Passive Aerobic Treatment of Net-Alkaline, Iron-Laden Drainage from a Flooded Underground

29 Pages·2017·1.42 MB·English

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USGS Staff -- Published Research US Geological Survey
2007
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Charles A. Cravotta III
US Geological Survey, PA Water Science Center, 215 Limekiln Rd, New Cumberland, PA, USA,
[email protected]
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Cravotta III, Charles A., "Passive Aerobic Treatment of Net-Alkaline, Iron-Laden Drainage from a Flooded
Underground Anthracite Mine, Pennsylvania, USA" (2007). USGS Staff -- Published Research. 275.
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MineWaterEnviron(2007)26:128–149
DOI10.1007/s10230-007-0002-8
TECHNICAL ARTICLE
Passive aerobic treatment of net-alkaline, iron-laden drainage
from a flooded underground anthracite mine, Pennsylvania, USA
Charles A. Cravotta III
Received:3April2007/Accepted:1May2007/Publishedonline:18August2007
(cid:1)Springer-Verlag2007
Abstract Thisreportevaluatestheresultsofacontinuous June 2005. During the first 12 months of operation, esti-
4.5-daylaboratoryaerationexperimentandthefirstyearof mateddetentiontimesinthetreatmentsystemrangedfrom
passive, aerobic treatment of abandoned mine drainage 9 to 38 h. However, in contrast with 80–100% removal
(AMD)fromatypicalfloodedundergroundanthracitemine of Fe2+ over similar elapsed times during the laboratory
in eastern Pennsylvania, USA. During 1991–2006, the aeration experiment, the treatment system typically
AMD source, locally known as the Otto Discharge, had removed less than 35% of the influent Fe2+. Although
flowsfrom20to270 L/s(median92 L/s)andwaterquality concentrations of dissolved CO decreased progressively
2
that was consistently suboxic (median 0.9 mg/L O ) and within the treatment system, the PCO values for treated
2 2
circumneutral(pH&6.0;netalkalinity>10)withmoderate effluent remained elevated (10(cid:1)2.4 to 10(cid:1)1.7atm). The ele-
concentrations of dissolved iron and manganese and low vatedPCO maintainedthepHwithinthesystematvalues
2
concentrationsofdissolvedaluminum(medians of11,2.2, less than 7 and hence slowed the rate of Fe2+ oxidation
and <0.2 mg/L, respectively). In 2001, the laboratory aer- compared to the aeration experiment. Kinetic models of
ation experiment demonstrated rapid oxidation of ferrous Fe2+oxidationthatconsidereffectsofpHanddissolvedO
2
iron(Fe2+)withoutsupplementalalkalinity;theinitialFe2+ were incorporated in the geochemical computer program
concentrationof16.4 mg/Ldecreasedtolessthan0.5 mg/L PHREEQC to evaluate the effects of detention time, pH,
within 24 h; pH values increased rapidly from 5.8 to 7.2, and other variables on Fe2+ oxidation and removal rates.
ultimately attaining a steady-state value of 7.5. The These models andthe laboratory aeration experiment indi-
increasedpHcoincidedwitharapiddecreaseinthepartial catethatperformanceofthisandotheraerobicwetlandsfor
pressure of carbon dioxide (PCO ) from aninitial value of treatment of net-alkaline AMD could be improved by
2
10(cid:1)1.1atmtoasteady-statevalueof10(cid:1)3.1atm.Fromthese aggressive, continuous aeration in the initial stage to
results, a staged aerobic treatment system was conceptual- decreasePCO ,increasepH,andaccelerateFe2+oxidation.
2
izedconsistingofa2 mdeeppondwithinnovativeaeration
and recirculation to promote rapid oxidation of Fe2+, two Keywords Aerobic wetlands (cid:2) Iron oxidation (cid:2)
0.3 mdeepwetlandstofacilitateironsolidsremoval,anda Carbon dioxide exsolution (cid:2) Kinetics modeling
supplementaloxiclimestonedrainfordissolvedmanganese
and trace-metal removal. The system was constructed, but
without the aeration mechanism, and began operation in Introduction
Problem
Electronicsupplementarymaterial Theonlineversionofthis
article(doi:10.1007/s10230-007-0002-8)containssupplementary
material,whichisavailabletoauthorizedusers. Drainage from abandoned coalmines degrades more than
8,000 km of streams in the northern Appalachian region,
C.A.CravottaIII(&) USA;manyofthesestreamsarefishlessorcontainfewfish
USGeologicalSurvey,PAWaterScienceCenter,
(U.S. Environmental Protection Agency 2006; Herlihy
215LimekilnRd,NewCumberland,PA,USA
et al. 1990). Although acidic pH (<4.5) and elevated
e-mail:[email protected]
123
MineWaterEnviron(2007)26:128–149 129
concentrations of dissolved sulfate, iron, and other metals from typical Bituminous Coalfield AMD. Studies at wet-
are common characteristics of abandoned mine drainage lands receiving AMD have indicated that the prescribed
(AMD), a substantial percentage of the AMD in the iron-removal rate of 20 g/m2/day can underestimate the
Northern Appalachian Region is iron-laden but not acidic areaneededforeffectivetreatment,particularlyaspHand/
(Cravottaetal.1999;HymanandWatzlaf1997;Kirbyand or concentrations of iron decrease (Kirby et al. 1999;
Cravotta2005a;RoseandCravotta1998).Forexample,in Tarutis et al. 1999; Watzlaf et al. 2004). Generally, flow
1999, 30% of 140 sampled AMD sources in the Bitumi- rates are higher and concentrations of acidity, sulfate, and
nous and Anthracite Coalfields of Pennsylvania had iron are lower for similar pH in the Anthracite Coalfield
concentrations of dissolved iron greater than the 7 mg/L compared to the Bituminous Coalfield of Pennsylvania
criteria for effluent from active mines in Pennsylvania (Cravotta2007).
(Commonwealth of Pennsylvania 2002) and had net-alka- The oxidation rate of iron in aqueous systems is com-
linewaterquality,withnear-neutralpH(6.0–7.5)(Cravotta plex and tends to increase with water temperature, pH,
2007; Kirby and Cravotta 2005b). Despite their net-alka- concentration of dissolved oxygen, concentrations of dis-
line water quality, the dissolved iron tends to precipitate solvedandsuspendediron,andbacterialactivity(Dempsey
downstream from the AMD sources, forming ochreous et al. 2001; Kirby et al. 1999; Stumm and Lee 1961;
encrustations that degrade the aquatic habitat (Earle and Stumm and Morgan 1996; Watzlaf et al. 2001 2004; Wil-
Callaghan 1998; Williams et al. 2002; Winland et al. liamson et al. 1992). In the presence of dissolved oxygen
1991). Various alternatives for treatment to remove the (O ), concentrations of dissolved iron tend to decrease
2
dissolved iron and associated metals from AMD before it because of oxidation and hydrolysis reactions that lead to
discharges to streams could be appropriate depending on the formation of various ferric-hydroxide and hydroxysul-
thevolumeoftheminedischarge,itsalkalinityandacidity fate solids, hereinafter denoted Fe(OH) :
3
balance, and the available resources for construction and
maintenance of a treatment system (Hedin et al. 1994; Fe2þþ0:25O2þ2:5H2O!FeðOHÞ3ðsÞþ2Hþ ð1Þ
Skousen et al. 1998). Information on the hydrologic vari-
Generally, the ferrous iron (Fe2+) oxidation step is rate-
ability and the site characteristics is needed by water-
limiting; ferric iron (Fe3+) hydrolysis and precipitation of
resource managers to identify effective remedial strategies
Fe(OH) arenearlyinstantaneous(SingerandStumm1970;
for the AMD. 3
Stumm and Morgan 1996). Despite initial pH values that
can be near neutral, some AMD containing dissolved iron
canhaveacidicpH(<4.5)aftercompleteoxidationbecause
Background
of the release of protons (H+) associated with iron
hydrolysis. As equation 1 proceeds to completion, the pH
Current guidelines for passive treatment of coalmine
willdecreaseunlessbufferedbyexcessalkalinity,indicated
drainage (Skousen et al. 1998; Watzlaf et al. 2004) were
by positive values for net alkalinity or negative values for
largely developed by the former U.S. Bureau of Mines
hotperoxide acidity (Kirbyand Cravotta 2005a,b).
(Hedin et al. 1994) on the basis of systems that were con-
The pH, alkalinity, and associated properties of AMD
structedfortreatmentofAMDintheBituminousCoalfield
also can change as a water sample equilibrates to atmo-
of Pennsylvania and West Virginia. These guidelines
spheric conditions because of the exsolution of dissolved
indicate different treatment strategies for net-alkaline and
carbon dioxide (CO ). Ground water and coalmine drain-
net-acidic water quality. For AMD that has net acidity 2
age commonly contain elevated concentrations of
(alkalinity < acidity; hot peroxide acidity > 0), treatment
dissolvedCO ,resultinginpartialpressureofCO (PCO )
systemsthatpromotealkalinityproductionareappropriate. 2 2 2
of10(cid:1)1.5to10(cid:1)0.5atminthevadosezone(Langmuir1997;
The alkalinity can be produced by limestone dissolution
Rose and Cravotta 1998). When exposed at the surface,
and, in some cases, sulfate reduction (e.g., Hedin et al.
excess CO will exsolve until dissolved CO concentra-
1994;Langmuir1997).Net-alkalineAMDdoesnotrequire 2 2
tions equilibrate with atmospheric PCO of approximately
supplemental alkalinity and can be routed directly to an 2
10(cid:1)3.5 atm. To explain effects on pH and alkalinity, the
oxidation pond(s) and/or aerobic wetland(s) where dis-
CO exsolution process may be described as:
solved iron is oxidized and precipitated from solution. 2
Typically, to determine the areal extent of an aerobic sys-
HCO(cid:1) $CO ðgÞþOH(cid:1); ð2Þ
tem, the average iron-loading rate of the AMD source 3 2
expressed in grams per day (g/day) is divided by an where equation 2 goes to the right. The pH will increase
assumed iron-removal rate of 20 g/m2/day, based on the as CO exsolves. Exsolution of CO can be promoted
2 2
work of Hedin et al. (1994). However, this areal-removal by aeration within a stream or aggressive air sparging
ratemaynotbeapplicableforAMDthatdiffersincharacter in a treatment system (Jageman et al. 1988). Unless
123
130 MineWaterEnviron(2007)26:128–149
accompanied by the precipitation of Fe(OH) , CaCO , or thebasisforthedesignofaerobicwetlandsfortreatmentof
3 3
other solids, the alkalinitywill notbeaffected by equation the Otto Discharge. The performance of the treatment
2 because bicarbonate (HCO(cid:1)) and hydroxyl (OH(cid:1)) ions system is evaluated on the basis of water-quality data
3
have equivalent capacity to neutralize acid; that is, they collected monthly during June 2005 to June 2006 at the
have equivalent alkalinity. On the other hand, if Fe(OH) inflow, outflow, and intermediate points within the treat-
3
precipitates in accordance with Eq. 1, alkalinity will ment systemand onthe receiving stream above andbelow
decrease and the pH also could decrease, but not neces- the discharge. Lastly, geochemical models of iron oxida-
sarily. For solutions containing dissolved iron and excess tion are used to evaluate detention time and chemical
CO , the pH could increase or decrease depending on the factors affecting iron oxidation and removal.
2
concentrationsoftheseconstituentsandtherelativeratesof
reactions (1) and (2).
Wetlands designed for removal of iron may be inade- Description of Study Area
quate for the removal of dissolved manganese and various
trace metals. Alkaline pH values may be necessary for The Otto Colliery Discharge (Otto Discharge; lat.
attenuation of these metals. At neutral to alkaline pH, iron 40(cid:2)3905800 N, long. 76(cid:2)1901400 W) near Branchdale,
and manganese oxides can be effective sorbents of dis- SchuylkillCounty,isoneofthelargestsourcesofAMDin
solved manganese, copper, cobalt, nickel, zinc, cadmium, theSchuylkillRiverBasinineasternPennsylvania(Fig. 1).
and other potentially toxic trace metals (Kooner 1993; The upper Schuylkill River Basin originates within the
McKenzie 1980). Such metals have been identified with Southern Anthracite Coalfield of the Appalachian Moun-
ochreous precipitates at AMD sites (Kairies et al. 2005; tain Section of the Ridge and Valley Physiographic
Webster et al. 1998; Williams et al. 2002; Winland et al. Province, where it is underlain by strongly folded and
1991). Adsorption of the cations by the hydrous iron and faulted sandstone, shale, siltstone, conglomerate, and coal
manganeseoxidesgenerallyisinterpretedtobetheprimary of the Llewellyn and Pottsville Formations (Wood et al.
mechanismforremovalofthemetals.Forexample,cobalt, 1986; Eggleston et al. 1999; Way 1999). Since the late
nickel, and zinc in AMD were attenuated under oxidizing 1800s, a total of 38 mapped coalbeds with average thick-
conditions within a buried limestone bed, but only after nesses from 0.3 to 2.5 m have been mined to depths
ironandmanganeseoxidesprecipitatedneartheinflowhad exceeding 1,000 m in the Southern Anthracite Coalfield
migrated downgradient where the pH was near neutral (Wood et al. 1986). Although several surface and under-
(Cravotta and Trahan 1999; Cravotta and Watzlaf 2002). ground anthracite mines presently (2007) are active, most
Althoughelevatedtemperaturescanproducefasterrates minesintheupperSchuylkillRiverBasinwereabandoned
of iron oxidation and more effective treatment in AMD before 1960 and are flooded. Fresh surface and ground
wetlands (Dempsey et al. 2001; Kirby et al. 1999), the water that enters the mines acquires acidity, sulfate, iron,
prolongedexposureofAMDtoambientairtemperaturesor and other metals and eventually discharges as AMD from
sunshine can produce temperature extremes that are not mine shafts, tunnels, and topographically low points
suitable for fish and other aquatic organisms. Directing (Growitzetal.1985;RobertKimballandAssociates2000;
effluentfromashallowwetlandthroughatrenchfilledwith Pennsylvania Department of Environmental Protection
limestone or other rock fragments can moderate the water 2003; Wood 1996). The metal-laden AMD contributes
temperature. Ideally, the treated AMD effluent quality and substantially to base flow but degrades stream-water
temperature would sustain fish and other aquatic life. quality and aquatic habitat of the Schuylkill River and its
upper tributaries. Consequently, the West Branch and
upper Schuylkill River are on the U.S. Environmental
Purpose and Scope ProtectionAgency(USEPA)303(d)listofimpairedwaters
in Pennsylvania (Pennsylvania Department of Environ-
This report presents information that could be useful to mental Protection 2003, 2004).
water-resource managers to assess remedial strategies for The Otto Mine operated from the early 1800s to 1934
net-alkaline, iron-laden AMD. The report describes field, and encompassed an area of approximately 20 km2 (Guy
laboratory, and computational methods and water-quality Mace, White Pine Coal Company, oral communication,
data collected by the U.S. Geological Survey (USGS) 2006).Duringthelifeofthemine,thousandsofmetrictons
during 2001–2006 for the evaluation of passive-treatment of anthracite were extracted from 13 named coal beds
strategiesandperformanceresultsatafloodedunderground (RabbitHolethroughBuckMountain)toamaximumdepth
anthracite mine in eastern Pennsylvania that is locally of60 mbelowsealevel(Egglestonetal.1999;GuyMace,
known as the Otto Discharge. A laboratory aeration WhitePineCoalCo.,oralcommunication,2006).Afterthe
experiment conducted in December 2001 is presented as Otto Mine closed, the underground voids flooded and the
123
MineWaterEnviron(2007)26:128–149 131
Fig.1 Maps showing location of the Otto Discharge. a Bituminous County,Pa.CoalfieldboundariesbasedondistributionofPennsylva-
and Anthracite Coalfields of Pennsylvania; b Abandoned mine nianbedrock(Bergetal.1980).MapofupperSchuylkillRiverBasin
drainage (AMD) in the upper Schuylkill River Basin, Schuylkill adaptedfromL.RobertKimballandAssociates,Inc.(2000)
present discharge developed from a production tunnel and Datumof1929(NGVD29)andflowsdowna470 mlong,
associatedshaft.TheuntreatedAMDexitsthetunnelatan rock-inlaid ditch to elevation 238 m above NGVD 29 at
elevation of 250 m above the National Geodetic Vertical Muddy Branch (Fig. 1). At their confluence, the Otto
123
132 MineWaterEnviron(2007)26:128–149
DischargecontributesthemajorityofstreamflowinMuddy coagulation and settling, plus oxidation rates could be
Branch, which flows eastward to West Creek and eventu- increased by sorption of Fe2+ and catalysis on Fe(OH)
3
ally the West Branch of the Schuylkill River. Below their surfacesatnear-neutralpH.Theabioticoxidationofsorbed
confluence, the water and streambed of Muddy Branch are Fe2+ (heterogeneous oxidation) generally is faster than the
a rusty orange-brown, owing to loading of iron and other oxidation of dissolved Fe2+ (homogeneous oxidation)
metals from the Otto Discharge. Despite the metals load- (Dempsey et al. 2001; Dietz and Dempsey 2002; Kirby
ing, the Pennsylvania Fish Commission (Chikotas and et al. 1999; Stumm and Sulzberger 1992; Tamura et al.
Kaufman2002)documentedhighdensitiesofnativebrook 1976).Enhancingironremovalbyheterogeneousoxidation
trout (Salvelinus fontinalis) in Muddy Branch downstream could improve treatment efficiency and produce a denser
from the Otto Discharge. The brook trout benefit from sludge.
sustainedinflows ofcold,near-neutralwaterfromtheOtto Landowner consent was obtained, detailed engineering
Discharge; however, populations of aquatic insects and was completed, construction permits were approved, and,
other fish species tend to be limited because of iron-oxide in June 2005, construction of the treatment system was
depositsinthestreambed(e.g.,EarleandCallaghan1998). completed by private-sector companies on behalf of the
Data collected by the USGS from 1991 to 2006 (this Schuylkill Headwaters Association, Inc. The ‘‘as-built’’
paper; Williams et al. 2002; Wood 1996) indicate that the system incorporated most basic features of the conceptual
OttoDischargerangeswidelyinflow(20–270 L/s;median design. However, to reduce construction costs, two novel
92 L/s) but is consistently net alkaline (pH = 5.7–6.1; provisions—the hydraulic aeration and recirculation in the
alkalinity = 34–70 mg/L CaCO ), anoxic to suboxic upperoxidationpondandtheperforatedflushpipinginthe
3
(<0.1–2.2 mg/L O ), and contaminated with sulfate loweroxiclimestonedrain—wereeliminated.Asshownin
2
(SO = 190–370 mg/L), iron (Fe = 3–16 mg/L), manga- Fig. 2, the AMD from the Otto Discharge was diverted
4
nese (Mn = 2–3 mg/L), aluminum (Al = 0.1–1.3 mg/L), fromtherock-linedditchatabout235 mdownstreamfrom
and other dissolved metals. On the basis of its net-alkaline thetunnelandspilledsequentiallyintotheoxidationpond,
waterqualityandflowcharacteristics,theeffluentfromthe the two shallow wetlands, and finally the oxic limestone
Otto Discharge could be treated with a passive aerobic drain. Rectangular weirs with adjustable freeboard were
wetland system for removal of iron and other metals (e.g., installed between each of the treatment cells to control
Hedin et al. 1994). water levels and facilitate flow measurements. At the sec-
ondwetland,anoverflowweirwasinstalledasasecondary
outlet in the event the oxic limestone drain would not be
Treatment System Design and Construction able to transmit the entire flow.
A passive aerobic treatment system was conceptualized to
facilitatetheremovalofiron,manganese,andothermetals Materials and Methods
from the Otto Discharge while maintaining the sustained
flows of oxygenated, cold water required by native brook Water-Quality Sampling and Analysis
trout in downstream reaches of Muddy Branch. The con-
ceptual treatment system would divert the existing Data on the flow rate and inorganic chemistry of the Otto
discharge with a low-head dam; promote aeration with an Discharge and Muddy Branch above and below the Otto
in-linehydraulicdevice;detaintheaeratedwaterwithinan Discharge were collected by the USGS over a range of
oxidation pond; promote deposition of iron-hydroxide hydrologicconditionsduring2001to2006.Oncetreatment
precipitates that form as a consequence of aeration within began, additional water-quality data were collected at the
the oxidation pond and two constructed, aerobic wetland outletsofeachofthetreatmentcells.Aftertreatmentwasin
cells; and remove manganese and trace metals in a flush- placefromlateJune2005toearlyJune2006,allthewater
able, oxic limestone drain that would add alkalinity and fromtheOttoDischargewasdivertedthroughthetreatment
mitigate temperature fluctuations. The proposed in-line system. Sample sites along the rock-lined diversion ditch
hydraulicaerationwouldtakeadvantageofgravityanduse from the Otto tunnel were at the inflow (Otto 235-m) and
venturinozzlesnearthepipeoutlettodrawairintothepipe below the outflow (Otto 470-m) of the treatment system
(e.g., Ackman and Kleinmann 1985), hence, supersatura- and,hence,usefultoindicatethe‘‘untreated’’and‘‘treated’’
tingtheinfluentwithatmosphericO beforedischargingto waterquality,respectively.Monitoringsitedescriptionsare
2
the first treatment cell. The aerated influent would be reportedin Table 1.
injected near the bottom of the oxidation pond to promote TheflowrateofMuddyBranchaboveandbelowtheOtto
the circulation of the water and Fe(OH) particles. The Dischargewasmeasuredbyusingawadingrodandpygmy
3
circulation of Fe(OH) particles could facilitate particle currentmeter(Rantzetal.1982a).Afterthetreatmentsystem
3
123
MineWaterEnviron(2007)26:128–149 133
Fig.2 Schematic plan of passive, aerobic treatment system con- stages;ababove,blbelow.Dataontheareaandvolumecapacityare
structedin2005attheOttoDischarge,Branchdale,Pa.Labelledpoint for the design dimensions, slopes, and surface-water elevations
symbols (e.g., Otto-235m) indicate water-quality sample sites; (ChristineHaldemanandClaytonBubeck,RettewAssociates,written
arrows indicate general flow direction through sequential treatment communication,2006)
wasinstalled,weirsattheoutletofeachtreatmentcellwere were assumed constant because only minor changes in
usedtoestimatetheflowratethroughoutthetreatmentsys- water depth were recorded at the weirs between cells
tem. The flow across the weir notch was estimated on the compared to the total depths of the cells.
basisofastandardequationforrectangularnotchgeometry At each sample site, temperature, pH, specific conduc-
(Rantzetal.1982b)adaptedformetricunits: tance (SC), dissolved oxygen (DO), and redox potential
Flowrate(cid:1)Ls(cid:1)1(cid:2)¼ð1:839(cid:2)widthÞ(cid:2)ðdepthÞ1:5 ð3Þ (Eh) were measured by using a multiparameter, sub-
mersible sonde. The sonde was calibrated daily in
where the width is 1.52 m and the water depth over the accordancewithstandardmethods(U.S.GeologicalSurvey
weir is in centimeters. To estimate detention time within 1997 to present). Field pH and Eh were determined by
each treatment cell, the volume of water contained within using a combination Pt and Ag/AgCl electrode with a pH
thecell(Fig. 2)wasdividedbytheflowrate.Thevolumes sensor. The electrode was calibrated in pH 4.0 and 7.0
Table1 Descriptionsofwater-qualitymonitoringsitesonMuddyBranchandtheOttoDischargetreatmentsystematBranchdale,Pa
USGSStationID Sitenameanddescription Longitudea Latitude Elevationbm
403958076191401 OttoMineTunnelatBranchdale (cid:1)76.32022 40.66620 250
0146784348 Otto235m‘‘Inflow’’ (cid:1)76.31939 40.66758 244
404001076191301 OttoTreatmentCell1‘‘OxidationPond’’ (cid:1)76.32033 40.66705 244
404006076191001 OttoTreatmentCell2‘‘Wetland1’’ (cid:1)76.31927 40.66803 244
404005076190901 OttoTreatmentCell3‘‘Wetland2’’ (cid:1)76.31956 40.66842 244
404008076190601 OttoTreatmentOxicLimestoneDrain (cid:1)76.31842 40.66875 244
0146784350 Otto470m‘‘Outflow’’ (cid:1)76.31828 40.66870 238
0146784338 MuddyBranch0.1kmaboveOtto (cid:1)76.31861 40.66889 244
0146784354 MuddyBranch1.0kmbelowOtto(PF101),SteinsMillRd (cid:1)76.30666 40.66750 232
0146784358 MuddyBranch1.8kmbelowOtto(PF102),BlackDiamondRd (cid:1)76.30278 40.66472 230
ApproximatesitelocationsshowninFig.2
a HorizontalcoordinateinformationisreferencedtotheNorthAmericanDatumof1983(NAD83)
b ElevationsreferencedtotheNationalGeodeticVerticalDatumof1929(NGVD29)
123
134 MineWaterEnviron(2007)26:128–149
buffer solutions and in ZoBell’s solution (Wood 1976, pp. described by Barbour et al. (1999) and Meador et al.
18–22; U.S. Geological Survey 1997 to present). Values (1993). Although benthic-macroinvertebrate samples also
for Eh were corrected to 25(cid:2)C relative to the standard were collected during the annual surveys, sample pro-
hydrogen electrode in accordance with methods of Nord- cessingandtaxonomicidentificationwerenotcompletedat
strom (1977). Unfiltered and filtered (0.45 lm pore size) the time of this report (2007).
samplesofwaterwereprocessedinthefield,transferredto
polyethylene bottles, preserved as appropriate, and trans-
ported on ice to the laboratory.
Laboratory Aeration Experiment
Thealkalinityand‘‘hotperoxide’’acidity(hotacidity)of
theunfilteredwatersamplesweretitratedusingsulfuricacid
AlaboratoryaerationexperimentwithAMDfromtheOtto
(H SO )and/orsodiumhydroxide(NaOH)tofixedendpoint
2 4 Discharge was conducted in December 2001 to determine
pH of 4.5 and 8.3, respectively (American Public Health
the potential rate ofironremoval andtoverify that thepH
Association 1998a, 1998b). Typically, alkalinities were
could be sustained near neutrality without supplemental
measured within 24 h of sampling, whereas acidities were
alkalinity.OnthemorningofDecember3,2001,rawwater
measured several days later at the USGS Water Science
and corresponding field measurements of water quality
CenterlaboratoryinNewCumberland,Pennsylvania.Con-
were collected from the discharge as it exited the Otto
centrationsofmajoranions(SO ,Cl)infiltered,unpreserved
4 Minetunnel.Atotalof30 Lofrawwaterwascollectedin
subsampleswereanalyzedbyionchromatography(IC),and
a66 Lcapacityplasticcoolerandimmediatelytransported
concentrationsofmajorcations(Ca,Mg,Na,K)andselected
to the author’s residence. An aquarium pump with porous
trace metals(Fe, Mn,Al,Ni,Zn)inunfilteredandfiltered,
stone diffuser set at the bottom of the cooler delivered a
acidifiedsubsampleswereanalyzedbyinductivelycoupled
continuousair-flowrateof1.5 L/min;therisingairbubbles
plasma optical emission spectrometry (ICP) at the Actlabs
promoted aeration and circulation of the water and sus-
laboratory in Toronto, Ontario, or the USGS Mineral
pended fine particles. The ambient temperature of the
Resources Laboratory (MRL) in Denver, Colorado (Crock
initial sample, 12.3(cid:2)C, was approximately maintained
etal.1999;FishmanandFriedman1989).
during the aeration experiment (10.5–13.7(cid:2)C). A multipa-
To supplement data on hot acidity, the net acidity was
rametersondewassubmergedatthebottomofthecoolerto
computed as mg/L of CaCO , considering positive acidity
3 record temperature, pH, SC, DO, Eh, and turbidity. Serial
contributions from pH and concentrations of dissolved
sampling was conducted to evaluate changes in alkalinity,
iron, manganese, and aluminum in mg/L (C , C , and
Fe Mn acidity, and concentrations of metals. At fixed time inter-
C , respectively), and negative contributions from alka-
Al vals,a500 mLpolyethylenebottlewasimmersedopposite
linity as mg/L of CaCO :
3 the aerator and its contents were processed for chemical
analysis. Alkalinity and ‘‘cold peroxide’’ acidity (cold
NetAcidity acidity)inunfilteredsamplesweremeasuredinUSGSNew
¼50(cid:3)10ð3(cid:1)pHÞþ2(cid:2)C =55:85þ2(cid:2)C =54:94þ3(cid:2)C =26:98(cid:4) Cumberlandoffice.Thecoldacidityincludescontributions
Fe Mn Al
from the dissolved CO plus dissolved iron and aluminum
2
(cid:1)Alkalinity ð4Þ butmayexcludecontributionsfrommanganese(Kirbyand
Cravotta 2005a, b); hot acidity was not measured because
Kirby and Cravotta (2005a, b) showed that net acidity
the computed net acidity (Eq. 4) was assumed to provide
computedwithEq.4iscomparableinvaluetothestandard
equivalent information. Concentrations of dissolved
methodhotaciditywheretheH SO addedtothesampleis
2 4 (0.22 lm filter) and total metals in acidified samples were
subtractedfromtheNaOH added(AmericanPublicHealth
measured by ICP at the U.S. Department of Energy labo-
Association 1998a).
ratory in Pittsburgh, Pennsylvania.
Annual aquatic ecological surveys were conducted in
October 2002, 2003, 2004, and 2005 on Muddy Branch at
one site immediately upstream and two sites downstream
from the confluence with the Otto Discharge. The two Geochemical Models of Iron-Oxidation Rates
downstream stations (PF 101 and PF 102 in Table 1) were
surveyed previously by the Pennsylvania Fish and Boat Variouskineticoxidationmodelsthatconsideredeffectsof
Commission (Chikotas and Kaufman 2002). Fish were detention time (travel time), pH, O , and Fe2+ concentra-
2
collected by electrofishing over a 150-m reach consisting tion were used to evaluate the data for the laboratory
of mixed riffle, run, and pool habitats at each stream site, aerationexperimentandtheOttotreatmentsystem.Stumm
held for measurement and identification, checked for and Lee (1961) proposed a widely cited rate law for the
anomalies, and then released in accordance with methods abiotic oxidation of Fe2+ under laboratory conditions:
123
MineWaterEnviron(2007)26:128–149 135
d(cid:5)Fe2þ(cid:6)=dt¼(cid:1)k(cid:5)Fe2þ(cid:6)½OH(cid:1)(cid:3)2PO ; ð5Þ thermodynamicdatabase usedbyWATEQ4Fwasused for
2
the PHREEQCsimulations.
where [Fe2+]denotes concentration (inmol/L)andkis the
rate constant, 1.33 · 1012L2/(mol2 atm s). Equation 5
indicates the abiotic oxidation of Fe2+ is strongly Results: Laboratory and Field Observations
dependent on pH; the rate increases by a factor of 100
witheachunitincreaseinpH.Incontrast,Williamsonetal. Laboratory Aeration Experiment
(1992) reported that the biological oxidation rate for Fe2+
in surface waters receiving AMD is independent of pH Initially, the Otto Discharge sampled for the aeration
overtherange2.5–6.0andfasterthantheabioticoxidation experiment was colorless and anoxic, with pH of 5.8 and
rate as indicated by: dissolved iron concentration of 16.4 mg/L. Aggressive
aeration promoted the rapid oxidation of dissolved Fe2+;
d(cid:5)Fe2þ(cid:6)=dt¼(cid:1)k(cid:5)Fe2þ(cid:6); ð6Þ freshly precipitated, suspended Fe(OH) particles tempo-
3
rarily changed the water to a deep orange-brown color. In
where the rate constant is 10(cid:1)3.61 s(cid:1)1. less than 24 h, the initial Fe2+ concentration of 16.4 mg/L
Forthisstudy,thetwooxidationmodelswereevaluated
decreasedtolessthan0.5 mg/L(Fig. 3a).Nevertheless,the
in a spreadsheet by solving the integrated form of Eqs. 5
concentrations of total iron declined slowly because of
and 6:
slow settling of Fe(OH) particles. The abundance of sus-
3
pended Fe(OH) particles was indicated by the turbidity,
½Fe(cid:3) ¼½Fe(cid:3) expf(cid:1)k0(cid:2)tg ð7Þ 3
t t¼0 which increased from zero to a maximum value of 230
wherek0 fortheintegratedformofEq.5includesspecified nephelometric turbidity units (NTU) after 12 h of aeration
constant values of pH and PO . For the specified initial and then gradually declined as the Fe(OH) settled from
2 3
concentration of Fe2+, the spreadsheet models computed suspension(Fig. 3a).After4.5 daysofcontinuousaeration,
the concentration of residual Fe2+ as a function of elapsed most Fe(OH) particles had settled to the bottom of the
3
time,t.Thespreadsheetmodels,equivalenttothemodeling cooler and the supernatant was again colorless.
approach used in the ‘‘AMDTreat’’ model for mine-drain- Once aggressive aeration began (1 h after collection),
age treatment (U.S. Office of Surface Mining Reclamation theDOincreasedtosaturation(Fig. 3c).Althoughthecold
andEnforcement2002),didnotconsidereffectsontherate temperature of the discharge water was maintained during
constant owing to variations in the iron concentration, pH, the aeration experiment (10.5–13.7(cid:2)C), aeration promoted
or temperature as oxidation progresses. evaporation and the exsolution of dissolved CO . The SC,
2
To evaluate interactions among the dissolved iron con- which indicates dissolved solute concentrations, declined
centration and oxidation state, pH, alkalinity, and other during the first 12 h of aeration due to the precipitation of
variables,thetwokineticmodelsofFe2+oxidation(Eqs.5, Fe(OH) and the conversion of HCO(cid:1) to CO :
3 3 2
6) also were used in the geochemical computer program
PHREEQC (Parkhurst and Appelo 1999). This model, Fe2þþ0:25O þ2HCO(cid:1)þ0:5H O¼FeðOHÞ þ2CO ðgÞ
2 3 2 3 2
given in Appendix A (in Electronic supplementary
ð8Þ
material), was adapted from ‘‘Example 9—Kinetically
controlled oxidation of ferrous iron’’ provided by the but thereafter increased in accordance with the sulfate
PHREEQC authors. Initial models for this study assumed concentrations (Fig. 3b). Because of evaporation, concen-
constant, equilibrium values of PO and/or PCO and lim- trations of sulfate increased by about 5% during the
2 2
ited concentrations of dissolved Fe3+ to equilibrium with experiment.
amorphous Fe(OH) . Subsequent models simulated atmo- Despitethenearlycompleteoxidationandhydrolysisof
3
sphericdisequilibriumwithrespecttoCO byincorporating iron, net-alkaline character was maintained during the
2
atermfortheobservedasymptoticdecreaseinPCO during aeration experiment. Initially, the Otto Discharge had pH
2
the aeration experiment. The geochemical program of5.8,coldacidityof122 mg/L,alkalinityof70 mg/L,and
WATEQ4Fversion2.63(BallandNordstrom1991),which net acidity (Eq. 4) of (cid:1)35.9 mg/L as CaCO3 (Fig. 3d, e).
isusefultocomputeequilibriumspeciationforlargesample Onceaerationbegan,thepHincreasedandthecoldacidity
sets, was used to compute the PCO2, PO2, and saturation decreasedowingtotherapidexsolutionofCO2(Eqs.2,8).
index (SI) values for selected minerals. The PCO was Within 12 h, the pH increased to values of 7.0 or more
2
computed on the basis of measured pH, alkalinity, and (Fig. 3d), ultimately attaining a steady-state value of 7.5.
temperature. The activities of ferrous and ferric species Protonaciditygeneratedwiththehydrolysisofiron(Eq.1)
werecomputedonthebasisofthemeasureddissolvediron, was neutralized by the initial excess alkalinity (Eq. 8),
Eh, andtemperature offreshsamples.For consistency, the causing the alkalinity to decline to a steady-state
123
136 MineWaterEnviron(2007)26:128–149
Fig.3 Time-series plots of the quality of water during laboratory acidity,andnetacidity;fmeasuredandestimatedpartialpressureof
aerationexperimentontheOttoDischargeconductedDecember3–7, carbon dioxide (PCO ) computed with Eq. 12; g, manganese;
2
2001. a Iron and turbidity; b sulfate and specific conductance; h aluminum; i nickel; j zinc. Values below detection limits were
cdissolvedoxygenandtemperature;dpHandEh;ealkalinity,cold plottedasnegative(belowaxis)
concentrationofapproximately30 mg/LasCaCO andthe (Fig. 3f). The inverse correlation between pH and PCO
3 2
net acidity to increase to approximately (cid:1)25 mg/L as indicates the potential for PCO to determine pH and,
2
CaCO (Fig. 3e). hence, Fe2+-oxidation rates. To compute PCO as a func-
3 2
The rapid, asymptotic increase in pH coincided with an tion of elapsed time for the aeration experiment, the
asymptotic decrease in the PCO from an initial PCO empirical log(PCO ) data were fit with an exponential
2 2 2
value of 10(cid:1)1.1 atm to a steady-state value of 10(cid:1)3.2 atm equation of the form:
123