatcouldbeattributedtothewater beingablendfromnearly1500wells(BeneTerraLLCdata).Water samplesforthisstudywerecollectedduringeachoffourfieldvisits in2010.Temperature, specificconductance, andpHwere mea-suredinthefield.Watersamplesforlateranalysiswerecollectedin cleanedpolyethylenebottles.Samplesforchemicalanalysiswere filteredto<0.2m,andaliquotsforcationanalysiswereacidified withnitricacid. Samplesfor anion analysiswerekept refriger-atedandinthedarkuntilanalyzed.Unfilteredwatersampleswere analyzedforalkalinitybytitrationusing1.6Nsulfuricacid(Hach Company,Loveland,CO,USA).
Cationand anion analyseswere completedat the U.S. Geo-logical Survey Laboratories in Denver, CO. Cation (Na, Ca, Mg, and K)concentrations weredeterminedby inductively coupled plasma-massspectrometryandSO42−,andCl−concentrationsby
ionchromatography(IC)attheU.S.GeologicalSurveyinDenver. Standardreferencesamples(WoodworthandConnor,2003)and standardspreparedfromlaboratoryreagentswereanalyzed along-sideunknowns.Concentrationsofreferencesamplesandstandards werewithin10%ofexpectedvalues.Charge-balanceerrorsonall sampleswere<12%.
2.2.2. Soilsamplingandmineralogicalanalysis
Aland-surface-basedelectromagneticgeophysicalsurveyofthe Platmakgrassandalfalfafieldsandsomeadjacentnon-irrigated landwascompletedinFebruary2010(Burtonetal.,2010).The resultingmapofsubsurfacebulksoilelectricalconductivitywas usedtoselectlocationsforsoilsampling.Twogroupsofsoilcores werecollected,whichweredesignatedas“solitary”or“transect” cores.Threesolitarysoilcoreswerecollectedinthealfalfafieldand threeinthegrassfield.Inbothfields,solitarycoreswerecollected atlocationsselectedtorepresenthigh,low,andintermediatebulk soilelectricalconductivity.Theproximityofsolitarycorestodrip tubingwasunknown.Forthetransectcoresampling,alocationof intermediateelectricalconductivitywasexcavatedinthealfalfa fieldtolocatetwoadjacentlinesofdriptubing.Withthose loca-tionsmarkedonthesurface,threetransectcoreswerecollected
alongatransectperpendiculartoandatknowndistancesfromthe driptubing.Corescomprisingthetransectwerecollectedwithone essentiallyinlinewiththedriptubing,anotherhalfwaybetween thelinesofdriptubing,andthelastspacedevenlybetweenthose twocores.Asecondsetofthreetransectcoreswascollectedat approximately7mdistancefromthefirsttransectalongthesame driptubing.Theprocesswasrepeatedinthegrassfield.Atotalof 18coreswerecollectedfromthegrassandalfalfafields(9cores each).Fivesoilcoreswerecollectedatlocationsonnon-irrigated land40–150moutsidetheSDIfieldstoserveascontrolsites.
Soilcores(45mmindiameter)werecollectedinplasticliners fromthealfalfaSDI,grassSDI,andnon-irrigatedcontrolsoilsby usingahydraulicsoilsampling,coringanddrillingmachine (Gidd-ingsMachine Company,Windsor,CO).1Coresweredividedinto
15-cmdepthincrementsfrom0to150cm,andinto30cm incre-mentsbetween150cmandbottomofthecore,usually450cm.One setofalfalfatransectcoresandtwocontrolcoreswerecollectedin February2010.AllothercoreswerecollectedinAugust2010.Soil coresweresplitinthefieldand469sampleswerecollected.Soil sampleswereairdriedinthelaboratorythendisaggregated,sieved to<2mm,andsplitbyusingaJonessplitterpriortoanalysis.
Asubsetof76soilsamplesfromtheFebruary2010sampling wasanalyzedbyquantitativeX-raydiffraction(XRD)todetermine mineralabundances.Sampleswerespikedwith20wt.%corundum, micronizedinisopropylalcohol,sieved,andplacedinside-packed powder mounts.Sampleswere scannedoneithera PANalytical “X’PertPro-MPDX-rayDiffractometeroraScintagX-1 Diffractome-ter”,andresultingspectrawereprocessedbyusingtheRockJock software,v.11(Eberl,2003).TheresultsofRockJockprocessingof XRDspectrawerenotnormalizedto100%.
2.2.3. Soilchemicalanalysis
Chemical properties of 389 soil samples were analyzed by Servi-TechLaboratories(Hastings,Nebraska).Soilcarbonate con-tent, reportedas CaCO3, wasdetermined bymeasuringthe pH of a mixture ofacetic acid and soil(Loeppert etal., 1984).EC, pHandconcentrationsofNa,Ca,Mg,andKweredeterminedfor soil extractsat 1:1water–soil weightratio. The1:1 water–soil ratio wasselectedfor ease ofcomparison to computer simula-tionsofsoilextracts.Tobetterassesstotal soilNaand gypsum content,waterextractableNaandSweredeterminedonsamples extractedovernightat20:1water–soilratios.Waterextractswere separatedfromsoilbyusingpaperfilters(Ahlstrom,#6420-0550) andelementalconcentrationsdeterminedbyinductivelycoupled plasma-atomicemissionspectroscopy.
ConcentrationsofCO2 in thesubsurface air-filledporespace weremeasuredinOctober2010attheeightcoringlocationsinthe alfalfaandgrassSDIfields.Steeltubing3.5mmindiameter,with perforationsonthesidesnearthetip,waspressed75cmdeepinto soil.Thetubingwaspurgedbydrawingouttwofullvolumesof gasandthenasampleofsoilgaswascollectedbyusinga25mL syringe.Theconcentrationof CO2 wasmeasuredinthefieldby injectionofthesampleintoanEGM-1infra-redgasanalyzer(PP Systems,Amesbury,Massachusetts,USA).
2.3. Geochemicalsimulations
2.3.1. Simulationofirrigationwaterandpre-irrigationsoil
composition
Geochemicalsimulationswereusedtoexaminetheinteractions betweenirrigationwaterandsoil.Fourdistinctscenariosofsoil geochemistry(Tables1and2)weresimulatedusingthePHREEQC 1Anyuseoftrade,firm,orproductnamesisfordescriptivepurposesonlyand
Table1
Summaryofpre-irrigationgeochemicalconditionsindifferentsimulatedscenariosforSDIsoilaboveandbelowthedriptubing.
Soilzonesimulated Scenario Soilgypsumcontent Water–soilratio(gg−1) EC(mScm−1) SAR pH ESP
Abovedriptubing 1–Gypsumabsent 0% 0.12 0.9 1.8 7.1 3%
Abovedriptubing 2– Gypsumpresent 0.001–1.4% 0.12 2.9 0.8 6.8 1%
Belowdriptubing 3–Acidifiedirrigationwaterleaching 2.4% 0.14 5.1 10.6 7.0 14%
Belowdriptubing 4–Non-acidifiedirrigationwaterleaching 2.4% 0.14 5.1 10.6 7.0 14%
Table2
SummaryofparameterssimulatedindifferentscenariosforSDIsoilaboveandbelowthedriptubing.
Soilzonesimulated Scenario Advection/transportof
SDIwater?
Rain/snow infiltration?
SDIwater
acidified
SDIwaterevapotranspiration
concentrationfactors
Abovedriptubing 1–Gypsumabsent No Yes Yes 1–15
Abovedriptubing 2– Gypsumpresent No No Yes 1–7
Belowdriptubing 3–Acidifiedirrigation
waterleaching
Yes No Yes 1.0,1.3,1.65
Belowdriptubing 4–Non-acidified
irrigationwater
leaching
Yes No No 1.0,1.3,1.65
software(version2.18.3)withthePitzerdatabase(Parkhurstand Appelo,1999).2Scenarios1and2refertosoilabovethedriptubing,
whereevapotranspirationconcentratessolutestoagreaterdegree. Scenarios3and4refertosoilbelowthedriptubing,whereexcess irrigationwater transportssolutesdownward towardthewater table(Fig.1).Allfourmodelssharedtwocommonsteps:simulation ofthechemicalcompositionofirrigationwater,andsimulationof thechemicalcompositionofpre-irrigationsoilwater.
WaterwiththecompositionofmeanCBMwaterdeliveredto thePlatmaksite(Table3)wassimulatedtoundergodegassingof CO2,asitdoesintheholdingpond.Then,acidificationofthewater topH6.0byusingsulfuricacidwassimulated.Water tempera-turewasassumedtochangetoasoiltemperatureof10◦Cupon applicationthroughtheSDIsystem.Concentrationofthesolutesin irrigationwaterwasthencalculatedtovaryingamountsofwater lossbyevapotranspiration.Aselectedrangeofsoluteconcentration factorswereusedforeachofthescenariossimulated.
Simulationofpre-irrigationsoilwatercompositionwasbased uponmeancontrolsoilcompositionbetween30and90cmdeep forscenarios 1 and 2,and control soilbetween90 and 450cm deepfor scenarios 3 and 4.Components includedin the simu-lationswere:soilmineralabundances,soilsolutioncomposition, cationexchangecapacity, andcomposition ofionsonexchange sites.Calcite,dolomite,andgypsum,werethephasesconsidered tobereactiveinthesimulations.Calciteanddolomite concentra-tionswerealwayssufficienttomaintainsaturationthroughoutall simulations.Calciteconcentrationwassetto3.6%and1.8%,and dolomitesetto2.2%and2.3%,aboveandbelowthedriptubing, respectively.Dolomitewasconsideredonlytodissolve,basedupon unfavorableprecipitationkinetics(ArvidsonandMackenzie,1997). Gypsumpresenceandconcentrationvariedacrossthefour scenar-iosasdescribedbelow.
Soil water composition in pre-irrigation soil was estimated byfirstenteringintoPHREEQCthemeancompositionofthe1:1 water–soilextractsfromthecontrol-sitesoilbetween30and90cm deepforscenarios1and2,andbetween90and450cmdeepfor scenarios3and4.Thenremovalofwaterrelativetosoluteswas simulatedtoreducethewater–soilratiofromthe1:1oftheextract torepresentthewatercontentincontrol-sitesoilatthetimeof sampling.Forscenarios1and2,meanwatercontentabove90cm wasused(0.12gg−1)(Bernet al.,2012).For scenarios 3 and 4 2 PHREEQCinputfilesforthespecificgeochemicalsimulationsdescribedare
pro-videdinanelectronicsupplementtothisarticle.
themeanwatercontentbetween90and450cmdeepwasused (0.14gg−1).Theresultingwatercompositionwasthenequilibrated at10◦CwiththemeanmeasuredCO2concentrationandsoil min-erals.Gypsumwasomittedfromthisequilibrationforscenario1 (Table1).
Therelativelyhighabundanceof2:1claymineralsfoundbythe XRDanalysissuggestthatcationexchangecapacity(CEC)willbe highinsoilsatthestudysite.AnaverageCECperwt.%claywas cal-culatedfortherelevantsoilseriesfromtheSoilSurveyGeographic Database(SoilSurveyStaff),andfoundtobe0.76±0.17cmolkg−1 per wt.% clay (n=97). Soil above the drip tubing at the SDI site averages36wt.%clay and soilbelow thedriptubing aver-ages24%.Multiplyingthetwoparameters yieldsanestimateof 27.0cmolkg−1fortheCECofsoilabovethedriptubing(scenarios 1and2),and18.2cmolkg−1fortheCECofsoilbelowthedriptubing (scenarios3and4).Forcomparison,theaveragevaluemeasured on36sedimentsamplesfromanotherPowderRiverBasinsitewas 24.5cmolkg−1(Healyetal.,2008).TheabundanceofNa+,K+,Ca2+ andMg2+ionsonexchangesiteswasthendeterminedby estab-lishingexchangepoolequilibriumwithpre-irrigationsoilwater.
Simulationofionequilibriumwithcationexchangesitesallows a calculation of the exchangeable sodium percentage (ESP). It shouldbenotedthatthewater–soilratioandotherparameterswill influencesimulatedESP.Additionally,theaffinitiesofthevarious cationsforthetypesofexchangesitespresentonthesuiteofclay mineralsandorganicmatterinPlatmaksitesoilsmaybedifferent thanthoseusedinthesimulations.Therefore,ESPvaluesfromthe simulationsmustbeevaluatedwithcaution.
2.3.2. Simulationofsoilwaterabovethedriptubing
Drier conditions are understood to draw irrigation water upwardintosoilabovethedriptubingandevapotranspiration con-centratesthesolutesthataccumulatethere(Bernetal.,2012).Two scenariosweresimulatedforsoilabovethedriptubing.The sce-nariosdifferinhavinggypsumbecompletelyabsent(scenario1) versushavinggypsumpresentinvariablequantities(scenario2) (Table1).Gypsumconcentrationsrangingfrom0.001%to1.4%were usedin scenario 2. Soluteconcentrationfactors, resultingfrom evapotranspiration,werevariedwithinthescenarios.Factors ran-gingfrom1to15wereusedinscenario1,andfactorsfrom1to7 wereusedinscenario2.Theevaporativelyconcentrated,acidified irrigationwaterwasaddedtosoilataratioof0.18gg−1, consis-tentwiththeaveragewatercontentmeasuredinSDIsoilinAugust 2010.Compositionsofwater–soilextractsat1:1ratiowere simu-latedforsoilsinscenarios1and2bycomputationallyincreasing
Table 3 Average water chemistry for CBM water ( n = 4) and acidified CBM water ( n = 4) from Table 4 and water chemistry simulated by using PHREEQC as described in the text. EC a(mS cm − 1) p H Alkalinity (mg L − 1 as CaCO 3 ) Cl (mg L − 1) SO 4 (mg L − 1)C a (mg L − 1) K (mg L − 1) Mg (mg L − 1) Na (mg L − 1) SAR b Averaged Platmak site waters CBM water 2.1 8.0 1204 8.5 <0.06 4.5 6.9 1.7 529 54 Acidified CBM water 2.6 5.9 223 9.1 972 5.1 7.4 1.7 568 57 Simulated waters CBM water 1.4 8.0 1204 8.5 0 4.5 6.9 1.7 529 54 Outgassed CBM water 1.4 9.3 1204 8.5 0 4.5 6.9 1.7 529 54 Acidified CBM water 1.6 6.0 327 8.5 841 4.5 6.9 1.7 528 54 Solute concentration factor 1.3 2.1 6.0 425 11.0 1093 5.8 9.0 2.2 687 61 Solute concentration factor 1.65 2.6 6.0 539 14.0 1388 7.4 11.4 2.8 872 69 Solute concentration factor 3 4.4 5.9 981 25 2522 13 21 5.1 1585 93 Solute concentration factor 7 9.4 5.9 2285 59 5878 31 48 12 3693 142 Solute concentration factor 15 17.7 5.8 4883 127 12,560 67 103 25 7892 208 a Electrical conductivity. b Sodium adsorption ratio.
thewaterproportioninthesoilsolution,andadjusting tempera-tureto25◦CandsettingCO2concentrationsto0.1%.Simulating1:1 waterextractsnormalizestheresultstoacommonsoilwater con-tentandalsoallowsdirectcomparisonofsimulationresultstosoil extractcompositionsmeasuredinthelaboratory.
Anadditionaleffectwastestedforscenario1.Ionspresentin soilsolutionwereremovedpriortosimulationofthe1:1water extract.Althoughsolutesbecomeconcentratedduringthe grow-ingseasonduetoevapotranspiration,aportionofthesamesolutes arevulnerabletoelutionduringdeepsoilwettingeventsthatoccur duringthenon-growingseason.Thissimulationprocedure gener-allyduplicatestheeffectofdilutewatersfromrainorsnowmelt infiltratingthesoilanddisplacingorelutingsoilwaterandtheions itcontains.Thesoluteconcentratingstepandsoluteelutingstep haveopposingeffectsonsoluteconcentrations,butincludingboth inthesimulationsisnecessarybecausetheyaretemporallydistinct atthestudysite.
2.3.3. Simulationofsoilwaterbelowthedriptubing
Scenarios3and4describeconditionsinsoilbelowthedrip tub-ingwhereexcessirrigationwaterisunderstoodtoresultinleaching ofsoil(Bernetal.,2012).Scenarios3and4areidentical,exceptthat irrigationwater isacidifiedinscenario3,andleftasunacidified CBMwaterinscenario4.Comparisonofthescenariosillustrates theeffectsofacidification.
Downwardmovementofwaterandsolutesinscenarios3and4 wassimulatedtooccurbypiston-flowandadvection,thetransport ofsoluteswiththeliquidwater(Table2).Thecolumnofsoilthrough whichthisoccurredinthesimulationconsistedof30,30-cmthick, 1-m2unitsofsoil,representingtheintervalfromaboutthedepth ofthedriptubing(90cm)toanapproximatewatertabledepthof 960cm.Irrigationwaterwasaddedtothetopmostsoilunit(90cm) atthemeanwater–soilratioinSDIsoilmeasuredinAugust2010 (0.27gg−1)andthesolutionassociatedwitheachunitwasshifted downwardtosimulatedownwardmovementbypiston-flow.As solutionsadvancedtheywereequilibratedwiththemineralsand exchangepoolineachsuccessiveunit.
Porevolumesofwaterpassingthroughthetopsoilunitwere calculatedbyconvertingestimatesofexcessirrigationwaterinthe alfalfafield(1658mmfirstyear,5048mmsixyears)(Bernetal., 2012)toakgm−2basisanddividingbythemassofwaterina 30-cmthick,1-m2unitofsoil(116kg).Thesimulationsconsiderexcess irrigationinthealfalfaSDIfield,althoughexcessinthegrassSDI fieldissimilar.Becauseofheavyirrigationratesatthestartofthe SDIoperation,itiscalculatedthat14porevolumesofirrigation waterhadpassedthroughthetopsimulatedsoilunitinthefirst year.Therefore,14coupledtransport/equilibrationstepswere sim-ulatedtodescribethefirstyearofleaching.Includingthelower irrigationvolumesinrecentyears,44transport/equilibrationsteps representedthesixyears ofirrigationprior tosoilsampling.To projectfuturechanges,107transport/equilibrationstepswereused torepresentfifteenyearsofaverageexcesswaterapplication.
Anexcessofirrigationwaterbelowthedriptubing suggests that irrigation water at those depths will be less concentrated by evapotranspiration, but the amountof concentration differs fromtwo linesof analysis.Totalwater balancefor thePlatmak SDI site suggests that solutes should concentrate overall by a factorof1.65(Bernetal.,2012).Incontrast,calculatedaqueousCl concentrationsinSDIsoil(8±5mgL−1)beneath thedriptubing arequitesimilartothoseinirrigationwater,suggestingnosolute concentration,orafactorof1.0(Bernetal.,2012).To accommo-datebothestimatessimulationsbelowthedriptubingwererun usingsoluteconcentrationfactorsof1.0and1.65,aswellasan intermediatefactorof1.3(Table2).
Thesimulationresultsandmeasuredsoilpropertieswere com-pared by using the composition of 1:1 water–soil laboratory
extracts simulated from scenario 3 at two different trans-port/equilibriumsteps.Thewater–soilratiowasadjustedto1:1, temperaturewasset25◦C,andCO2concentrationwassetto0.1% toreflect laboratoryconditions.Extractsweresimulatedforthe topmostsoilunit(90cmdepth)afterthe14th(firstyear)and44th (sixthyear)transport/equilibrationstep.Extractsweresimulated fortheentiredepthprofileforthesixthyeartocoincidewithsample collectionatthePlatmaksite.
3. Results
3.1. Analyticalresults
3.1.1. Waterchemistry
CBMwaterpumpedtothePlatmaksitewasNa-HCO3typeand hadECrangingfrom2.0to2.2mScm−1(Table4).TheCBMwater atthePlatmaksitecontainsslightlyhigherconcentrationsofNa andlowerconcentrationsofCaandMgthanaverageCBMwater foundinthePowderRiverBasin(Riceetal.,2002).Asaresult,the SARvalues(48–59)areatthehighendoftheexpectedrange.The combinationofECandSARplacethewaterslightly beyondthe severerestrictioncategoryandthereforethiswaterposesa signifi-cantriskofsoildegradationevenwithspeciallydesignedcropping management(AyersandWestcot,1985).ConcentrationsofSO42− are<0.06mgL−1andClispresentinconcentrationsbetween7and 10mgL−1.
WaterinthestoragepondhashigherpHthanthesuppliedCBM waterduetoCO2outgassing(Table4).HigherSO42−concentrations
inthepondareattributedtoreturnofacidifiedwaterfromflushing oftheSDIfiltrationsystem.Thewaterdeliveredtothefieldsreflects in-lineacidificationbyH2SO4.ThetargetpHis5.5,butvaluescan rangefrom5.5to6.1(Table4).Afteracidification,irrigationwater isalteredtobeaNa-SO4type;theSARisunchanged.
3.1.2. Mineralabundances
Notable differences in the mineral composition of SDI and controlsoilswerenotapparentin theXRDresults. Thesumof crystalline materials identified in samples averaged a total of 91±2wt.%. One contributing factor to low totals is the pres-enceof X-rayamorphous material,including organiccarbon in generaland visiblequantitiesofligniteinparticular.Major sili-catemineralsshowedlittlesystematicvariationwithdepth,and averaged abundances are presented. Quartz is the most abun-dant(41±5wt.%)mineral,andasuiteofclaymineralsaccountfor 34±6wt.%ofthesoil.Amongtheseclayminerals,illite/muscovite phasesdominate(18±3wt.%),followedbysmectite(7±2wt.%), kaolinite(6±1wt.%),andchlorite(4±1wt.%)(Table5).Feldspar contentaverages8±1wt.%andincludespotassiumfeldsparand plagioclase.
Incontrasttothesilicateminerals,abundancesofcalciteand dolomite(CaMg(CO3)2)variedsystematicallywithdepth(Fig.2). Calciteanddolomitegenerallyarenotdetectedinsoil0–15-cm deep,andat somesites arealsoabsentfromsoil15- to30-cm deep.Manyofthehighestcalciteconcentrations,upto7.8%,were foundbetweenthe30-and60-cmdepth,whichareinterpreted tobe pedogenic calcite accumulation or redistribution (Fig.2a andc).Belowthe60-cmdepth,calciteconcentrationsvarywidely (0.3–9.1%).In contrast,mostdolomiteconcentrationsbelowthe 60-cmdeptharebetween2and3%(Fig.2bandd).
Total carbonate concentration measured by the acetic acid methodcorrelatedwellwiththesumofcalciteanddolomite con-centrationsmeasuredbyXRD(R2=0.92,p<0.01).Theacetic-acid methodalwaysestimatedcarbonatemineralcontentinexcessof calciteasdeterminedbyXRD,butlessthancombinedcalciteand
dolomite.Thediscrepancy is likelydue toincompletedolomite Table
4 Measured properties of waters at the Platmak SDI site. Sample type Collected Temperature ( ◦C) Conductivity (mS cm − 1) pH Alkalinity (mg L − 1 as CaCO 3 ) Cl (mg L − 1) SO 4 (mg L − 1) Ca (mg L − 1) K (mg L − 1) Mg (mg L − 1) Na (mg L − 1) SAR a CBM water 17-Feb-2010 11.2 2.2 8.0 1272 8.0 <0.06 3.2 4.4 1.7 558 59 CBM water 27-May-2010 18.4 2.2 8.0 1228 10 <0.06 4.8 6.4 2.0 585 57 CBM water 10-Aug-2010 23.3 2.0 7.9 1117 8.6 <0.06 5.7 11 1.5 517 50 CBM water 14-Oct-2010 20.3 2.1 8.0 1198 7.1 <0.06 4.3 5.8 1.5 454 48 Holding pond 17-Feb-2010 6.3 2.3 8.1 1171 8.2 91 3.2 4.5 1.8 538 59 Holding pond 27-May-2010 23.0 2.2 8.6 1127 9.4 71 4.7 6.6 2.1 571 55 Holding pond 10-Aug-2010 27.3 2.2 8.4 1046 9.2 82 4.4 11 1.5 527 55 Holding pond 14-Oct-2010 17.0 2.2 8.1 1137 8.1 91 4.8 3.3 1.5 564 57 Acidified water 17-Feb-2010 7.6 2.6 6.1 281 10 944 5.2 5.7 1.8 510 49 Acidified water 27-May-2010 18.5 2.5 6.0 217 9.2 904 4.8 5.9 2.0 636 68 Acidified water 10-Aug-2010 25.4 2.5 5.5 106 8.9 1090 5.8 14 1.7 549 52 Acidified water 14-Oct-2010 15.5 2.5 6.1 288 8.0 950 4.7 3.6 1.5 576 59 a Sodium adsorption ratio.
Table5
ClaymineralabundancesasdeterminedbyquantitativeX-raydiffractiononselectsamplesofcontrolandalfalfaSDIsoilfromtheFebruary2010sampling.
Illite/muscovite(%) Smectite(%) Kaolinite(%) Chlorite(%) Totalclayminerals
identifiedbyXRD(%)
Meanandstandarddeviation 18.0±3.0 7.1±2.4 5.8±1.3 3.6±0.8 34.5±6.1
Range 11.7–27.2 2.9–14.0 3.2–9.6 2.2–5.7 22.4–51.6
Fig.2.SoilmineralcontentasdeterminedbyquantitativeX-raydiffractiononselectsamplesfromtheFebruary2010sampling:(a)calciteinalfalfaSDIsoil;(b)dolomitein
alfalfaSDIsoil;(c)calciteincontrolsoil;(d)dolomiteincontrolsoil.Horizontaldashedlinesshowthe92-cm-depthofdriptubinginSDIfields.
dissolutionbytheacetic-acidmethod.Measurementsoftotal car-bonatebetween30cmand450cmdeepbytheaceticacidmethod rangedfrom1.7to15.5wt.%CaCO3.BasedontheXRDandacetic
acidresults,calciteanddolomiteareconsistentconstituentsofthe soilbelowthe30-cmdepth.
Gypsum abundance also varies with depth. Gypsum mea-suredbyXRDrangedfromnotdetectedto6.6wt.%.Theaccuracy of thetechniqueis questionableat low gypsumconcentrations (<1wt.%).Toestimatetheabundanceofgypsuminsampleswith lowconcentrations,XRDresultswereexaminedbycomparingthe gypsumcontenttothatestimatedby20:1water–soilextractions. Regressionof concentrationestimatesfrom thetwotechniques foundgoodcorrelationsforbothSDI-irrigated(R2=0.99,p<0.01,
slope=0.95) and control soils (R2=0.98, p<0.01, slope=0.84).
Basedupontheconsistencyofthetwomethodsformostsamples, the20:1waterextractablesulfurdataisusedtoestimatesoil gyp-sumcontent.Soillessthan90-cm-deepgenerallycontainslittle gypsum,whiledeepersoilhaswidelyvariable,butgreater concen-trations(Fig.3).Baseduponwater-extractablesulfur,soilgypsum concentrationsdeeperthan90-cmrangedfrom0.1to12wt.%,and averaged2.1±1.8wt.%forSDIsoiland2.4±1.7wt.%forcontrol sitesoil(Fig.3).Stringersandnodulesofgypsumwerevisiblein sampleswithgreatergypsumconcentrations.Incontrast,gypsum concentrationsinsoillessthan90cmdeeprangedfrom0.02to 8.6wt.%andaveraged0.2±0.4wt.%forSDIsoiland0.5±1.9wt.% forcontrolsitesoil.
3Soilchemistrydataareprovidedinanelectronicsupplementtothisarticle.
3.1.3. Compositionofsoilextractsolutions3
TheECof1:1watersoilextractsshowcommonpatternsin con-trolandSDIsoils.Aboveaboutthe90-cmdepth,ECofcontrolsoil extractionsolutionisrarelygreaterthan0.5mScm−1 andalfalfa SDIsoilisrarelygreaterthan1.2mScm−1(Fig.4).GrassSDIsoilhas morevariability,withafewsamples>2mScm−1butthe major-ityarebelowthatvalue(Fig.4).Incontrast,controlsoilsamples belowthe90-cmdepthhave1:1solutionECmostly>2mScm−1 andrangeupto6mScm−1.AlfalfaandgrassSDIsoilsalsohave values>2mScm−1forthemajorityofsamplesbelowthe90-cm depth,andrangeupto4.9mScm−1.TheECofthe1:1water–soil extractswerecorrelatedwithconcentrationsofsulfurextractedat 20:1water–soilratiofromallsoiltypes(R2=0.60,p<0.01).The cor-relationbetweenECandextractablesulfurmightbehigherexcept forthesuggestionofgypsumsaturationinsome1:1extracts. Cor-relationofECwithextractablesulfur,andextractablesulfurwith gypsum,indicatesthatgypsumpresenceisamajorcontrolonEC.
ThepHvalues of 1:1water–soilextractsincontrolsoilsfall generallyintherangeof7.3–8.4,withafewlowervaluesin near-surfacesoil,andafewhighervaluesnearthe45-cmdepth(Fig.4). Bycomparison,SDIsoilshavehigherpHvalues,ranginggenerally from7.4upto9.1,andagainthelowestvaluesweremeasuredon soilsfromnearthegroundsurface.
SARvaluesof1:1water–soilextractsfromSDIsoilsclearlyshow theeffectsofirrigationwithhigh-SARwaterwhencomparedtoSAR valuesofnon-irrigatedcontrolsoils(Fig.5).Controlsoil0–30cm deephadSARvaluesof0.1–0.5.SARvaluesareonlyslightlyhigher inalfalfaandgrassSDI-irrigatedsoilsbetween0-and30-cmdepth, rangingfrom0.5 to0.7and 0.3to1.2, respectively.Below that depth,SARvaluesofalfalfaandgrassSDIsoilsriseabruptly,while controlsoilschangelittle.InalfalfaSDIsoil,SARvaluesrangeupto 13,withhighervaluescommoninthe45–135-cmdepthinterval.
Fig.3.Estimatedweightpercentgypsumconcentrations,calculatedfrom water-extractablesulfurasdescribedinthetext:(a)alfalfaSDIsoil;(b)grassSDIsoil;(c) non-irrigatedcontrolsoils.Horizontaldashedlinesshowthe92-cm-depthofdrip tubinginSDIfields.
IngrassSDIsoil,SARvaluesrangeupto12.8,andthehighest val-uesaremostcommoninthe30–105-cmdepthinterval.Amarked declineinSARtovaluesbetween2and4inthedeeperportions oftheSDIsoilprofilesisassociatedwiththeincreasesinECand extractablesulfurandisattributabletogypsumdissolution.
Naextracted at20:1 water–soilratiomaybeanincomplete measureofwater-solubleNainsoilbecausesomeNawouldbe retainedoncationexchangesites.However,dissolutionofgypsum andcalciteinthesamplesprovidesadditionalCa2+ionstoexchange forNa+,makingtheNa+extractionmorecomplete.Therefore,Na extractedat20:1water–soilratioseemsareasonableestimateof totalwatersolubleNa.Controlsoilscontained20–49mgkg−1Nain
thetop45cm(Fig.5).Sodiumconcentrationsarelargeratgreater depths,rangingupto395mgkg−1Na.Bycomparison,SDIsoils con-tainedslightlymoreNainthe0–30cm-depths,rangingfrom54to 87mgkg−1NaforalfalfaSDIsoiland34–168mgkg−1Naforgrass SDIsoil.SubstantiallygreaterNaconcentrationsinSDIsoil com-paredtocontrolsoilaremoreapparentbelow30cmdepthgrass SDIsoilandat45cmdepthinalfalfaSDIsoil(Fig.5).MaximumNa concentrationsinthealfalfaSDIsoiloccuratandbelowthedrip tubingandrangeupto747mgkg−1.Ingrassfieldsamples max-imumNaconcentrationsarefoundabovethedriptubingwhere theyrangeupto834mgkg−1.
UsingbulkdensitydatafromBernetal.(2012),Naextracted at 20:1water–soilratio totals 3.0±0.2 and 2.8±0.1kgm−2 for the0–450-cmdepthintervalin thealfalfaand grass SDIfields, respectively. Control soil contained 1.1±0.4kgm−2 of Na in thesame interval. Abovethedrip tubing, Naextracted at20:1 water–soilratiototals0.50±0.07and 0.68±0.11kgm−2 forthe alfalfa and grass SDI fields, respectively. Controlsoil contained 0.13±0.07kgm−2ofNainthatsameinterval.Forcomparison,the irrigationwaterhasadded4.6and3.6kgm−2 ofNatothealfalfa andgrassfields,respectively(Bernetal.,2012).
IrrigationwaterhasthepotentialtoaddSO4toSDIsoil,dueto acidificationwithsulfuricacid,butalsocanpotentiallyleachSO4 fromthestartingsoilcompositionasaresultofgypsumdissolution. IfacidificationhadbeenconsistentovertheoperationoftheSDI system,approximately8.2and6.4kgm−2ofSO
4wouldhavebeen addedtothealfalfaandgrassfields,respectively.Assumingall20:1 waterextractablesulfurwaspresentasSO4,controlsoilcontained 74±13kgm−2ofwaterextractableSO
4inthe0–450cminterval. Bycomparison,totalwaterextractableSO4waslowerinthealfalfa (57±14kgm−2)andgrass(69±22kgm−2)SDIfields.
The concentrationof CO2 in soil gas atthe 75-cm depthin October2010averaged3.9±1.1vol.%(1S.D.)withoutsignificant differences between the grass and alfalfa fields (p=0.74, two-sampleKolmogorov–Smirnovtest).
3.2. Simulationresults
3.2.1. Simulatedirrigationwaterandpre-irrigationsoil
composition
Theconcentrationofsolutesinwaterisamajorinfluenceon thesimulatedinteractionsbetweenwaterandsoil.BothECand SARincreasewithwaterremovalbyevapotranspiration(Table3). AcidificationoftheCBMwaterslightlyincreasesitsEC,buthaslittle effectonSAR(Table3).Noeffectsfromplantuptakeorreleaseof ionsareincludedinthesimulationofevapotranspiration.
Table1presentsgeochemicalattributesofsoilwater in pre-irrigation soil for the four simulated scenarios. Compared to concentrationsmeasuredin1:1water–soilextractsfromcontrol soil,ECand SAR are higher in thesimulated pre-irrigationsoil water.Thedifferencesareattributedtohigherconcentrationsof ionsincontrolsoilwater,reflectingthelowwater–soilratios.Lower pHreflectsthehigherCO2concentrationsmeasuredinSDIfields (3.9±1.1%)comparedtoconditionsforlaboratoryextracts.
3.2.2. Simulatedsoilwaterabovethedriptubing
Twomajorpatternsemergefromthesimulationsofsoilwater above the drip tubing. First, scenario 1 (gypsum-absent) and scenario2 (gypsum-present) evolve tohave distinctlydifferent compositions.ThisisevidentinthelackofoverlapinplotsofEC versusSAR,andECversuspHforthe1:1water–soilextractsforthe twoscenarios(Fig.6aandb).GreaterECinscenario2isattributed togypsumdissolution.The Ca2+releasedfromgypsumdirectly decreasesSARbutalsodecreasespHbysuppressingdissolutionof calciteanddolomite.Second,increasesinthesoluteconcentration factor, resultingfrom evapotranspiration,consistently increases
Fig.4. Chemistryof1:1water–soilextractsfromthePlatmakSDIsiteplottedbydepth:(a–c)electricalconductivityforalfalfaSDI,grassSDIandcontrolsoils,respectively; (d–f)pHforalfalfaSDI,grassSDIandcontrolsoils,respectively.HorizontaldashedlineinplotsforalfalfaandgrassSDIsoilsrepresentthedepthofthedriptubing.
SARforsoil(Fig.6a).Nonetheless,soilSARisdramaticallylower compared to that predicted by direct evaporation of irrigation water prior to interaction with soil. SAR values for simulated irrigationwaterswithsoluteconcentrationfactorsof1,7,and15 are54,142,and208, respectively(Table3), whilenosimulated 1:1water–soilextractSARvaluesexceed14.ThelowerSARarises fromadsorptionofsomeNa+onexchangesitesandreleaseofCa2+ andMg2+fromsoilminerals.
The effect of soil water flushing by infiltration of rain and snowmelt is apparentin scenario1. Flushingdisplaces thesoil water,whichwasderivedfromevapotranspiration-concentrated irrigationwater.ECdecreasesinthatportionofthesoilbecause theconcentratedionscontainedinthesoilwateraretransported intheelutingwater.Incontrast,theSARchangeslittle,becauseNa+ isonexchangesitesisreleasedasnewwaterreleasesCaandMg fromcarbonatemineraldissolution.
Fig.5. MeasuresofNaabundanceatthePlatmakSDIsiteplottedbydepth:(a–c)sodiumadsorptionratiosfor1:1water–soilextractsalfalfaSDI,grassSDIandcontrolsoils, respectively;(d–f)Naextractedby20:1water–soilextractsandexpressedonadrymassbasisforalfalfaSDI,grassSDIandcontrolsoils,respectively.Horizontaldashedline inplotsforalfalfaandgrassSDIsoilsrepresentthedepthofthedriptubing.
Fig.6.ComparisonsofoutputvaluesforsimulationsofSDIsoilabovethedrip tub-inginscenarios1and2:(a)ECandSARinsimulated1:1extracts;(b)ECandpH insimulated1:1extracts.Scenario1(gypsum-absent)variedthedisplacementof ionsinsoilsolution,simulatingflushingbyinfiltratingrainorsnowmelt.Scenario2 (gypsum-present)variedtheamountofgypsumpresentinsoil.Bothscenarios var-iedthefactorbywhichevapotranspirationconcentratedtheacidifiedCBMwater andthosefactorsareindicatednexttolinesmarkingtheirchemicaltrends.
Scenario2 variestheamount ofgypsumpresent fromtrace amounts (0.001%) up to that sufficient to achieve gypsum saturationina1:1water–soilextract(1.4%).Varyinggypsum con-centrationhasrelativelylittleinfluenceonSAR,butsubstantially affectstheEC.Further,SARvaluesforagivenevapotranspiration factorarelowerinscenario2byonlyabout1–2,comparedtothe scenario1.TheminimaleffectsofgypsumonSARcanbeattributed to the precipitation of calcite resulting from reaction of Ca2+ suppliedbygypsumdissolutionandavailablecarbonatespecies. Calcite is predicted to form as gypsum dissolves, resulting in decreasingpHandalkalinitywithincreasinggypsum.Allsolutions simulatedforscenarios1and2areslightlytomoderately oversat-uratedrelativetodolomite,withsaturationindicesrangingupto 0.5.Inhibitingdolomiteprecipitationinthesimulationsresultsin retentionofMginsolution,andconsequentlylowerSARvalues.
Simulationresultsforsoilabovethedriptubingcompare favor-ablytomeasuredECand SAR valuesin 1:1water–soilextracts (Fig. 7).Samplesof irrigated soilbetween 30 and120cm deep havecombinationsofECandSARvaluesthatfallalmostentirely withinthedomainsdescribedbyscenarios1and2.Soildownto the120-cmdepthisincludedintheassessmentbecausethe tran-sitionbetweensoilzonesisnotexpectedtobeabrupt(Fig.1).Most measuredvaluesfallinthegypsum-absent(scenario1)domain,as wouldbeexpectedbaseduponthelackofgypsuminthiszonein bothSDIandcontrolsoil(Fig.3).MostsamplesfromalfalfaSDIsoil havelowECvaluesthatindicatedisplacementofsoilsolutionions, indicatingthatperiodicflushingbyrainandsnowmeltisan impor-tantprocessinthosesoils.GrassSDIsoilhasmorescatterwithinthe
Fig.7. PlotsofECandSARof1:1water–soilextractsforsamplesfrombetween 30and120cmdeep:(a)alfalfaSDIsoil;(b)grassSDIsoil.Controlsoilvaluesfor thesamespanofdepthsareshownforcomparison.TheECandSARresultsfrom geochemicalsimulationscenarios1and2ofsoilabovethedriptubingdepictedin
Fig.6aareshownforcomparison.
gypsum-absentdomain,possiblyindicatinglessflushing.The high-estSARvaluesmeasuredinsoilsamplesfallwithinthescenario1 domainandrangeupto>13,comparedtomeasuredSARvalues ran-gingupto∼7inthescenario2domain(Fig.7).Thecorrespondence oflowsoluteconcentrationfactorsandgypsumpresenceis consis-tentwiththegreatestabundanceofgypsumoccurringbelowthe 90-cmdepth(Fig.3)wherelessevaporativeconcentrationofSDI soluteswouldbepredicted.BothalfalfaandgrassSDIsoilsare pre-dictedtohaveSARvalues>12inthescenario1domain,forwhich thesimulationsindicateasoluteconcentrationfactorof>13.The ESPindicatedforthosesamplesbythesimulationsis>15%.
AplotofECandpHmeasuredvalueson1:1water–soilextracts showsthattheydonotfallperfectlywithinthedomainofsimulated values(Fig.8).However,wheredifferencesexist,theyare gener-allylessthan0.4pHunits.Irrigationwithunacidifiedwaterwould generatepHvaluesgreaterthanthoseseenwithacidifiedwaterin scenarios1and2.HighpHvaluesmeasuredin1:1extractsmay thereforebedetectinglapsesinacidificationthathavebeennoted bytheoperatorandweremorecommoninearlyyearsofoperation.
3.2.3. Simulatedsoilwaterbelowthedriptubing
Geochemicalsimulationsofsoil-watercompositionbelowthe driptubinginscenarios3and4includedownwardmovementof excessirrigationwaterwhichresultsinsoluteleaching.To sim-ulatedthetransportofsolutestemporalandspatialcomponents wereincludedinthemodeling.Scenarios3and4alsoassumea constantandrelativelyhighratioofirrigationwatertosoil.For thatreasonweinitiallyexaminechangesinsoilwater composi-tion,ratherthanextracts,throughthesimulations.Resultsfromthe simulationsusingasoluteconcentrationfactorof1.3aredescribed; otherfactors(1.0and1.65)wereconsideredinthecalculationsto
Fig.8.PlotofECandpHfor1:1water–soilextractsforsamplesfrombetween30 and120cmdeepfromalfalfaSDI,grassSDI,andcontrolsoils.TheECandpHresults fromgeochemicalsimulationscenarios1and2ofsoilabovethedriptubingdepicted inFig.6bareshownforcomparison.
provideameasureoftheevapotranspirationconcentrationfactor’s influence.
Scenario3simulatesirrigationwateracidifiedtopH6.0.The uppermostsoilunitinthesimulationispositionedatthedrip tub-ingand thechemicalchanges thereare similartochanges that happenindeeperunits.Simulatedpre-irrigationsoilwaterinthe uppermostunithasaSARof10.6andECof5.1mScm−1. Concen-trationofsolutesintheirrigationwaterbyafactorof1.3produced aSARof61andanECof2.1mScm−1 (Table3).Equilibrationof thissolutionwithsoilduringearlystepsofthesimulation,yields aSARof7andECof∼4.1mScm−1inthenewsoilwater(Fig.9). Thechangeincompositionasirrigationwaterbecomessoilwater islargelydrivenbygypsumdissolution.However,unlikethe sim-ulationsofsoilabovethedriptubing,dolomitedissolutionplaysa moresignificantrole.Whilegypsumanddolomitedissolve,calcite precipitatesfromCa2+releasedbygypsumanddolomite.Alower SARinsoilwaterderivedfromhigh-SARirrigationwater,compared tonativesoilwater,isattributedtolowerdissolvedNa concentra-tionsintheirrigationwater,asbothwatersequilibratewiththe samesoilminerals.
Downwardtransportofsolutesbyexcessirrigationwater even-tuallyresultsindepletionofgypsumintheuppermostsimulated soilunit.Modelingpredictsthedepletioniscompletebeforethe endofthesecondyearofsimulatedirrigation,andtheeffectson pH,ECandSARarepronouncedbythesixthyear(Fig.9).Without gypsum,soil-waterECdeclinesabruptlytoaround2.4mScm−1and SARvaluesrise.TheriseinSARistemperedinitiallybyexchange of Na+ forCa2+ and Mg2+oncation exchange sites. Eventually, exchangeableCa2+andMg2+aredepletedbycontinueddownward flowofexcessirrigationwaterandSARrisesto>14(Fig.9). Follow-inggypsumdepletion,calcitecontinuestoformfromCa2+supplied fromexchange sites, but at a decreasingratefor severalmore transport/equilibriumsteps.WithdepletionofexchangeableCa2+, calcitebeginstodissolve.Thedeclineincalciteprecipitationand subsequentdissolutionbothcausethepHofsoilwatertoincrease (Fig.9)anddissolutionofdolomitetodecrease.Underthese con-ditions,thelimitedcalciteanddolomitedissolutionaretheonly processesactingtolowertheSARofirrigationwater.Onlyafter gypsumisdepletedintheoverlyingsoilunitabovedoes substan-tialgypsumdissolutionbeginintheunitbelow.Thispredictsthat theeffectsofgypsumdepletionwillproceedsequentiallydownthe simulatedsoilcolumn.
Model results generated by using the 1.65 and 1.0 solute concentrationfactorsbracketthoseobtainedfromthe1.3factor. Thehigherconcentrationfactorof1.65generateshigherEC,SAR,
Fig.9.Changesinthesoilwatercompositionoftheuppermostsoilunit(90–120cm depth)forgeochemicalsimulationsofsoilbelowthedriptubing,scenarios3and 4:(a)ECandtransport/equilibriumstep;(b)SARandtransport/equilibriumstep; (c)pHandtransport/equilibriumstep.Scenario3(CBMwateracidifiedtopH6.0 withsulfuricacid)andscenario4(unacidifiedCBMwater)aredistinguishedinthe legend.Simulationsusingasoluteconcentrationfactorof1.3forirrigationwater areplottedasblacklines,whilesimulationsusingfactorsof1.0and1.65areshown asgraylinesandlabeledinthefigure.Stepscorrespondingtothesimulatedfirst, sixth,andfifteenthyearofirrigationattheSDIsitearelabeled.Abruptbreaksinthe trendsmarkthestepwheregypsumisfullydepletedfromthemodelunit.
andpHvalues,andthe1.0factorpredictslowervaluesforthese parameters(Fig.9).Forsimulationsofacidifiedirrigationwater,the effectofsoluteconcentrationisparticularlynoticeableforSARafter gypsumdepletion.Inthatsituation,SARrisesto21whenusingthe 1.65factor,butonly12usingthe1.0factor.Althoughthesimulated ESPvaluesmustbeevaluatedwithcaution,theyprovideabroad indicationofthechangescausedbythesodicwater.ESPis7,9, 12%fortheconcentrationfactors1.0,1.3and1.65,respectively,in simulatedsoilwhilegypsumispresent.Aftergypsumdepletionthe ESPvaluesarepredictedtoincreaseto15,20and25%,respectively. SimulationofirrigationwithunacidifiedCBMwaterinscenario 4generallyyieldslowervaluesofsoil-waterECandhighervalues ofpHandSAR(Fig.9).Inthisscenario,abundantbicarbonatein unacidifiedwatercombineswithCa2+ionsreleasedasgypsum dis-solvestoprecipitatecalcite.Greatercalciteprecipitationincreases gypsumdissolutionand gypsumisdepleted morequickly.Asa
result,changesinsoilwatercompositionassociatedwithgypsum depletionarepredictedfortheuppermostsoilunitearlyinthe sec-ondyearofirrigation,ratherthanlateinthesecondyear(Fig.9). After15yearsofsimulatedirrigationwithunacidifiedwaterand aconcentrationfactorof1.3,a180-cm-thicksection(6modeled soil units)is depleted of gypsumcompared toa 120-cm-thick section(4modeledsoilunits)withacidifiedwater.Inthe gypsum-depletedunits,leachedbyunacidifiedirrigationwater,soil-water ECis1.8mScm−1andSARis33aftersixyearsofirrigation(Fig.9) whileESPis55%.Suchvalueswouldbeexpectedtocausenotable reductionsinsoil hydraulicconductivity(McNeal and Coleman, 1966).Withcontinuingirrigation,SARandESPcontinuetoincrease, ultimatelyreaching38and 60%,respectively.Usingthe1.0 and 1.65concentrationfactorswillyieldarangeofSARvaluesfrom 22to46andECfrom1.4to2.2mScm−1aftersixyearsofirrigation. Thehigher-SARlower-ECconditionsinthesescenariosaredriven largelybythehigherpHandalkalinityofunacidifiedwater,which suppresscarbonatemineraldissolutionaftergypsumdepletion.
Returningtoscenario3,simulationsof1:1extractsolutionEC, pH,SARandextractableNaatthesixth-yeartimestepareplotted inFig.10.MeasuredvaluesfromsamplesofcontrolandalfalfaSDI soilbelow90-cmdepthareplottedforcomparison.Thevariability inmeasuredvaluescontrastswiththesimplepatternsgenerated bythesimulations.Thisistobeexpectedaspatternsofunsaturated flowthroughtheheterogeneoussoilarelikelytobetemporallyand spatiallycomplex.Thesimulations,bycomparison,consistof uni-formandunidirectionalflowalongwitharequirementtoachieve chemicalequilibriumpriortotransport.Additionally,gypsum con-tentin thestudiedsoilsisquitevariable(Fig.3), andthesame islikelytrueforCEC,whereasuniformvalueswereusedforall soilunitsatthestartofthesimulations.Nonetheless,measuredEC tendstobelowestandpHandSARgenerallyhavetheirhighest valuesinthe100–150-cmzoneimmediatelybelowthedrip tub-ing,aspredictedbyinthesimulations.Measured1:1water–soil extractableNa concentrations throughoutthe lower profileare generallybracketedbythesimulationsusingdifferentsolute con-centrationfactors.Overall,themodeledvaluescomparefavorably withmeasuredvaluesandthemodelexplainsbasicspatialpatterns observedinsoilcomposition.
In addition to sodicity, root zone salinity is a concern for long-termsustainabilityoftheirrigationmethoddescribedhere. Application of water in excess of crop demand keeps salinity lowerneartheapplicationpoint,butcomputersimulations sug-gestthatsoluteswillaccumulateabovethedriptubingovertime (Bern etal., 2012).Alfalfayields candecline when soil salinity exceeds2.0mScm−1,asassessedbysoilsaturatedpastes(Maas
andGrattan,1999).Somebromesandfescuesaremoderately tol-erantofsalinity,butDactylisglomeratayieldscandeclinewhensoil salinityexceeds1.5mScm−1(MaasandGrattan,1999). Consider-ingtheECmeasuredabovethedriptubingin1:1water-soilextracts (Fig.4),salinityintherootingzoneshouldbemonitoredrelative tothese thresholds.The issuecouldbecome more acute when theSDIsystemisshutdown;intheabsenceofirrigationwater, naturalprecipitationratesmaybetoolowtoflushaccumulated solutesfromthesoilzone.Asaresult,salinityofsoilwatermay increase.
4. Discussion
Additionof NatotheSDI soilsviairrigationwaterfromthe driptubingburied92cmdeephasnotresultedinlargeNa enrich-mentsintheshallowestportionsofthePlatmakalfalfaorgrass SDIfields.ThisisevidentinboththeSARvaluesandtotalwater extractableNameasuredinSDIsoil0–30cmdeep(Fig.5).Thisis insharpcontrasttositesusingCBMwatersforsurfaceirrigation
elsewhereinthePowderRiverBasin,wherethehighestSAR val-uesarefoundatandnearthesoilsurface(Ganjegunteetal.,2005, 2008).Highsodicityandlowsalinityatthesoilsurfacecancause clayswellinganddispersionandtherebyreduceinfiltrationrates aswellascauseproblemswithcrusting,aeration,andtrafficability (OsterandJayawardne,1998).Thoseproblemsarelikelytobe pro-nouncedinPowderRiverBasinsoils,suchasthoseatthePlatmak SDIsite,thatcontainabundantswellingclays(Table5)(Chaudhari andSomawanshi,2004).MinimalaccumulationofNaatthe sur-face,evenaftersixyearsofirrigation,givesthedeepSDIsystem describedhereanadvantageoversurfaceirrigationschemesthat usecompositionallysimilarwatersevenfollowingsimilar chemi-caltreatments(Johnstonetal.,2008,2011).However,measureable accumulationofNalocally withinsomenear-surfacesoilinSDI fieldsindicatesthatfieldandirrigationheterogeneitiesmayyield problemareasovertime.
Below30cmdepth,moresubstantialamountsofNaaddedby irrigationwaterhaveaccumulatedinSDIsoils(Fig.5).Theamount ofNaincontrolsoilbetween0and92cmdepthcanbesubtracted fromNainalfalfaandgrassSDIsoilstoobtaintheamountofNa presumablyaccumulatedduetoirrigation.Thoseamountsarea fractionofthetotal Naaddedby irrigationwater.Only 8%and 15%oftotaladdedNahasaccumulatedabovethedriptubingin thealfalfaandgrassSDIfields,respectively.Thecombinationof low-ECandhigh-SARconditionsinirrigatedsoilbetween45and 150cmdepthcouldpotentiallyimpactsoilstructureand perme-ability(Figs.4and5).Whilelessproblematicthanifsimilarchanges hadoccurredatthesoilsurface,suchchangesdohaveimplications forsoilandirrigationmanagement.Therefore itisimportantto understandhowsuchconditionsdevelopandpredicthowtheymay evolve.Thecompanionpaper(Bernetal.,2012)documented con-trastingpatternsinthemovementofirrigationwaterandsolutes betweensoilaboveandbelowthedriptubing.Thecomputer sim-ulationsofsoilgeochemistrydescribedhereillustratehowthose patternsof waterand solute movementtranslateintodifferent pathwaysfor thedevelopmentofhighsodicityand lowsalinity conditionsinSDIsoil.
Soilabovethedriptubingreceivesirrigationwaterandsolutes drawnupwardbyevaporativedemand.Lossesofwatervia evapo-transpiration concentrate the solutes. Computer simulations of soilgeochemistrydemonstratehowincreasingsolute concentra-tionbyevapotranspiration drivesboth water (Table5)and soil SAR values higher (Fig.4). Comparison betweenthe simulated and measured soil chemistry suggest solute concentration fac-tors ranging up to ∼13 for irrigation water (Fig. 5). Although theprogressionofincreasingSARvaluesisconsistentwithwater loss,theSARincreasesdonotcorrelatedirectlywithincreasesin soilconcentrations oftheconservativeanion Cl−,which isalso derivedfromirrigationwaterandmeasuredinSDIsoil(Bernetal., 2012).For example, maximum concentrationfactors calculated fromextractableCl−are4.5and26forthealfalfaandgrassSDIsoils, respectively.Thelackofcorrelationcanbeattributedtodecoupling ofthemovementofNa+relativetoCl−byretardationofNa+ trans-portasaresultofcationexchangeaswatermovesthroughtheSDI soil.Thesecondtransporteffectinfluencingsalinityandsodicity conditionsabovethedriptubingisinfiltrationofdiluterainand snowmelt.ThatinfiltrationlowersECbyremovingionsheldinsoil solution,althoughtheNa+oncationexchangesitesmaintainsSAR relativelyunchanged(Fig.4).Athirdfactoraffectingsoilabovethe driptubingisitslowgypsumcontent(Fig.3).Comparisonof sce-nario1andscenario2simulationsclearlydemonstrateshowthe lackofgypsumpredisposessoilinthatzonetolowerECandhigher SAR(Fig.4).Insummary,upwardmovementofirrigationwater, evapotranspirationconcentrationofsolutes,infiltrationof precip-itationwaters,andalackofgypsumareresponsibleforhigh-SAR andlow-ECconditionsinsoilabovethedriptubing.
Fig.10.Depthprofilesofcompositionofextractsat1:1water–soilratioforsamplesfromalfalfaSDIandnon-irrigatedsoilcores:(a)EC;(b)SAR;(c)pH;(d)extractableNa onadrymassbasis.Sampledataareplottedaspointsversusbottom-of-sampleincrementdepth.Outputfromscenario3ofthecomputersimulationsofsoilbelowthedrip tubingaftersixyearsofsimulatedirrigationareplottedassolidlines.Simulationresultsusingasoluteconcentrationfactorof1.3forirrigationwaterareshownasablack line.Resultsfromusingfactorsof1.0and1.65areshownasgraylinesandarelabeledinthefigure.
Insoilbeneaththedriptubing,excessirrigationwaterandits downwardmovementdriveadifferentsetofprocessesthatalso resultinhigh-SARlow-EC conditions.Onebenefitoftheexcess irrigationwaterbelowthedriptubingisthatevapotranspiration doesnot concentrate solutestothe levelsseen above the drip tubing.Relativelylow soluteconcentrationfactorsrangingfrom 1.0to1.65keepSARfromattaininghighervaluesinsoilbeneath thedriptubing(Fig.10).Thehighconcentrationsofgypsuminthis zonemaintainhighECand lowSAR,butexcessirrigationwater dissolvesthatgypsumandpercolatesdownward,transportingthe solutesdeeperintheprofile.ThesimulationsshowthatECdrops abruptlyandSARincreasesrapidlytoanewsteady-statevaluein soilwheregypsumbecomesdepleted(Fig.9).Continuedirrigation, equilibrationofirrigationwaterwithsoil,anddownwardtransport ofpost-equilibrationsolutesthereforegeneratethehigh-SARand low-ECconditionsinsoilbelowthedriptubing.
ThesimulationofirrigationwithunacidifiedCBMwaterin sce-nario4illustratesthevalueoftreatmentwithsulfuricacid.Atthe onsetofirrigation,acidificationhasrelativelylittleinfluenceon SARorEC(Fig.7).However,byreducingprecipitationofcalcite, andconcomitantlossesofCa2+,acidificationreducesthequantity ofgypsumdissolvedbyagivenquantityofirrigationwater.The depletionofgypsumandonsetofhigh-SAR,low-ECconditionsthat followarethereforedelayedbythesulfuricacidtreatmentofCBM water.Aftergypsumbecomesdepletedinagivensectionofsoil,
acidification becomes crucial topreventingsubstantial changes insoilchemistry. Withoutacidification,SARvalues rise precipi-touslyandECdropstorelativelylowvalues(Fig.7).Substantial declinesinsoilhydraulicconductivityareanticipatedasaresult (McNealandColeman,1966).Withacidification,bothSARandEC aremaintainedatmoremoderatevalues,decreasingthelikelihood ofclaydispersionandswelling.UnacidifiedCBMirrigationwater wasalsoconsideredinavariationonsimulationscenario1(results notshown).SARvalueswereelevatedby0.2–4comparedtothose forscenario1,whereirrigationwaterwasacidified.ECvalueswere lowerbyasmuchas1.9mScm−1.Acidificationofirrigationwateris thereforecrucialinlimitingthedevelopmentofhigh-SAR,low-EC conditionsbothaboveandbelowthedriptubing.
Itisworthconsideringtheoriginanddistributionofthenative gypsumpresentatthePlatmakSDIsite,asitissocrucialin main-taining lowersoilSAR, and higherEC. Oxidation ofpyrite from localgeologicsourcesandreactionofresultingsolutionswith avail-ablecalciteisonepossibleorigin.However,pedogenicgypsumin thearidBigHornBasin140kmtothewestisidentifiedashaving aneoliansourcewithdepositionatratesof0.01–1.2kgm−2ky−1 (Reheis, 1987). Gypsumat the Platmak SDI site may therefore have a similar eolian source, but the quantities present sug-gestlongeraccumulationtimesorgreaterdepositionrates.More likely than either of those scenarios is that gypsum derived from both processes hasbeenredistributed withinthe Powder
