arsenic redistribution between sediments and water near a highly contaminated source

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Arsenic Redistribution between Sediments and Water near a Highly Contaminated Source ALISON R. KEIMOWITZ,* , ,‡ YAN ZHENG, STEVEN N. CHILLRUD, BRIAN MAILLOUX, , | HUN BOK JUNG, § MARTIN STUTE, , | AND H. JAMES SIMPSON ,‡ Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, New York 10964, Department of Earth and Environmental Sciences, Columbia University, New York, New York 11367, School of Earth and Environmental Sciences, Queens College, Flushing, New York 11367, and Department of Environmental Science, Barnard College, Columbia University, New York, New York 10027 Mechanisms controlling arsenic partitioning between sediment, groundwater, porewaters, and surface waters were investigated at the Vineland Chemical Company Superfund site in southern New Jersey. Extensive inorganic and organic arsenic contamination at this site (historical total arsenic >10 000 μgL -1 or >130 μM in groundwater) has spread downstream to the Blackwater Branch, Maurice River, and Union Lake. Stream discharge was measured in the Blackwater Branch, and water samples and sediment cores were obtained from both the stream and the lake. Porewaters and sediments were analyzed for arsenic speciation as well as total arsenic, iron, manganese, and sulfur, and they indicate that geochemical processes controlling mobility of arsenic were different in these two locations. Arsenic partitioning in the Blackwater Branch was consistent with arsenic primarily being controlled by sulfur, whereas in Union Lake, the data were consistent with arsenic being controlled largely by iron. Stream discharge and arsenic concentrations indicate that despite large-scale groundwater extraction and treatment, >99% of arsenic transport away from the site results from continued discharge of high arsenic groundwater to the stream, rather than remobilization of arsenic in stream sediments. Changing redox conditions would be expected to change arsenic retention on sediments. In sulfur-controlled stream sediments, more oxic conditions could oxidize arsenic- bearing sulfide minerals, thereby releasing arsenic to porewaters and streamwaters; in iron-controlled lake sediments, more reducing conditions could release arsenic from sediments via reductive dissolution of arsenic- bearing iron oxides. Introduction Arsenic is a toxic metalloid occurring naturally in soils and sediments throughout the world (1). Its mobility is controlled largely by pH and redox changes, and can be present at high concentrations in both natural and contaminated environ- ments under reduced, circumneutral conditions as well as oxidized and reduced alkaline environments such as Mono Lake (2). Arsenic has been widely used in agricultural and manufacturing applications (3) and has thus become a common anthropogenic pollutant. In the subsurface, arsenic mobility is strongly influenced by redox conditions. Release of arsenic from sediments into associated waters has been attributed to reductive dissolution of iron oxyhydroxides to which arsenic had been sorbed (2, 4), although arsenic and iron solubility have been shown to be decoupled in some environments (5-7), indicating that this mechanism is not solely responsible for high arsenic concentrations in reducing groundwaters. Sulfur species have also been shown to influence dissolved arsenic concentra- tions, particularly under strongly reducing conditions (8) where sulfide can complex arsenic and thereby increase arsenic solubility (9, 10). Sulfide can also remove arsenic from solution by precipitation of sulfide mineral phases (11) or sorption to iron sulfide minerals (12). O’Day et al. (13) recently proposed a conceptual model for arsenic mobility in reducing shallow sediments, which characterizes the behavior of arsenic as either sulfur- controlled, as observed in this study in the Blackwater Branch (BWB), or iron-controlled, as observed in Union Lake (UL). This published model (13) indicates that under high-iron conditions, aqueous sulfide is rapidly consumed in the formation of iron sulfide minerals (14, 15), while under low- iron conditions, aqueous sulfide is available to complex aqueous arsenic and to form solid arsenic-sulfide minerals. The control of arsenic mobility by either iron or sulfur is determined by the iron/sulfur ratio. This work expands upon this conceptual model. Under low-iron, sulfur-controlled conditions, removal of arsenic from solution occurs primarily through formation of solid arsenic sulfides (16), with aqueous thioarsenites as potential intermediates (10). Under high-iron, iron-controlled condi- tions, the consumption of sulfide by available iron prevents formation of aqueous thioarsenites (17). Arsenic oxyanions sorb to the iron sulfides that have formed (18), and further pyritization may subsequently occur (19-21) wherein strong As-Fe bonds form (12, 22). The Blackwater Branch and Union Lake (both contaminated by the Vineland Chemical Com- pany) are classified within this system, as are other environ- ments (4, 8). Little is currently known about the geochemical behavior of thioarsenites under natural conditions, although the implied presence of these compounds in both aqueous and sedimentary phases in BWB sediments indicates a need for a better understanding of their behavior. Site Overview The Vineland Chemical Company manufactured arsenical biocides in Vineland, NJ from 1950 to 1994. During this time arsenic salts were stored improperly, thereby introducing large-scale arsenic contamination to soils and groundwater. A groundwater extraction and treatment plant operating since 2000 removes 6000-7500 m 3 day -1 of highly contaminated groundwater and subsequently discharge treated effluent to the BWB, a small freshwater stream that runs along one side of the site (Figure 1); there are no tributaries to the BWB between the Vineland Chemical site and the Maurice River. Since manufacture of arsenicals commenced, the BWB has delivered arsenic downstream into the Maurice River, Union Lake, and Delaware Bay (23). The associated sediments in these surface water bodies are now noteworthy reservoirs of * Corresponding author phone: (854)365-8793; e-mail: [email protected]. Lamont-Doherty Earth Observatory. Department of Earth and Environmental Sciences, Columbia University. § School of Earth and Environmental Sciences, Queens College. | Department of Environmental Science, Barnard College. Environ. Sci. Technol. 2005, 39, 8606-8613 8606 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 22, 2005 10.1021/es050727t CCC: $30.25 2005 American Chemical Society Published on Web 10/12/2005

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Arsenic Redistribution betweenSediments and Water near a HighlyContaminated SourceA L I S O N R . K E I M O W I T Z , * , † , ‡

Y A N Z H E N G , † , § S T E V E N N . C H I L L R U D , †

B R I A N M A I L L O U X , † , | H U N B O K J U N G , §

M A R T I N S T U T E , † , | A N DH . J A M E S S I M P S O N † , ‡

Lamont-Doherty Earth Observatory of Columbia University,61 Route 9W, Palisades, New York 10964, Department ofEarth and Environmental Sciences, Columbia University, NewYork, New York 11367, School of Earth and EnvironmentalSciences, Queens College, Flushing, New York 11367, andDepartment of Environmental Science, Barnard College,Columbia University, New York, New York 10027

Mechanisms controlling arsenic partitioning betweensediment, groundwater, porewaters, and surface waterswere investigated at the Vineland Chemical CompanySuperfund site in southern New Jersey. Extensive inorganicand organic arsenic contamination at this site (historicaltotal arsenic >10 000 µg L-1 or >130 µM in groundwater)has spread downstream to the Blackwater Branch,Maurice River, and Union Lake. Stream discharge wasmeasured in the Blackwater Branch, and water samplesand sediment cores were obtained from both the stream andthe lake. Porewaters and sediments were analyzed forarsenic speciation as well as total arsenic, iron, manganese,and sulfur, and they indicate that geochemical processescontrolling mobility of arsenic were different in thesetwo locations. Arsenic partitioning in the Blackwater Branchwas consistent with arsenic primarily being controlledby sulfur, whereas in Union Lake, the data were consistentwith arsenic being controlled largely by iron. Streamdischarge and arsenic concentrations indicate that despitelarge-scale groundwater extraction and treatment, >99%of arsenic transport away from the site results from continueddischarge of high arsenic groundwater to the stream,rather than remobilization of arsenic in stream sediments.Changing redox conditions would be expected to changearsenic retention on sediments. In sulfur-controlled streamsediments, more oxic conditions could oxidize arsenic-bearing sulfide minerals, thereby releasing arsenic toporewaters and streamwaters; in iron-controlled lakesediments, more reducing conditions could release arsenicfrom sediments via reductive dissolution of arsenic-bearing iron oxides.

IntroductionArsenic is a toxic metalloid occurring naturally in soils andsediments throughout the world (1). Its mobility is controlled

largely by pH and redox changes, and can be present at highconcentrations in both natural and contaminated environ-ments under reduced, circumneutral conditions as well asoxidized and reduced alkaline environments such as MonoLake (2). Arsenic has been widely used in agricultural andmanufacturing applications (3) and has thus become acommon anthropogenic pollutant.

In the subsurface, arsenic mobility is strongly influencedby redox conditions. Release of arsenic from sediments intoassociated waters has been attributed to reductive dissolutionof iron oxyhydroxides to which arsenic had been sorbed (2,4), although arsenic and iron solubility have been shown tobe decoupled in some environments (5-7), indicating thatthis mechanism is not solely responsible for high arsenicconcentrations in reducing groundwaters. Sulfur species havealso been shown to influence dissolved arsenic concentra-tions, particularly under strongly reducing conditions (8)where sulfide can complex arsenic and thereby increasearsenic solubility (9, 10). Sulfide can also remove arsenicfrom solution by precipitation of sulfide mineral phases (11)or sorption to iron sulfide minerals (12).

O’Day et al. (13) recently proposed a conceptual modelfor arsenic mobility in reducing shallow sediments, whichcharacterizes the behavior of arsenic as either sulfur-controlled, as observed in this study in the Blackwater Branch(BWB), or iron-controlled, as observed in Union Lake (UL).This published model (13) indicates that under high-ironconditions, aqueous sulfide is rapidly consumed in theformation of iron sulfide minerals (14, 15), while under low-iron conditions, aqueous sulfide is available to complexaqueous arsenic and to form solid arsenic-sulfide minerals.The control of arsenic mobility by either iron or sulfur isdetermined by the iron/sulfur ratio.

This work expands upon this conceptual model. Underlow-iron, sulfur-controlled conditions, removal of arsenicfrom solution occurs primarily through formation of solidarsenic sulfides (16), with aqueous thioarsenites as potentialintermediates (10). Under high-iron, iron-controlled condi-tions, the consumption of sulfide by available iron preventsformation of aqueous thioarsenites (17). Arsenic oxyanionssorb to the iron sulfides that have formed (18), and furtherpyritization may subsequently occur (19-21) wherein strongAs-Fe bonds form (12, 22). The Blackwater Branch and UnionLake (both contaminated by the Vineland Chemical Com-pany) are classified within this system, as are other environ-ments (4, 8). Little is currently known about the geochemicalbehavior of thioarsenites under natural conditions, althoughthe implied presence of these compounds in both aqueousand sedimentary phases in BWB sediments indicates a needfor a better understanding of their behavior.

Site OverviewThe Vineland Chemical Company manufactured arsenicalbiocides in Vineland, NJ from 1950 to 1994. During this timearsenic salts were stored improperly, thereby introducinglarge-scale arsenic contamination to soils and groundwater.A groundwater extraction and treatment plant operating since2000 removes 6000-7500 m3 day-1 of highly contaminatedgroundwater and subsequently discharge treated effluent tothe BWB, a small freshwater stream that runs along one sideof the site (Figure 1); there are no tributaries to the BWBbetween the Vineland Chemical site and the Maurice River.Since manufacture of arsenicals commenced, the BWB hasdelivered arsenic downstream into the Maurice River, UnionLake, and Delaware Bay (23). The associated sediments inthese surface water bodies are now noteworthy reservoirs of

* Corresponding author phone: (854)365-8793; e-mail:[email protected].

† Lamont-Doherty Earth Observatory.‡ Department of Earth and Environmental Sciences, Columbia

University.§ School of Earth and Environmental Sciences, Queens College.| Department of Environmental Science, Barnard College.

Environ. Sci. Technol. 2005, 39, 8606-8613

8606 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 22, 2005 10.1021/es050727t CCC: $30.25 2005 American Chemical SocietyPublished on Web 10/12/2005

contaminant arsenic, providing potential for human exposureand ecological damage. Projected total costs for remediationof the highly contaminated Vineland Chemical site are ∼$100million over three decades, making it one of the mostexpensive interventions at a NPL site (24).

MethodsField Methods. Stream discharges were measured on July11, 2004, at four locations along the BWB (Figure 1) using aUSGS-type AA current meter. Two cross-channel transectswere taken per location. For each transect, velocity mea-surements were made twice at points spaced 0.5-1 m apartacross the stream at 0.6 times the local depth. At these andother locations, surface waters were passed through 0.45 µmfilters, acidified to 1% with Optima grade nitric acid, andreturned to the laboratory for high resolution inductivelycoupled plasma mass spectrometry (ICPMS) analysis. Allconcentrations described herein as dissolved were passedthrough 0.45 µm filters, although this fraction may containcolloids.

On July 11, 2004, two hand piston cores were collectedfrom fine-grained bottom sediments of the BWB adjacent toa swampy area (Figure 1). These fine-grained, highly reducedsediments represent one of two major types of sedimentsfound on the BWB bottom; the other type is primarily sand.The proportions of these two types are not known. On July12, 2004, three push cores were collected in one of the deepestparts of UL; this ∼5 m deep area was located via bathymetricmap and depth finder. The ∼30 cm cores collected reached

a gravel layer underlying this man-made lake (Figure 1). Theentire UL water column was oxic (dissolved oxygen >7 mgL-1) at the time of core collection. One core from each locationwas sectioned within 1 h of collection into 2 cm slices (thedepth of which is reported as the central depth) under anitrogen atmosphere. Each section was centrifuged for 20min at 5000 rpm, separating porewater and sediment.Centrifuge tubes were opened in a nitrogen atmosphere, andporewaters were filtered with 0.45 µm filters. Residualsediments were sealed and placed on dry ice, and a subsampleof the porewaters was immediately analyzed by differentialpulse cathodic stripping voltammetry (DPCSV) for As(III)and total inorganic As (25). Residual porewaters were acidifiedto 1% with Optima grade nitric acid and returned to thelaboratory for ICPMS analysis.

Laboratory Methods. Sediments were frozen until labo-ratory analysis. Selected samples were thawed in a nitrogenatmosphere, and aliquots were separated for gamma count-ing, sequential extraction, and X-ray absorption near edgespectroscopy (XANES). The gamma counting aliquot wasdried, ground to a fine powder, and sealed in airtight, 100-mL aluminum cans or 4-mL plastic vials for radionuclideanalysis. Measurement of 137Cs and other gamma-emittingradionuclides were made using either an intrinsic germaniumor a lithium-drifted germanium detector; sediment coresobtained immediately adjacent to cores collected for pore-water analyses were also gamma counted. Gamma countingdata were used to elucidate sedimentation rate.

FIGURE 1. Map of the Vineland Chemical Co. site, showing the location of Vineland within New Jersey, the general region includingcoring locations marked with stars, and the NPL site with surface water sampling locations numbered. Bar charts show BWB dischargeand arsenic concentrations with water sampling locations marked. The EPA standard shown is the 0.13 µmol L-1 (10 µg L-1) standardeffective in 2006. Flow in the Maurice River (MR) was obtained from the USGS gauging station in nearby Norma, New Jersey.

VOL. 39, NO. 22, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 8607

The XANES aliquot (consisting of damp solids) remainedfrozen under nitrogen until analysis at the National Syn-chrotron Light Source (Brookhaven National Laboratory,Upton, NY) on beamline X-11A with an unfocused spot sizeof approximately 10 mm H by 0.5 mm V and a Si(111) crystal.Standard spectra were collected in transmission mode withan ionization chamber, and sample spectra were collectedin fluorescence mode with a Lytle detector after a Ge filter.Samples were wet packed into custom-made polycarbonatesample holders and then sealed with Kapton tape. Next, 6-10scans were averaged for each sample. A sodium arsenatestandard in Kapton tape was placed between the 2 and 3 in.ionization chamber to account for any drift during each run.Normalized and derivative XANES spectra were fit with alinear combination of the following sodium salt standards:arsenate, arsenite, monomethyl arsenate (MMA), and di-methyl arsenate (DMA) as well as arsenopyrite and thio-arsenites. The thioarsenite category included both realgar(AsS or As4S4) and orpiment (As2S3) standards; data were notsufficient to distinguish between these minerals. In thesamples, peaks attributed to thioarsenites could be due tothese minerals, sorption of aqueous thioarsenites (10), oramorphous arsenic sulfides, so these species will be referredto here as thioarsenites; these species all show coordinationbetween As-S, not between As-Fe (12). Optimal fits weredetermined by minimizing the square of the error betweenthe observed and calculated spectra between 11.85 and 11.96keV. Because of the relatively low concentrations of MMAand DMA in the samples, the data for these species arecombined.

For the sequential extraction, 0.2-0.3 g wet sediment,equivalent to 0.02 g-0.07 g dry sediment, was weighed intoamber vials under nitrogen. A solution 1 M in sodiumphosphate, 0.1 M in ascorbic acid, and adjusted with sodiumhydroxide to pH 5 was degassed with nitrogen; 10 mL wasadded to each sediment sample. The vials were sealed withgastight septa and mixed for 24 h on a wrist-action shaker.The samples were then centrifuged for 10 min at 5000 rpm,and aqueous phases were analyzed for As(III) by DPCSV.Residual extract was retained for ICPMS analysis (26). Thesediments were washed with 15 mL of deionized water,centrifuged, and the rinse was discarded, possibly leading toa small but systematic underestimation of total elementalconcentrations in sediments. Fifteen milliliters of 1.2 MOptima grade HCl was then added to each vial, and thesamples were shaken for 1 h and centrifuged. The HCl extractswere retained for ICPMS analysis (27). Finally, previouslyleached sediments were digested completely using nitric acidfollowed by perchloric and hydrofluoric acids (28). Somevolatilization of arsenic could have occurred during this step(29), which would lead to systematic underestimation ofarsenic in these sediments, but this is unlikely because thesediments were oxidized by this point in the digestionprocedure. These latter digests were refluxed repeatedly ona hotplate until no visible sediment remained, dried down,and then diluted for ICPMS analysis. A laboratory blankshowed all elements below the analytical limits of detection,two sets of duplicate digests gave results consistent within10%, and duplicate analyses of one digest gave variability<5%.

Elemental Analyses. Differential pulse cathodic strippingvoltammetry (DPCSV) was used to analyze arsenic speciation;As(III) and As(III+V) were quantified in porewaters, and As-(III) was quantified in the phosphate sediment extractionstep. An Eco-Chemie µAutolab voltammetric apparatus(Brinkmann Instruments, Westbury, VT) equipped with aHMDE working electrode, a Pt auxiliary electrode, and anAg/AgCl/3 M KCl double-junction reference electrode wasused. As(III) is selectively deposited on the electrode, and toanalyze As(III+V), dissolved arsenic was reduced with 1 M

L-cysteine in batch mode. Samples were quantified bystandard addition, and relative standard deviations for allsamples (N g 2) were <5% (25); the recovery for spikedphosphate extracts (N ) 2) was 98%.

Inductively coupled plasma mass spectrometry (ICPMS)with a high-resolution Axiom Single Collector instrument(Thermo Elemental, Germany) was used to quantify sulfur,manganese, iron, and arsenic in streamwaters, porewaters,and all sequential sediment extraction steps. 115In was addedto each sample as an internal response standard, and 115In,75As, 57Fe, 55Mn, and 34S were analyzed with the instrumentset to ∼12 000 resolving power, sufficient to clearly separateAr-Cl from 75As. Peak areas were corrected for sensitivitydrift with 115In before quantification. Three point internaland external calibration curves were run every ∼20 samples,and each sample and standard was run in triplicate. Runswere accepted only if R values of internal curves were >0.97and standard deviations between triplicates were <10%(typically <5%). Uncertainties reported for data in profilesof dissolved species represent the standard deviation ofmultiple (N ) 3-15) ICPMS runs of the same samples.

ResultsStream Data. The locations where stream discharge wasmeasured and/or stream samples were obtained are shownin Figure 1; from the upstream to the downstream location,total dissolved arsenic concentrations increased from 22 (4 to 272 ( 36 nM (errors reported were propagated fromanalytical uncertainty, analytical duplicates, and samplingduplicates) and stream discharge increased from 207 ( 5.5to 950 ( 25 L s-1 (N ) 4). For comparison, discharge in theMaurice River measured at the nearest USGS gauging stationin Norma, NJ, was 2464 L s-1 (30). Because there are notributaries to the BWB stream between the most upstreamand downstream locations, the increase in flow is assumedto be from groundwater discharge.

Sediment Core Data. All cores obtained in both locationswere dark brown to black, loosely packed, and fine grained(silt and clay sized particles) except for one core obtained inthe BWB, which contained medium sands. Analytical dif-ficulties with extracting porewaters from this core preventeddetailed analysis, but the general profile of arsenic is similarto the BWB core described in detail herein. No laminationsor other vertical structures were visually evident.

Figure 2 shows porewater profiles, sequential sedimentextraction data, and XANES data from the BWB and UL cores.137Cs data are consistent with a mean sediment accumulationrate in UL of ∼2.5 mm year-1; in the BWB, most cores obtainedindicate some vertical mixing (well-mixed or distorted bomb137Cs peaks), with a mean sediment accumulation rate of >6mm year-1. In neither location does the maximum insediment arsenic concentrations correspond with the yearsof highest arsenic production in the Vineland Chemical plant(∼1960-1977) (23, 31), suggesting that sediment-boundarsenic profiles are controlled by redox conditions. Severalsections of the BWB sediment core have measurable sedi-ment-bound MMA and DMA, which may correspond toperiods of release of these species and sorption to sedimentminerals (32). A porewater arsenic profile from UL in 1993is quite similar to the one shown here (31). Note that at thissite, arsenic contamination is extreme: porewater arsenicconcentrations are 1.2-165 times the EPA drinking waterstandard of 10 µg L-1 (0.13 µM), and sediments contain 7-135times typical arsenic levels for uncontaminated fine-grainedseiment (∼10 mg kg-1 or 0.13 mmol kg-1) (2).

Relationships between Data Sets. Porewater arsenic wasanalyzed by both DPCSV and ICPMS, and the total arsenicvalues were identical within the uncertainties of the mea-surements. Concentrations of As(III) and As(III+V) measuredby DPCSV were identical within the uncertainties of the

8608 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 22, 2005

measurements except above 11 cm in the UL core, wheresignificant As(V) was present. Speciation of phosphate-extractable arsenic by DPCSV indicates that ∼100% of arsenicis present as As(III) except in the following three zones: above5 cm in the BWB core, at 31 cm in the BWB core, and above9 cm in the UL core.

Speciation of sediment-bound arsenic was done byXANES, by DPCSV speciation of phosphate-extractablearsenic, and by sequential extraction, each step of whichputatively attacks selected arsenic phases. These three datasets agree well; the only data quality problems observed wereat 21 and 29 cm depth in the UL core. These samples yieldedmore phosphate-extractable As(III) by DPCSV than totalphosphate-extractable arsenic by ICPMS. This could reflectmeasurements near the ICPMS detection limit (∼5 µg L-1 inthe original sample or ∼0.5 µg L-1 in samples after dilutionfor analysis) or the fact that the ICPMS measurements weremade >1 week after extraction, when they may be lessaccurate due to precipitation of arsenic solids (33). XANESdata indicate that reduced arsenic, either as arsenite or asthioarsenites, represents >70% of arsenic in all samplesexamined. Although the XANES and phosphate extractionexamine somewhat different reservoirs of arsenic, bothindicate predominantly reduced arsenic in the sediment(Figure 2).

DiscussionBlackwater Branch Core. This core can be subdividedqualitatively into three zones: suboxic from 0 to 3 cm, anoxicfrom 3 to 11 cm, and sulfidic from 11 to 31 cm. Neither Ehnor dissolved oxygen (DO) data are available to confirm these

classifications; however, these divisions are descriptive ofand supported by the geochemical conditions observed.

In the suboxic zone, the geochemistry is probablycontrolled by diffusion of DO from the oxygen-saturatedstreamwater and by sediment deposition on the streambottom. The relatively oxic nature of this zone is supportedby the relatively large percentage of sediment iron extractedby HCl, containing primarily iron oxides (27), although noorange-colored sediments were observed. The low dissolvedarsenic concentrations in this depth interval are consistentwith effective sorption onto these oxides (34) (Figure 2).

Sediment deposition probably provides a large portion ofsediment sulfur, manganese, iron, and arsenic to thisuppermost zone of the core. Particle deposition could accountfor local maxima of sediment sulfur and arsenic at 3 cm ifupstream, reduced, sulfidic sediments were resuspended andsubsequently redeposited at this location more rapidly thanoxidation could occur. The local maximum of porewatersulfur at 1 cm could be explained by oxidation of redepositedsulfidic sediments. The maxima of iron and manganese inthe sediment at 3 cm are probably due to a combination ofparticle deposition and redox trapping (i.e., capture of redox-sensitive metals at a redox boundary in the sediment) ofdiffusive porewater fluxes of these elements from the anoxiczone below.

In the anoxic zone from 3 to 11 cm, geochemicalconditions appear to reflect a reducing but not stronglysulfidic environment. There are broad maxima of dissolvediron and manganese consistent with these elements beingin primarily soluble (+2) oxidation states. Dissolved man-

FIGURE 2. Depth profiles from the core taken in the Blackwater Branch (top row) and Union Lake (bottom row); error bars represent 1standard deviation (N ) 2-4 replicates). All arsenic values are total arsenic measured by ICPMS unless otherwise noted. In the XANESAs panel, “thioarsenites” is used as a blanket term, which includes any amorphous or crystalline compound with bonds between reducedsulfur and arsenic. Note that the manganese, iron, sediment arsenic, and XANES arsenic plots have different x-axis scales for the BWBand UL. The HClO4/HF digest from the 3 cm depth in the BWB core was lost.

VOL. 39, NO. 22, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 8609

ganese and iron concentrations gradually decrease below∼7 cm.

In the sulfidic zone below 11 cm, sulfur concentrationsand speciation dominate the geochemical conditions. Thesource of sulfur at these depths may be groundwater influx:the hydraulic gradient between groundwater and streamwaterin this section of the BWB indicates potential for groundwaterdischarge. Additionally, total sulfur concentrations in thetwo deepest porewater samples, 105 ( 4 µM, are in relativelygood agreement with total sulfur concentrations in ground-water from four nearby wells, 168 ( 85 µM; these concentra-tions are sufficient to support significant sulfate reduction.At 31 cm, the largest fraction of arsenate was observed in theporewaters and on the sediments, consistent with an influxof suboxic or oxic groundwaters. However, the high fractionsof As(III) in the porewaters and of thioarsenites detected byXANES in the sediments above 31 cm are consistent withreducing conditions in this zone of the core. The decreasein porewater sulfur concentrations between 31 and 11 cm isconsistent with reduction of incoming groundwater sulfateto sulfide in this depth interval and precipitation of somefraction of this sulfide (14).

Union Lake Core. The Union Lake core can be subdividedinto two depth intervals: the suboxic zone from 0 to 3 cmand the anoxic zone from 3 to 29 cm. In the suboxic zone,oxygen should diffuse into the core from the overlyingoxygenated lake water, although no orange iron-oxide typesediments were observed. This is consistent with lowerdissolved iron and manganese concentrations observed atdepths less than 5 cm and with sediment maxima for theseelements at 3 cm caused by trapping at a redox boundary,especially in the HCl extractable iron fraction, that is,amorphous iron oxides (27). A major fraction (>35%) ofsediment arsenic in this zone is arsenate, also consistentwith relatively oxic conditions in this zone of the core. Sulfur(probably in the form of sulfate) appears to be diffusing intothe core from the lake water above; in uppermost porewatersand lake bottom waters concentrations of total S measuredby ICPMS are similar (77 and 87 µM, respectively) (31).

The geochemical environment below ∼3 cm in the ULcore indicates anoxic conditions. In this anoxic zone,dissolved total sulfur concentrations decrease downward andsediment sulfur concentrations increase, consistent withdiffusion of sulfate from above, reduction of sulfate to sulfide,and subsequent precipitation of sulfide minerals in deepersediments (22). In this anoxic depth zone, dissolved ironconcentrations are constant and high (∼300 µM), consistentwith strongly reducing conditions and insufficient sulfide toremove a large fraction of iron via iron sulfide precipitation.Amorphous FeS, a relevant phase under low-temperaturereducing conditions (14), is very undersaturated when thesaturation index is calculated assuming all dissolved sulfuris sulfide and using the most recent data of Benning et al.for the Ksp (35).

Mechanisms Controlling Arsenic Mobility. Arsenic mo-bility in the BWB and UL was expected to be either iron-controlled or sulfur-controlled (13). In the BWB, Fe/S ratioswere low in sediments and porewaters implying sulfur-controlled arsenic mobility; in UL, Fe/S ratios were highimplying iron-controlled arsenic mobility (Figure 3). To morequantitatively examine controls on arsenic mobility, pore-water arsenic data from each sediment core were examinedfor covariance with porewater manganese, iron, and sulfur;with these elements and with arsenic from each step of thesediment extractions; and with linear combinations of twoof these variables.

In the Blackwater Branch sediment core, the covariance(Table 1) between porewater and arsenic phosphate-extract-able As(III) (but not total phosphate-extractable arsenic, R) 0.43, p > 0.05) implies that this is a primary reservoir of

sediment-water exchange. The correlation of porewaterarsenic with both porewater and sediment sulfur supportedthe designation of the geochemical conditions within thiscore as sulfur-controlled (Table 1). Correlations betweenporewater arsenic and linear combinations of two variables,as shown below for the UL core, were all weaker (R < 0.4)than these univariate correlations. Below 3 cm in the BWBcore, that is, if the porewater sulfur peak at 1 cm is ignored,the R value between porewater arsenic and sulfur increasesto 0.84 (p < 10-4). The UL core, by comparison, showed aninverse correlation between dissolved arsenic and sulfur(Table 1) consistent with dissolved sulfur and arsenicchemistry in this core being controlled by different factors.The positive correlations between dissolved arsenic anddissolved and phosphate-extractable iron in the UL core areconsistent with iron-controlled arsenic mobility in this core.

Sulfur-controlled arsenic mobility in the BWB core andiron-controlled arsenic mobility in the UL core can also beexplained by examining geochemical conditions in each. Inthe BWB sediment core, arsenic concentrations are consis-tently high (>10 µM) only in the depth interval below 11 cm,where total dissolved sulfur concentrations are elevated, andmay support sulfate reduction. At sulfide concentrationsabove 50 µM, thioarsenite species dominate, and the totalsolubility of arsenic is increased (10). Both manganese and

FIGURE 3. Comparison of Fe/S ratios from the Blackwater Branch,the Clark Fork River, Montana (8), Union Lake, and Balmer Lake,Ontario (4). Sediment data are displayed in the upper panel andporewater data in the lower panel; only data from below the depthof oxygen penetration are shown. The BWB and the Clark ForkRiver are sulfur-controlled environments, and UL and Balmer Lakeare iron-controlled environments. The horizontal line across eachbox represents the median value, the upper and lower limits ofeach box represent the 25th and 75th percentiles, error bars represent10th and 90th percentiles, and outlier points are shown.

TABLE 1. Significant (p < 0.01) Correlations betweenPorewater Arsenic and Other Variables

Correlations with BWB [As]aq Correlations with UL [As]aq

[As(III)]PO4 R ) 0.67 [S]aq R ) -0.65[S]aq R ) 0.70 [Fe]aq R ) 0.67[∑S]sed R ) 0.69 [Fe]PO4 R ) 0.64

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iron are elevated in porewaters well above the depth at whicharsenic concentrations are elevated; this suggests that thedecrease in arsenic concentrations at above 11 cm isassociated with the decrease in sulfur concentrations at thissame depth, and not with redox conditions that controlconcentrations of iron and manganese. The correlation ofporewater arsenic and sediment sulfur (Table 1) implies someexchange between dissolved arsenic and sulfur-bearingsediment phases, most likely sorbed or amorphous thio-arsenites. Large fractions of solid thioarsenites observed byXANES (Figure 2) support this conclusion and are consistentwith previous predictions (13).

If arsenic mobility in the UL core is iron-controlled, asindicated by the data examined thus far, then arsenicoxyanions would be expected to be sorbed to minerals (22).Phosphate and HCl extractable iron are plausible phases towhich arsenic might be sorbed, and which therefore couldcontrol arsenic levels in UL porewaters. Data for porewaterarsenic in UL correlated best with the following linearcombination of two sediment variables: [As]aq ) A[Fe]PO4 +B[Fe]HCl, where A and B are adjustable empirical constantsderived by optimizing the equation to maximize correlationand minimize residuals between predicted and measuredporewater arsenic (A ) 0.057, B ) -0.012). This equationimproved the correlation with observed porewater arsenicconcentrations to an R value of 0.85 (p ) 0.007). The signsof the coefficients also agree with expectations: phosphateextractable iron phases, that is, those containing weaklybound arsenic, are a source of arsenic to porewaters (thusA > 0); while HCl extractable iron phases, that is, iron oxidesknown to tightly sorb arsenic oxyanions (34, 36), are a sinkof arsenic from porewaters (27), and B < 0. XANES data showa high proportion of sediment arsenite throughout this UnionLake core and an increasing proportion of thioarsenites withdepth, supporting the conclusions that arsenic is primarilypresent as sorbed oxyanions in this iron-controlled core,possibly with some transformations to arsenic- and sulfur-containing minerals with depth. Data indicate that arsenicis sequestered in this core via sorption, which may allowredox-controlled movement of arsenic within the sedimentcolumn, similar to that seen in Coeur d’Alene Lake (37). Thismobility would be inhibited if Fe-As-S phases that areintermediates to pyritization (12, 38) were forming, butXANES data do not indicate formation of Fe-As bonds (12),consistent with no formation of these phases.

Comparison of the Blackwater Branch and Union LakeCores. In the sulfur-controlled BWB core, 57% (median value)of sediment arsenic was phosphate extractable as comparedto 82% of sediment arsenic in the UL core. Phosphate is itselfan oxyanion, which releases arsenic by competition forsurface sorption sites (27); the greater proportion of arsenicreleased from the UL core by phosphate extraction isconsistent with phosphate more effectively displacing arsenicoxyanions than thioarsenites. This difference may alsoindicate that the arsenic-sulfur species formed in the BWBare more recalcitrant than the sorbed arsenic oxyanionsprimarily present in the UL core.

A fraction of arsenic sulfides was observed by XANES onboth cores: 50-90% in the BWB core and 10-60% in the ULcore. Within thioarsenites, realgar (AsS) may be moreprevalent in the UL core because this system is so iron-rich,and, conversely, orpiment (As2S3) may be more prevalent inthe BWB core because this system appears to be sulfur-controlled and (relatively) iron poor (13), although the XANESdata do not allow us to confirm these predictions. XANESdata are consistent with precipitation of thioarsenites in theBWB (not with sorption of arsenic to iron sulfides, e.g., (12))and weaker sorption of arsenic oxyanions without subsequentmineralization in UL.

The most obvious geochemical differences between thesetwo cores are the high sulfur in the porewaters of the BWBcore and the very high iron in the porewaters and sedimentof the UL core (Figures 2 and 3). The total sulfur (sedimentplus porewaters) in the BWB core is only ∼1.5 times higherthan that in the UL core. The total iron in the UL core,however, is 3-10 times higher than that in the BWB core(Figure 3). This implies that the high iron in the UL coreremoves sulfide from solution, preventing build-up of highaqueous sulfur concentrations and resultant formation ofaqueous thioarsenites. The data in this study do not addressthe cause of the high iron in UL sediments, but one possibilityis that particle fluxes in the BWB have lower iron abundancesthan those delivered via the Maurice River to UL.

Comparison with Previously Published Data. The datapresented herein are consistent with predictions made byO’Day et al. (13) that with lower Fe/S ratios, arsenic wouldbe sulfur-controlled, e.g., the BWB, and with relatively highFe/S ratios, it would be iron-controlled, e.g., UL. For the datain this study, both porewater and sediment Fe/S ratios fitthis prediction (Figure 3).

Iron and sulfur (sulfate + sulfide) concentrations insediment and porewaters were estimated. These estimateswere made from figures published for the Clark Fork River,Montana (8), and from the August 1999 data (the only datasetwith sulfide measurements) from Balmer Lake, Ontario (4).Conditions in porewaters of the Clark Fork River weredescribed as sulfidic (8), and arsenic mobility in the sedimentsbeneath Balmer Lake was attributed to dissolution of ironphases (4). Sediment Fe/S ratios fit the paradigm describedabove; the two sites that may be classified as iron-controlled,Balmer Lake and UL, have similar and high sediment Fe/Sratios, while the sulfur-controlled Clark Fork River and BWBhave much lower sediment Fe/S ratios. The porewater Fe/Sratios for the Clark Fork River and the BWB are also similarto one another and relatively low. However, the porewaterFe/S ratios of Balmer Lake do not fit the paradigm of highFe/S ratios at iron-controlled sites. This is likely due to miningimpacts on this lake and resultant very high porewater sulfateconcentrations of up to ∼350 mg L-1 (4).

The geochemical environments at the Clark Fork River(8), Balmer Lake (4), and East Palo Alto (13) are all appreciablydifferent from those near the Vineland Chemical site.However, classifications based on Fe/S ratios appear to beinstructive as to mechanisms influencing arsenic mobility.These mechanisms are significant because the type of eventsthat might mobilize arsenic would be somewhat different;in iron-controlled environments, anoxia of overlying waterswould release arsenic by altering speciation of sorbed arsenicspecies or dissolution of host minerals, whereas in sulfur-controlled environments, exposure of sediments to oxicconditions could release arsenic by dissolution of authigenicarsenic sulfide phases.

Sources of Offsite Arsenic Transport. The BWB serves asan integrator of most of the arsenic transported downstreamfrom the Vineland site. There are four plausible sources ofarsenic transported downstream in the BWB: (a) erosionand overland transport of arsenic-bearing soil minerals, (b)effluent from the pump and treat plant which is dischargedto the stream, (c) remobilization of sediment-bound arsenicin the BWB, and (d) continued high-arsenic groundwaterdischarge from the Vineland Chemical Company site intothe BWB. Total arsenic transported downstream was esti-mated from the streamflow and arsenic concentrationmeasurements (Figure 1) to be ∼22 ( 3 mol (1.64 kg) day-1.Overland transport can be eliminated as a primary sourceof arsenic to the BWB due to the dry conditions on the dayof stream sampling and the absence of surface watertributaries to the BWB between the site and the Maurice

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River (30). Plant effluent contributes ∼0.64 mol day-1, lessthan ∼3% of the total downstream transport.

An upper bound on remobilization of sediment-boundarsenic can be estimated by calculating the diffusive flux ofarsenic from the porewaters using Fick’s Law (39). Thisdiffusive flux is ∼4.5 µmol arsenic m-2 day-1 or 35 mmolarsenic day-1 (using stream length of 1.4 km and width of 5.5( 0.2 m). Even if the diffusive flux from porewaters were anorder of magnitude greater than this estimate, it wouldrepresent only∼1.6% of total downstream arsenic flux. Similarstream and porewater measurements were made in thesummer of 2003 (40), and the diffusive flux was estimated as∼200 mmol day-1, <1% of the 31 mol day-1 total estimatedBWB downstream arsenic flux. Groundwater discharge tothe BWB is the only remaining potential source of arsenicto the stream, and we conclude that this pathway ispredominantly responsible for offsite arsenic transport.

Future Implications. One of the main remediation goalsfor this site is cessation of offsite transport of arsenic (23).Considerable progress has been made towards this goal since1993, when P&T operations commenced. Total offsitetransport has been reduced from 54 to 22 ( 3 mol day-1, andarsenic concentrations at surface water sampling location 6(Figure 1) have dropped from 1440 to 270 ( 36 nM (41). Thisdecrease is due largely to decreases in groundwater dischargeresulting from groundwater extraction and treatment; ongo-ing (2004-2006) soil washing is predicted to further decreasecontamination in remaining groundwater discharges (42).

While diffusion of arsenic from sediment porewaters is arelatively minor source of arsenic to surface waters underthe geochemical conditions studied, this may not always bethe case. As groundwater contamination is further reduced,release of sediment-bound arsenic may become an increas-ingly large proportion of offsite arsenic fluxes. Additionally,temporal variations in the system (from diurnal to annual)not captured in the data presented here may also increasefluxes of sediment-bound arsenic to the BWB. Because thissystem is sulfur-controlled, arsenic is present primarily asauthigenic solid thioarsenites and not as sorbed species. Thebehavior of these thioarsenite minerals under various redoxconditions is difficult to predict without further investigationsinto their properties. However, it seems likely that greatestarsenic releases might occur under oxic conditions whereauthigenic arsenic sulfide minerals would be dissolved,similar to arsenic releases in mining situations (2).

Mobilization of arsenic from Union Lake sediments isalso of concern. In this iron-controlled system, sorption ofarsenic oxyanions to sediments is the primary mechanismof arsenic immobilization, indicating that sorption sites couldeventually saturate. However, currently arsenic transport tothe lake is dominated by particle fluxes, and thereforeincreases in arsenic would not be expected to outpaceincreases in sorption sites. UL has often become anoxic duringthe later part of summer (41), which allows greater releaseof arsenic from the sediments (4); bottom water arsenicconcentrations during summer 1993 anoxia were ∼700 nM(41). Such anoxic events could represent a pathway for humanexposure to arsenic, both by intake of lake water by swimmersas well as by redistribution of arsenic from deep sedimentsto near shore sediments more accessible to people. To predictarsenic behavior in iron-controlled UL and sulfur-controlledBWB, further research into the phases present is indicated.

AcknowledgmentsWe would like to thank EPA region 2, the Army Corps ofEngineers, and Sevensen, Inc., for access to the VinelandChemical Company Site, and James Ross, Matt Nanes, SandySantillan, Saugata Datta, and Yi He for field and analyticalassistance. Use of the National Synchrotron Light Source,Brookhaven National Laboratory, was supported by the U.S.

Department of Energy, Office of Science, Office of BasicEnergy Sciences, under Contract No. DE-AC02-98CH10886;Kumi Pandya provided assistance with synchrotron experi-ments. Funding was provided by NIEHS Grants P42 ES10349and P30 ES09089. This is LDEO publication #6823.

Supporting Information AvailableA figure showing XANES data and six tables showing thedata used to generate figures in this manuscript. This materialis available free of charge via the Internet at http://pubs.acs.org.

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Received for review April 14, 2005. Revised manuscript re-ceived August 22, 2005. Accepted August 29, 2005.

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