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document Historic, archived Do not assume content reflects current scientific knowledge, policies, or practices. \ ^ c United States Adding Sodium Hydroxide Department ofAgriculture To Study Metal Removal ForestService in Intermountain Research Station a Stream Affected by Acid Research Paper INT-465 Mine Drainage June 1993 Michael C. Amacher u4s( Ray W. Brown Janice Kotuby-Amacher Angelee Willis uJ THE AUTHORS RESEARCH SUMMARY MICHAEL C. AMACHER is a soil chemist with the Dis- Fisher Creek, a stream affected by acid mine drainage turbed Lands Rehabilitation Research Work Unit at the in the Beartooth Mountains of Montana, was studied to Forestry Sciences Laboratory in Logan, UT. He holds determinethe extentto which copper (Cu) and zinc (Zn) B.S. and M.S. degrees in chemistry and a Ph.D. degree would be removed from stream waterwhen pH was in- in soil chemistry, all from The Pennsylvania State Uni- creased by a pulse ofsodium hydroxide (NaOH). The versity. He joined the Intermountain Research Station creek, nearCooke City in southwestern Montana, has a in 1989. He studies geochemical weathering in dis- pH of 3.4. Watertemperature, pH, and concentrations turbed environments and develops methods for mitigat- of dissolved and particulate elements were measured ing damage to watersheds from mining and other land- belowthe pointwhere NaOH was injected. MINTEQA2, use activities. a chemical equilibrium speciation program, was used to tRuArbYedW.LaBnRdsORWeNhabiislaitpaltainotnpRheysseioalrocghisWtowritkhUtnhietDaitst-he csaolrcbuelda,teantdheparemcoiupinttatoefdepahcahseesledmuernitnginthdeisNsoalOveHd,pualds-e. Forestry Sciences Laboratory in Logan, UT. He holds Ferrihydrite (Fe(OH)3(am)) controlled Fe3+ concentrations eacBo.lSo.gydefgrroemethinefUonrievsetrrsyiatyndofaMnoMn.tSa.nad,egarnedeainPhr.aDn.ge tnhurmou(gAhl)ouptretchiepisttauteddyaptertihoedb.egAinhnyidnrgooufstohxeiNdeaOofHalpuumlis-e, dissolved during the middle ofthe pulse when the pH degree in plant physiology from Utah State University. He joined the Intermountain Research Station in 1965. exceeded 10, precipitated atthe end ofthe pulse, and He studies plant-water relations in harsh, disturbed en- dissolved afterthe pulse asthe stream reacidified. The vironments and develops reclamation methods for lands form ofthe Al hydrous oxide could not be identified. Pre- disturbed by mining and other land-use activities. cipitation of pyrochroite (Mn(OH)2) and brucite (Mg(OH)2) controlled dissolved manganese (Mn) and magnesium JANICE KOTUBY-AMACHER is the director of Utah (Mg) levels during the NaOH pulse. Dissolved Cu and State University Analytical Labs and is a cooperator Zn concentrations were reduced to <0.01 mg/Lduring on many research projects with the Disturbed Lands the NaOH pulse. MINTEQA2 predicted that Cu and Zn Rehabilitation Research Work Unit. She holds a B.S. would be adsorbed by hydrous Fe oxide atthe beginning degree in chemistry from Muhlenberg College, an M.S. and end ofthe NaOH pulse. Atthe high pH levels during degree in environmental resource managementfrom the middle ofthe pulse they should have been desorbed. The Pennsylvania State University, and a Ph.D. degree MINTEQA2 also predicted thatthe stream was under- in marine sciences from Louisiana State University. saturated with respectto Cu(OH) and Zn(OH) through- 2 2 Besides directing an environmental analytical labora- out the pulse. Because Cu and Zn were completely re- tory, she conducts research on the fate of pollutants in moved from the stream waterduring the entire pulse, the environment. notjustthe beginning and end, the mechanism respon- ANGELEE WILLIS is a contracttechnician with the Dis- siblefor removing them was postulated to be coprecipi- turbed Lands Rehabilitation Research Work Unit. She tation with Fe(OH)3(am). Although the pH adjustment is a B.S. degree candidate in biology at Utah State Uni- study indicated that precipitated Fe(OH)3(am) could rap- versity and researches methodsto mitigate environmen- idly remove Cu and Zn from a stream affected by acid tal damage from mining and other land-use activities. mine drainage, the pH should be maintained in an opti- mal range (7 to 8.5) to maximize removal by adsorption. The use oftrade orfirm names in thispublication is forreaderinformation anddoes not imply endorsementby the U.S. DepartmentofAgriculture ofanyproductorservice. Intermountain Research Station 324 25th Street Ogden, UT84401 1 CONTENTS Page Page Introduction 1 Precipitation of Hydrous Metal Oxides 8 Metal Sulfide Oxidation Mechanisms 1 MINTEQA2 Predictions of Dissolved Element TPrreevaetnmteinotnooffAMMetDal Sulfide Oxidation 21 ImpliCcoantcieonntsrfaotriMoentsal Removal From Acid 1 Previous Studies 2 Mine Drainage-Affected Streams 13 Present Study 3 Summary and Conclusions 14 Materials and Methods 3 References 14 pH Adjustment 3 Appendix A: Water Analysis Data for pH Sample Analysis 4 Adjustment Study at Fisher Creek, MT, Data Analysis 4 on August 30, 1992 16 Results and Discussion 5 Appendix B: Particulate Analysis Data for pH 5 pH Adjustment Study at Fisher Creek, MT, Changes of Element Concentrations as on August 30, 1992 17 pH Changes 6 Adding Sodium Hydroxide To Study Metal Removal in a Stream Affected by Acid Mine Drainage Michael C. Amacher Ray W. Brown Janice Kotuby-Amacher Angelee Willis INTRODUCTION Moses andothers 1987; Nordstrom 1982). Ferriciron and oxygen are the principal oxidants: Acid mine drainage (AMD) and acidic streams with highconcentrationsofmetals from metal sulfidedepos- FeS2(s) + 3.502(aq) + H2 = Fe2+ + 2SOf-+ 2H+ (1) its oxidized afterbeingexposedbyminingarecommon FeS (s) + 14Fe3+ + 8H = 15Fe2+ + 2S02"+ 16H+ (2) 2 2 throughoutthe Western UnitedStates (Amacherand Fe3+ oxidizes pyrite much fasterthan does, but others, in press; Fillipek and others 1987; Theobald the process is thoughtto beginwith oxid2ationby and others 1964). Many ofthese metal sulfide depos- dissolved oxygen. its arelocatedin mountainous regionswhere normally Ferrousiron(Fe2+ producedduringpyriteoxidation ) AprMisDtinceonwtaatienrisnhgehdisghhacvoencbeenetnrasteivoenrseloyfmiemtpaalcstesducbhy coxaindiitzseelpfybreitoexiadciczoerddibnygt2otroeaFcet3i*.onT(h2a).tFTeh3e* ceannsutihnegn as manganese (Mn), iron (Fe), copper (Cu), zinc (Zn), autoxidation cycle generates considerable amounts and aluminum (Al) from mine sites abandoned long ofacidity. The rate ofoxidation ofFe2+ by is very ago (Amacher and others, in press). Ofthese metals, slowandisindependentofpHbelow apHofa2bout3. Cu is the mosttoxic to aquatic fife (Forstner and Above pH 3, however, the oxidation rateincreases Wittman 1983). Restoringwatersheds impacted by withincreasing pH. AMD to anything approaching a pristinAeMsDtate is un- Certainmicroorganisms suchas Thiobacillusferro- likely, giventhe difficultyincontrolling andthe oxidans catalyze the oxidation ofFe2+ by greatly remote, ruggedlocatioAnsMoDfmanyhigh-elevationsites. increasingthe oxidationrate. Underacidicc2o,nditions, Methods to control can generally be classified microbiallycatalyzedoxidationofFe2* is more impor- into twotypes: (1) prevention atthe source and(2) tantthan oxidation thatis not microbially catalyzed. treatmentto remove acidity, metals, and sulfur. Pre- Under neutral or alkaline conditions, oxidation that vention generally seeks to keep metal sulfide depos- is notmicrobiallycatalyzedbecomes significant. Tem- its from oxidizing; drainage waters thenwill nothave perature and pH also influence the growth ofmicro- the acidity andmetals concentrationstypicalofAMD. organisms that catalyze oxidation ofFe2+ Besides Treatment may include raisingthe pHto precipitate . catalyzingreactions in water, Thiobacillusferrooxi- hydrousmetal oxides ofFe,Al, andMn, andto adsorb dans may also help oxidize pyrite on the surface of other metals (Cu and Zn) onthe metal oxides. Wet- exposedminerals. AMD landsystems remove metals andsulfurfrom by uptake by plants, adsorptionby organic matter, and Prevention ofMetal Sulfide Oxidation reducingsulfates to sulfides that precipitate (U.S. Bureau ofMines 1988). These control strategies have Prevention generally involves keeping metal sul- metwith varyingdegrees ofsuccess. Before we dis- fides from comingin contact withthe that begins cuss them in more detail, we needto examine the the autoxidationcycle. Measuresinclude2sealingadits chemistry ofmetal sulfide oxidation. tokeepairfrom comingincontactwithmetal sulfides, burying miningwastes that contain metal sulfides, Metal Sulfide Oxidation Mechanisms and addingchemicals to inhibit the growth and ac- tivity ofmicroorganisms that catalyze the oxidation Themechanisms andfactors influencingtherates of ofpyrite (U.S. Bureau ofMines 1988). metal sulfide oxidation are knownprimarilyfrom stud- Sealingunderground mine workings tolessen iesofpyriteoxidation(Brown andJurinak 1989a,b; sulfides' exposure to air seems to be a good control 1 strategy. Unfortunately, once sulfides have been ex- headwater stream ofthe Clark's Fork ofthe posedto air,the autoxidationcycle is difficultto arrest. Yellowstone River. NorandaMinerals Exploration The Fe3+ produced by oxidation ofFe2+ continues to and Crown Butte Mines, Inc., are exploringthe area oxidize pyrite (Stumm andMorgan 1981). In addition, aroundFisherandHendersonMountains forpossible it is rarely possible to completely seal offall exposed operation ofagoldmine. Theirmine permit applica- sulfidesfrom air. To completelycontrolAMD produc- tionis being processed. Reclamation plans for the tion, all exposed sulfides must be returned to areduc- areahavebeendeveloped; theAMD controlmeasures ing environment. Deep burial ofsulfide-containing proposed forthe oldMcLaren and GlengarryMines wastes, especially when they are flooded, attempts to aswell asthenewmineinclude closingportals, back- restore a reducing environmentwhere sulfides will filling pits, and saturating a proposed tailings im- remain stable. poundmentwithwaterto keep metal sulfides intheir reduced state. AMD Treatment of Previous Studies AMD Usinglime totreat doesnotcontroltheproblem atits source; itrequires continual additionoflimeto The Intermountain Research Station has been con- raise pH and precipitate or adsorb metals from AMD. ductinglong-term researchinthe BeartoothMountains The hydrous metal oxides ofFe, Al, and Mn will pre- to identifythe factors limitingreclamation and to de- cipitate as pHincreases; they adsorb othermetals such velop methods that will repair impacts from past and as Cu and Zn (Dzombak and Morel 1990; Kinniburgh presentmining. Overthelongterm, wehope torestore, and Jackson 1981). at least partially, an intact, functioning ecosystem. Recently, wetlands have been evaluated for their Research begun recently attempts to assess the im- ability to treat water affected byAMD (U.S. Bureau pact ofAMD onwatersheds in the area, identify how ofMines 1988). In aerobic-anaerobicwetlands, metals metals are removed from streams receivingAMD, de- AMD AMD are removedfrom by fourprocesses: (1) accumu- velop models to accurately predict impacts on lationin plant communities growingin the aerobic streams, and evaluate methods ofpreventingor miti- surfacelayer, (2) adsorptiononorganicmatterinthe gatingAMD. Researchusinghydrologicmass balance aerobic surfacelayer, (3) precipitation and adsorption calculations, geochemical modeling, and sediment onmetal oxides inthe aerobiclayer, and(4) precipita- analysis has identifiedthe processes controllingmetals tion as relatively insoluble metal sulfides inthereduc- concentrations inthese streams (Amacher andothers ingenvironment ofthe anaerobic lower layer. How- 1991a,b, in press). Iron concentrations are controlled ever, metals are introduced into the food chainwhen by precipitation ofhydrous Fe3+ oxide (ferrihydrite), they are taken up by plants, and loading rates must photoreduction ofhydrous Fe3+ oxide coatings on bekeptlowto avoid overwhelmingthewetlands' abil- stream sediments, and microbially mediated oxida- ity to absorb metals. tion ofFe2+ Aluminum concentrations appear to be . In anaerobic wetlands no plant communities are controlled by dilution from uncontaminated inflows needed to treatAMD. Sulfur-reducingmicroorgan- or precipitation ofhydrous Al oxides. Copper concen- isms reduce sulfate to sulfide and precipitate metals. trations are controlledbydilution orby adsorption on Because metal sulfides are so insoluble, all but very hydrous Fe3+ oxides. Manganese and S0 concentra- 4 low levels ofmetals can be removed from the water. tions are controlled by dilution from inflows. Much The major disadvantage ofusing anaerobicwetlands ofthis research is ongoing. is thatAMD loadingrates mustbe relatively lowto One ofthe potentially most significant findings is avoid exceedingthe rate atwhichthemicroorganisms thatthe hydrous Fe3+ oxide coatings on stream sedi- canreduce sulfate. Furthermore, anaerobic wetlands ments adsorbedCu from stream waterswhenthe pH are sensitive to temperature because they depend on was greater than 5.5 (Amacher and others 1991a, in growthofmicroorganism populations. Anaerobicwet- press). Asinflows added alkalinitytothe streams, Cu lands must be insulated to operate duringwinterin concentrations associated with the hydrous Fe3+ ox- coldclimates, andAMD loadingrates mustbe reduced ides in stream sediments increased. Maximum con- during winter to avoid exceedingthe system's capac- centrations ofCu in sediments were found several AMD ity to reduce sulfate. thousandmeters downstream from the sources. AMD Acidmine drainage from theMcLaren and Glengarry This suggests a possible mitigation strategyto Mines, two old abandoned Cu mines on opposite sides augmentothermethods proposedforthe area. Even ofFisher Mountain in the Beartooth Mountains of ifprevention methods such as closing portals, back- Montana, contaminate the headwaters ofDaisy and filling pits, buryingwastes, and saturatingtailings Fisher Creeks. Daisy Creek, affected by the McLaren impoundments are effective, some AMD may still Mine, is a headwater stream ofthe Stillwater River; flow into Daisy and Fisher Creeks. Wetlandsmay AMD Fisher Creek, affected by the Glengarry Mine, is a playsome roleintreating inthesewatersheds. 2 However,theireffectiveness will be limited bythe of7to8.5wastargeted. MINTEQA2 calculations and toxicity ofAl and Cu to wetland plants, the needto sediment analysis (Amacher and others 1991a, in insulate the wetlands for most oftheyear, and the press)indicatedthis isthe pHrangeformaximum Cu needforrelativelylowAMD loadingrates. Ifthe pH adsorptionbyhydrous Fe3+ EventhoughNaOHwould AMD . ofthe streams affectedby couldbe raisedbyin- notbe used as a final treatment to adjust the pH of creasingthe alkalinityoftheirinflows, thenhydrous these streams, itcan be used to test the ability ofhy- Fe3+ oxide coatings on stream sediments would scav- drous Fe3*oxideto scavenge Cuunderfieldconditions. enge Cufrom stream water. Themetal precipitation and adsorption zone can be confined to a relatively MATERIALS AND METHODS AMD shortreach ofstream near the source, extend- ingthe buffer zone between stream reaches affected FisherCreekis locatedabout5 km northofCooke byAMD and downstream reaches containingaquatic City, MT, in the Gallatin National Forest. Fisher life. Creekis one ofthe headwater streams that form The best way to raise the pH ofstreams affected the Clark's ForkoftheYellowstone River. Itbegins AMD by isto increasethe alkalinity ofuncontami- in a glacial cirquejust below Lulu Pass (2,963 m) natedtributaries. Puttinglimestonerockinthelower betweenFisher and ScotchBonnetMountains. Acid reaches ofthesetributaries wouldbethe cheapestand mine drainage from the adit ofthe Glengarry Mine, easiestway to increase their alkalinity. Both Daisy an abandoned copper mine, flows into Fisher Creek and Fisher Creeks have numerous tributaries that afew hundred meters below the glacial cirque. The AMD substantially increase their flow. Mosttributaries has severelyimpactedthe stream withconcen- inupstreamreaches neartheAMD sources havelow trations ofMn, Fe, Cu,Al, andsulfur(S)that are sub- alkalinitybecausetheydonotflowthroughcalcareous stantiallyhigherthan those in the area's relatively rock. Tributaries fartherdownstream contributemost pristinenaturalwaters (Amacherandothers, in press). oftheincomingalkalinity;thisiswhere mostofthe Cu is adsorbed(Amacher 1991a, inpress). Ifthechannels pH Adjustment ofthe upstream tributaries were lined with crushed limestonerock, the increased alkalinitycouldneutral- Twentyliters of10 M NaOH were added to Fisher izetheAMDinthemain streams. Thelowerends of Creek about 350 m belowthe Glengarry Mine adit at these tributaries are accessible byroads, so the lime- a rate of1 L/minbeginningat 10:33 a.m. onAugust30, stonerockcouldbereadily placedthere. Addinglime- 1992. Water samples were collected at a point about m stonerockto the main stream channels affectedby 280 belowthe injection point at 2-min intervals for AMD would not work as well, because precipitating the next80min. Additional sampleswere collected90, hydrous Fe3+ oxides would coat the rock, sealingit 105, 120, 150, and 180 min after injection. The last from the water. sample was collected at 1:35 p.m. Each 40-mL sample ofstream water was collected Present Study witha 50-mLplasticsyringe. The samplewasimme- diatelyfilteredthrougha47-mm polycarbonatemem- Before trying this approach, we wantedto assess branefilterholdercontaininga 0.2-fim polycarbonate whether precipitatinghydrous Fe3+ oxides would scav- membrane filter. Three aliquots offiltered sample enge Cu from stream water as pH increased. In gen- were pipetted into 16- by 125-mm plastic snap-cap eral, such adsorption reactions are quite fast (Sparks testtubes for Fe2+ (10 mL), reactive Fe (Fe2+ + Fe3+) and Zhang 1991), so Cu should be removed rapidly. (5 mL), and total elemental analysis (10 mL). The We conducted apH adjustment study atFisherCreek membrane filter holder was disassembled, andthe to test this hypothesis. Apulse ofNaOH was applied membrane filter was placed into a 22-mL plastic to the stream to rapidly increase pH and precipitate scintillationvial. hydrous Fe3+ oxide, whichwould adsorb Cu. We meas- The pH andtemperature ofthe stream water were uredthe amounts ofvarious elements in the dissolved monitored continuously with a Ross combination pH and particulate phases duringthe NaOH pulse. We electrode (Orion No. 811500) and automatictemper- used MINTEQA2, a chemical equilibrium speciation ature compensator (OrionNo. 917001) connectedto program, todetermine the possible solidphases formed anOrionModel SA250 portable pHmeter. Themeter duringthe pulse. Ifthe precipitation and adsorption andelectrodeswerecalibratedusingpH 4.00 and 7.00 reactions occurred sufficientlyfastrelativetothe mix- buffers immediately before the start ofthe NaOH in- ingandtransport rates within the stream, near equi- jection. Thecalibrationwas rechecked afterthe exper- libriumconditionscouldbemaintained. Then,chemical iment. The measured pH had changed only 0.01 pH equilibrium speciation models couldbe usedto evalu- unit from the buffervalues. ate possible controls onthe concentrations ofmetals The stream discharge was determinedatthe sample in the dissolved and particulate phases. ApH range collection point before NaOH injection by measuring 3 Fisher Creek's cross-sectional area and flow velocity indissolved, adsorbed, and precipitated phases as using aMarsh-McBirney Model 20ID portable water thepHwas systematicallyvaried. Dissolvedelement currentmeter. concentrationsjustbefore arrivaloftheNaOH pulse (14 min afterNaOH began to be added) were used Sample Analysis as initial input values forthe pH sweep simulations. Amounts ofelements in dissolved, adsorbed, and pre- Fe2+ andreactive Fe were measured in the filtered cipitated phases were calculated at pH levels corre- watersamples usingthe 2,2-bipyridine method spondingto differenttimes (min) afterthe injection (Skougstad andothers 1978). Colordevelopmentre- ofNaOH: agents were addedinthe field after all the samples Minutes pH Minutes pH werecollected. Sample absorbencies weremeasuredat 520 nm inthe laboratory. To determine Fe2+ 0.5 mL 14 3.41 50 9.78 , of0.2 percent 2,2'-bipyridine solutionwas added to 20 3.88 52 8.14 one ofthe 10-mLaliquots ofeach sample. One mLof 22 4.61 54 6.25 deionized waterwas addedto each ofthe aliquots, 24 6.32 56 5.39 followed by 1 mL of4.3 M sodium acetate buffer solu- 26 10.34 58 4.90 tion (290 gofsodium acetate trihydrate dissolvedin 28 11.22 62 4.54 deionized water and diluted to 500 mL). 34 11.73 74 4.25 To determine reactive Fe, 5 mL ofdeionized water 40 12.15 105 4.05 was pipettedinto eachofthe 5-mLaliquots ofsample. 46 11.60 180 3.66 Afteradding0.5 mLof0.2 percent2,2'-bipyridine solu- 48 11.00 tion and 1 mLof10percenthydroxylaminehydrochlo- The measurements covered the full range ofpH ride solution (10 g ofNr^OH HCl dissolvedin deion- NaOH ized water plus 4 mL ofconcentrated HC1 diluted to lleovweeldstoobspreercviepditdautreidnugritnhgethe pH psuwleseep. wSeorleidwsusalt-ite mL l1o0w0edtowsittahnddefioronaitzleedaswtat3e0r)m,int.heTahleinqu1otmsLweorfe4.a3l-M h(yFder(oOxHi)d2e),(fAelrr(iOhHy)dr(iatme))(,Feg(ibObHs)i3t(eam()A)l,(aOHl)umijnuurmban- wsoadsiduemtearcmetianteedbausfftehresdoilfufteiroenncweabsetawddeeedn.reFaecrtriivceiFreon iitdee((AC1u0(OHHS)04),bpryurcoi3cther(oiMtge((OMHn)(OHa)n2d),pcoorptple3a)rn,dhiytderox- and Fe2+ 2), 2), The con.centrations oftotal dissolvedAl, calcium t(oCaf(oOrHm)d2u).riTnhgestheewberrieeftphaesssoalgied pofhatsheesNmaoOstHlpiuklesley. (Ca), Cu, Fe, potassium (K), magnesium (Mg), Mn, Supersaturationwithrespect to other solid phases sodium(Na), S,andZnwere determinedintheremain- mayhavebeen attained, butkineticconstraints make ing 10-mL aliquot ofeach sample usinginductively it unlikelythat these phases would have formed dur- coupled plasma atomic emission spectrophotometry ingthe briefNaOH pulse (Stumm and Morgan 1981). (ICPAES) after acidification with 100 ^L ofconcen- Adsorption ofCu, Zn, and Ca on hydrous Fe3+ oxide HN0 A trated 3. complete listingofthedissolvedele- duringthe pH sweep simulations was modeled using mentconcentrations is given in appendixA. the diffuse-layermodel (DLM) ofDzombak and Morel asTsoocMdieatteedrmeilneemetnhtesa,m2o0umntLooffpa0r.t2i5cMulaNteHFgeOoHxiHdCelan—d (to19m9o0)d.elTahdesohrypdtrioounsaFsea3+fouxnicdteiopnaorfapmeHtearreslniesetdededin 0.25 HC1 solution(ChaoandZhou 1983)was added table 1. Twoadsorptioncaseswere considered. Inthe toeachscintillationvialcontainingamembrane filter, firstcase, thetotal dissolvedFe presentinthe stream andthe vials were allowed to stand overnight while just before the NaOH pulse arrivedwas assumed to the particulate Fe oxides dissolved. The resulting so- precipitate asferrihydrite duringthe pulse. Thiswas lutionswere analyzedbyICPAES. Acompletelisting equivalentto 12.3 mg/L ofparticulate Fe as Fe ofthe particulate elementconcentrations is givenin Inthe second case, adsorptionbythe hydrous F2e3+3. appendix B. oxide coatings on streambed sediments was also considered. Adsorption by such coatings seems likely Data Analysis giventhe stream's shallow depth; much ofthe water columnis in contactwiththe streambed as numerous Temperature, pH, and element concentrations in eachofthewatersampleswereinputintoMINTEQA2 rciefnftlersatiinodincoafteh.ydIrtoiussneFcee3s+soaxriydetotheasttiwmiallteacscoomuentcofno-r version3.0(Allisonandothers 1990), achemical equi- adsorptionbyboth suspended and streambedhydrous librium speciation program, to calculate ionic species Fe3+ oxide. Because we had no way to measure the activities, ion activityproducts (IAPs), andsaturation contribution ofstreambedhydrous Fe3+ oxide, we se- indexes forvarious solid phases. lected an arbitraryvalue of10 times the suspended Inaddition,simulationswererunusingthepH sweep featureofMINTEQA2tocalculate amountsofelements particulate Fe (123 mg/L as Fe2 3). This gives arela- tive comparison ofthe change in adsorption expected 4