arsenic mobilization in the hyporheic zone of a contaminated stream

WATER RESOURCES RESEARCH, VOL. 35, NO. 11, PAGES 3441-3450, NOVEMBER 1999

Arsenic mobilization in the hyporheic zone of a contaminated stream

Sonia A. Nagorski and Johnnie N. Moore Murdock Environmental Biogeochemistry Laboratory, Department of Geology, University of Montana, Missoula

Abstract. Arsenic behavior was examined in a contaminated stream by sampling the dissolved (<0.45/•m) arsenic and metals in surface water, shallow hyporheic zone water, and adjacent ground water. Surface water was oxic and slightly basic, and ground water was anoxic and acidic. Hyporheic zone water had pH values of 6-7, dissolved oxygen concentrations mostly between 0 and 3 mg L -1, and mean concentrations of most metals inbetween surface and ground water sample concentrations. However, arsenic and iron were enriched in the hyporheic zone. Most of the hyporheic zone dissolved arsenic was in the form of As(III), which is considered to be more toxic to some organisms than As(V). In the oxic surface water, 20% of the total dissolved As was found to occur in its reduced form. We hypothesize that upon burial and reduction of Fe-oxyhydroxides in the streambed, sediment-bound arsenic is transferred into the dissolved phase as As(III), and it is subsequently released into the surface water, where it does not immediately reoxidize. A continual flux of reduced As to the surface water maintains As(III) concentrations above that expected in oxygenated surface waters.

1. Introduction

The hyporheic zone has been defined broadly in the litera- ture as the saturated subsurface area connected to a stream

channel that shares with it some biological, chemical, or physical characteristics [Williams and Hynes, 1974; Triska et al., 1989; Hendricks and White, 1991; Valett, 1993; Brunke and Gonser, 1997]. This loosely defined zone is rapidly gaining recognition as both a key ecological niche crucial to the health of stream biota, as well as a major site of exchange, metabolism, and storage of particulates and solutes in rivers [Bencala, 1984; Grimm and Fisher, 1984; Stanford and Ward, 1988; Vaiett, 1993; WDrman, 1998]. Most studies of the physical and chemical dynamics of the hyporheic zone have generally concluded that surface and ground water interact extensively and that the hyporheic zone plays a major role in storage of stream solutes [Munn and Meyer, 1988; Valett et al., 1990; Castro and Horn- berger, 1991; Triska et al., 1993]. The exchange of nitrogen, oxygen, and organic material between surface water and hyporheic zones has been documented, and many researchers con- tend that hyporheic zones are important sites for nutrient cycling [Jacobs et al., 1988; Findlay et al., 1993; Holmes et al., 1994; McMahon et al., 1995; Findlay and Sobczak, 1996; Pusch, 1996]. In addition, studies of biologic communities in hyporheic zones have shown they are important sites of refuge, dwelling, and development for many freshwater fauna [Pugsley and Hynes, 1986; Stanford and Ward, 1988; Williams, 1989; Ward and Palmer, 1994; Brunke and Gonser, 1997].

Relatively little progress has been made in understanding the physical and chemical dynamics of hyporheic zones [Ben- cala, 1993]. In particular, there are many unanswered ques- tions about the role that the hyporheic zone plays in storing metals or metalloids and supplying them to surface and/or ground water [Benner et al., 1995; Harvey and Fuller, 1998]. A

Copyright 1999 by the American Geophysical Union.

Paper number 1999WR900204. 0043-1297/99/1999WR900204509.00

metalloid of particular environmental concern is arsenic, and published research to date has not addressed the important issues of how arsenic behaves in the hyporheic zone, what processes control its mobility or fixation, and the effects of those processes on surface water concentrations of arsenic. Arsenic can be toxic to aquatic organisms, and arsenic contamination in surface and ground waters is of growing concern in the United States [Eisler, 1994]. Many rivers and adjacent aquifers have elevated arsenic concentrations from mining wastes [Moore and Luoma, 1990] or from natural arsenic sources, such as volcanic rocks and thermal springs [Stauffer et al., 1980; Nimick et al., 1998; Wilke and Hering, 1998]. Although there is much known about the geochemistry of arsenic, this knowledge has not been applied to understanding arsenic mobility in the hyporheic zone. A better understanding of this issue is essential for assessing the effects of arsenic transfer from water and sediment into the food chain of organisms dependent on the hyporheic zone. It may also be important for understanding the controls on solute arsenic concentrations in surface water and ground water adjacent to rivers elevated in arsenic. These issues ultimately are important for the manage- ment of drinking water resources, because arsenic, even at low levels, is a known carcinogen in humans and is toxic to humans and aquatic biota at higher levels [Eisler, 1994].

2. Research Site

Silver Bow Creek, at the headwaters of the Clark Fork River in western Montana, was chosen owing to the large chemical differences between ground water and surface water and elevated metals concentrations in streambed sediments [Bennet et al., 1995]. The site is heavily contaminated as a result of over a century of large-scale mining in Butte, Montana, located !8 km upstream. During mining, over 100 million metric tons of mining waste was released into the creek [Andrews, 1987], and much of it was carried downstream by major floods at the turn of the century and was deposited in wide stretches of the floodplain [Nimick and Moore, 1991]. Along the study site

3441

3442 NAGORSKI AND MOORE: ARSENIC MOBILIZATION

portion of the creek, up to 2 m of arsenic and metal-rich mine tailings intermixed with sediments cover a several hundred meter wide area of the river's floodplain [Bennet et al., 1995• Lucy, 1996; Shay, 1997].

Floodplain soil samples from the site contain up to 1100 mg/kg arsenic [Lucy, 1996; Shay, 1997]. Acidic, anoxic, and high-metal concentration ground water flowing through these highly contaminated soils and sediments comes into contact with the slightly basic (pH = 7.5-8.5), oxic, and relatively low metal concentration surface water. A thin (<50 cm) interactive hyporheic zone (defined by Triska et al. [1989] as containing <98% but >10% channel water) underlies the streambed where physical mixing and chemical transformation of these waters occurs [Benner et al., 1995; Nagorski, 1997].

The water table lies between 0 to 1.5 m below land surface, depending on topography and temporal variability, and is hy- drologically connected with the creek [Shay, 1997]. Although the surface water is less contaminated than the groundwater, it is devoid of most aquatic life with the exception of certain microorganisms [Wielinga et al., 1994], algae, and an extremely depauperate aquatic insect population.

3. Methods

Hyporheic zone samples were collected at Silver Bow Creek in 1995 and again in 1998. The 1995 project was a more general survey of ground water-surface water interaction at Silver Bow Creek, and arsenic speciation analysis was not performed. On the basis of the findings from the 1995 study, two of the three sites were resampled in 1998 to specifically examine arsenic behavior and speciation in the hyporheic zone, surface water, and ground water. Improved analytical techniques with lower detection limits were used for the 1998 sampling, and As(III) concentrations were determined separately from total As.

3.1. The 199 5 Project Methods

Three sites, spaced • 1/3 km apart along the study area, were divided into three transects, spaced 5 m apart. At each transect, the shallow (<30 cm) hyporheic zone was sampled at 3-5 points within the streambed. Transects were sampled monthly from June to August 1995, during variable flow conditions (800-1600 L s -•) in the creek. Shallow floodplain piezometers were surveyed and sampled concurrently for geochemical data and potentiometric levels by Shay [1997].

Flexible, air-tight, small-diameter (1 cm OD) polyethelene tubing was used for obtaining the hyporheic zone samples. The lower 5 cm of the tubes was perforated and covered with a fine nylon screen. With the help of a steel rod to form an opening in the bed sediment, the tubes were installed by manually inserting them into the bed sediment to a depth of 15-25 cm beneath the streambed surface. Wooden stakes •30 cm long were rubberbanded to the tubes to help maintain their stabil- ity. Approximately 1-2 m of tubing remained above the surface so that samples could be drawn.

Each hyporheic zone water sample was taken by manually drawing up water through an acid washed (6 N HC1 for 2 hours and rinsed three times in aleionized water) 60 cm 3 syringe attached to the tubing. All samples were purged with one tube and one syringe volume before samples were collected for analysis. Small sample volumes (80-100, mL), the•minimum needed for chemical analyses, were taken in order to minimize the integration of water from areas far from the tubes. Samples were immediately pushed through a 0.45-/xm filter set in an

acid-washed filter housing and into an acid-washed bottle for cation analysis and glass bottles for anion analysis. It is ac- knowledged that the <0.45 /zm filters allow for passage of some colloidal material. Each cation sample was acidified with trace-metal grade HNO3 to pH < 2 immediately following filtration.

Three depth-integrated surface water samples were taken across each transect on each sampl.ing date. Dissolved oxygen (D.O), pH, and conductivity were measured in situ for surface water. For hyporheic zone and ground water samples, these parameters were measured by first pouring an additional 40-50 mL of sample into a beaker open to the atmosphere and setting the meters in the subsample for measurement. A lim- itation to the ground water data set is that Shay [1997] collected D.O. data for only 10 of the 29 ground water samples.

In the laboratory, the acidified samples were analyzed using a Thermo Jarrel-Ash Inductively Coupled Argon Plasma Emis- sion Spectrometer (ICAPES) for total As, Ca, Cd, Co, Cu, Fe, Mg, Mn, Mo, Na, Ni, Pb, Si, Sr, Ti: and Zn.

Artificial substrates were used to sample the solid phase, in order to bypass the problems encountered when trying to core coarse-grained stream sediments and delineating coating his- tory on variably sized sediments [B.enner et al., 1995]. Twenty- seven bead columns were constructed using 40-cm-long poly- carbonate tubing (1 cm OD and 0.6 cm ID), which were slotted (1 mm width, using a band saw) on two sides in 2-3 mm intervals. Aluminosilicate beads (2-mm nominal diameter) were inserted into the columns, with plastic dividers every 10 cm to minimize vertical integration of water flowing through the columns. The acid-washed, assembled columns were inserted into the substrate so that 30 cm of the column was below

the channel bed surface and 10 cm was exposed to the surface water. Three bead columns were placed in each transect, with each column placed <10 cm from the hyporheic zone water samplers. After 52 days, the bead columns were removed from the streambed and taken back to the laboratory, where they were sectioned into 4-7 segments, depending on the amount of visible coating on the columns. Coatings on 1 g of bead from each section were extracted in 5 N HC1 for 1 hour. The digest solutions were analyzed for metals using the ICAPES.

Seventeen streambed sediments were taken from the top 1-2 cm of the stream sediment and were wet-sieved in the field

through a 63-t•m mesh screen using ambient stream water. The samples were stored on ice for transport to the laboratory, where they were immediately centrifuged, decanted, and dried overnight at 70øC. A microwave aqua-regia digest was performed on the sediment samples and metal concentrations were determined by ICAPES.

3.2. The 1998 Project Methods

Two of the three sites used in the 1995 project were resampled for the 1998 data set. Ten new hyporheic zone samplers were installed at random locations at each of the sites, at depths of ---20 cm into the streambed. Hyporheic zone and surface water samples were taken in the same manner as for the 1995 data, with the exception that samples taken for ICP analysis were acidified to pH < 2 with trace-metal grade HC1 instead of HNO3. Separate 30-mL samples were collected for As(III) analysis, and each sample bottle was filled to capacity to minimize oxidation, and the samples were not acidified. Piezometers were sampled using a peristaltic pump, and all ground water samples were filtered and acidified in the field.

In the laboratory, the samples were analyzed for As(III) by

NAGORSKI AND MOORE: ARSENIC MOBILIZATION 3443

Hydride Generation Atomic AbsorPtion Spectrometry (HGAAS) (using a pH 3 citrate buffer and a borohydride solution containing 0.6% (by volume) NaBH4 and 0.4% NaOH) within 3 days of sampling. The practical quantification limit for As was 0.2 /zg L -•. Arsenate standards were run during the arsenic(III) analysis to check for interference. The instrument could not detect any (<0.2/zg L -•) arsenite signal from a 10/zg L- • arsenate standard and read only 0.6/zg L- • As(III) on a 100/zg L -• arsenate standard. This interference falls within the instrument error range and was therefore ig- nored. Total arsenic and metals were analyzed using the ICAPES. As(V) was assumed to be the difference between total As and As(III). Ultrasonic nebulization (EPA Method 200.15) was used on samples with arsenic concentrations too low to be detected with the cyclone nebulizer (EPA Method 200.7) (PQL = 5/zg L- • for ultrasonic nebulization; 65/zg L- • for cyclone nebulization.)

3.3. Quality Assurance/Quality Control

Field duplicates, instrument replicates, spikes, U.S. Geolog- ical Survey (USGS) standards (T-143, T-145, and T-107), field blanks, and laboratory blanks were evaluated for quality assurance/quality control on both the 1995 and 1998 data sets. Field duplicates, which had higher variability than instrument duplicates, compared mostly within 10%. However, several elements in the 1995 data set were different by up to 20% (As, Cu, and Pb). It is speculated that not enough time (<15 min) was allowed between resampling to allow replacement water to equilibrate with the hyporheic zone or that the extra amount of water withdrawn for a duplicate sample may have been derived from more distant areas with potentially different chemical signatures. Iron duplicates in the surface water with concentrations of <1 mg L -• were an average of 27% different, representing the highest error in the dataset. However, in the hyporheic zone, where concentrations were higher, duplicates were an average of 6.0% different. In the !998 study, sites were resampled hours apart to allow more chance for reequilibra- tion, and field duplicates were more similar. Specifically, As duplicates were an average of 2.2% different from one an- other, and Fe duplicates were 5.8% different. As(III) replicates on the HGAAS were an average of 8.2% different. Spike recoveries were within 85-115% on the ICAPES and on the

HGAAS. Three USGS water standards were checked routinely during analysis on both the ICAPES and HGAAS for accuracy. All elements were within the reported ranges for the 1998 data set, while for the 1995 data set, most elements were within 10% of the reported value. All laboratory blanks were below detection for all elements. Elements analyze d in the field blanks were either entirely below detection or contained levels of Ca, Mg, Na, and Zn that were insignificantly low compared to concentrations in environmental samples.

3.4. Microcosm Experiment

Samples of the bed material from the visibly (orange- colored) oxidized, upper 3 cm of the streambed were taken to construct microcosms to determine if arsenic could be released

from the sediment into the dissolved phase simply by estab- lishing less oxic conditions. Three 250-mL bottles were filled ---2/3 with bed sediment (seived to <4 mm) and the rest with surface water. These bottles were capped, refrigerated, and opened for extraction of water samples by syringe and direct measurement Of pH and dissolved oxygen after 24 hours and again after 13 days. Samples were filtered in the same manner

as were the other samples, and they were analyzed for As(III) and total As as above.

4. Results

4.1. Results of 1995 Data

4.1.1. Water. Dissolved (<0.45/•m) arsenic and iron concentrations in hyporheic zone samples (n = 75) were significantly higher than concentrations in both the surface water (n = 54) and ground water (n = 29) (p < 0.005 for both As and Fe, for all pairwise comparisons of the distributions by the nonparametric Wilcoxon rank sum test). None of the ground water and surface water samples had detectable (>65 /•g L -•) arsenic. The mean pH ofwater samples collected from the hyporheic zone was 6.6 (standard deviation (s.d.) = 0.5), which is an intermediate value between groundwater (mean = 4.6; s.d. = 1.0) and surface water (mean = 7.7; s.d. = 0.4) at the site. Mean D.O. in the hyporheic zone samples was 2.3 (s.d. = 1.6) mg L -•, compared to 7.2 (s.d. = 1.3) mg L -• in the surface water and 1.0 (s.d. = 0.1) mg L -• in the 10 ground water samples. Mean concentrations of most metals were generally intermediate between mean surface and ground water concentrations (Figures la and lb).

The highest levels of dissolved arsenic and iron at the site were found in the hyporheic zone and did not follow the trends of any of the other constituents (Figures lc and ld). (The sampling distributions for both arsenic and iron were not nor- mal, and as a result, summary data for arsenic and iron are reported in terms of medians and ranges, and the nonparametric Wilcoxon Rank Sum Test is used for comparing distributions.)

Dissolved arsenic in the hyporheic zone samples was above the detection limit (65/•g L -•) in 55% of the samples. Arsenic concentrations ranged from <65 to 2700/•g L -•. Assigning a value of one half the detection limit to those samples below detection, the median concentration of arsenic in the hyporheic zone was 80/•g L-•. Of the hyporheic zone s•tmpleg with detectable arsenic, the median arsenic concentration was 200 /•g L -•, mean pH was 6.6 (s.d. = 0.4) and mean D.O. was 1.6 mg L -• (s.d. = 1.0). The arsenic concentrations were clearly related to D.O. and pH. Arsenic was elevated only in samples with a pH between 6 and 7 and, with a few exceptions, D.O. less than 3.5 mg L -• (Figures 2a and 2c).

The range of iron concentrations in the hyporheic zone samples was between 0.15 and 400 mg L -• (median 38 mg L-•). Many of the samples with high Fe also contained the relatively high As and D.O. concentrations, suggesting the pos- sibility that the outliers are associated with colloids. The range of iron concentrations in the ground water also was large, between 0.08 and 42.77 mg L -• (median = 19.2 mg L-•). The range in surface water was much smaller; concentrations fell between 0.05 and 0.46 mg L -•, with a median of 0.21 mg L- • (Figures 2b and 2d).

The identification of conservative elements (Ca and Mg, which fell on a linear mixing curve, R 2 = 0.90) allowed for the calculation of physical mixing ratios within the hyporheic zone [Faure, 1998] (Figure 3a). By using the mixing equation below, the percent of groundwater in the hyporheic zone water samples could be calculated [Benner et al., 1995; Tdska et al., 1989]:

y = 100([HZ] -[SWl)/([GW] - [SW])

where


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Figure 1. Boxplots of concentrations in ground water (G.W.) hyporheic zone (H.Z.), and surface water (S.W.) of dissolved (a) calcium, (b) manganese, (c) arsenic, and (d) iron collected in 1995. Boxes contain the middle 50% (interquartile range) of the concentrations, with the median represented by the line within each box. Whiskers span the 10th and 90th percentiles. Outliers (values above the 90th percentile and below the 10th percentile) are represented by open circles.

y percent groundwater in the hyporheic zone water. [HZ] concentration of Ca or Mg in the hyporheic zone. [SW] concentration of Ca or Mg in the surface water. [GW] concentration of Ca or Mg in the groundwater.

The ground water end member values used in the equation were taken from the average concentrations of the floodplain piezometer samples. Since most of the piezometers were in the vicinity of sites 1 and 2, their ground water values were used in the mixing equations for hyporheic zone samples at the two sites. However, the Ca and Mg concentrations at site 3 were different enough from the other two sites that a separate ground water value had to be used for its mixing calculations. This value was taken from a lone nearby piezometer --•1 m away from the stream which was assumed to represent the local ground water interacting with the stream. The surface water values used in the equation were those measured on the day of the hyporheic zone sampling. The error associated with the surface water measurements was relatively minimal (<3%), and so the mean surface water Ca and Mg concentrations were used.

The percent ground water was calculated for each hyporheic zone sample by taking the average of the values obtained by using both Ca and Mg values in the equation. Examination of all the resulting calculations using the mean ground water and surface water concentrations indicated that an average of 63% (_+8%) of the hyporheic zone samples contained >75% surface water. The reported variability was derived by computing separate sets of mixing equations using the mean _+ 1 standard

deviation of the ground water concentrations measured in the piezometers.

The results of the mixing calculations also indicated that many metals (e.g., Mn and Fe) and As were not acting conservatively in the hyporheic zone, as they did not form a linear relationship with either Ca or Mg (Figures 3b, 3c, and 3d).

4.1.2. Bead columns and bed sediment. Examination of

the bead coatings formed in contact with the solute constituents within the hyporheic zone helped to further define the geochemistry. Twenty-two of the 27 bead columns exhibited a red-orange zone of precipitation at the streambed-surface water boundary. The precipitation zone typically spanned no more than 5 cm of the bead column. Beneath the precipitation zones, where the columns penetrated deeper into the substrate, and above the zones, where the columns protruded into the surface water, the beads did not have visible coatings. Analysis of the composition of the bead coatings indicated that the precipitation zones were elevated in metals over those areas without visible coatings and that the change in metal concentrations with column length was very abrupt (on the millimeter scale). The precipitation zones were generally tens or hundreds of times more enriched in Fe than the nonprec- ipitation zones. On 20 of the bead columns, As concentrations were at a maximum along the precipitation zones where Fe concentrations were also at their highest, irrespective of the size of the precipitation zone (Figure 4).

On the basis of aqua regia microwave digests, As concentrations in the <63 •m size fraction of the streambed sedi-


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Figure 2. (a) Arsenic versus pH, (b) iron versus pH (c) arsenic versus dissolved oxygen, and iron versus dissolved oxygen in the ground water (crosses), hyporheic zone water (squares), and surface water (diamonds), from the 1995 data set.

ments ranged from 358 to 549 ppm (mean 438 ppm). Iron concentrations were 3.1-6.3% (mean 4.4%).

4.2. Results: 1998 Data

4.2.1. Water. Results of the 1998 study support and ex- pand on those from 1995. As in the 1995 data set, the concentrations of most metals in the hyporheic zone fell within the range of surface water and ground water concentrations, as illustrated by Ca and Mg (Figures 5a and 5b). Again, dissolved As and Fe were present in low concentrations in ground water and surface water, but in relatively high concentrations in the hyporheic zone (Figures 5c and 5d). Ground water had total dissolved As values from <5 to 28 tag L -• (median = 10 tag L-•). Surface water arsenic values ranged from 8 to 12 tag L- • (median = 10 tag L-•). In the hyporheic zone, where D.O. levels were all <4.7 mg L -• (mean 1.7 mg L -•) and mean pH was 6.6 (s.d. = 1.0) units, dissolved As was significantly concentrated many times over the surface water and ground water (p < 0.01, using Wilcoxon rank sum test pairwise comparisons). Of the 20 samples collected from the hyporheic zone, 18 had detectable (>5 tag L-•) total As, with concentrations ranging from 29 to 483 tag L -•, with a median value of 75 tag L -•.

Speciation analyses showed that the percentage of As(III) in the different environments varied (Figure 6). As(III) made up >60% of the total As in 15 of the 20 hyporheic zone samples. The median percent of As(III) in the hyporheic zone samples with detectable As was 78%, with an overall range of 7 to 100%. Ground water samples with detectable arsenic also had a large range (5-100%) of As(III), although the median percentage was 15%. In the surface water samples, the percentage of total As that was in the reduced form ranged from 15 to 25% (median = 19%), even though the surface water was highly oxic.

Again, iron concentrations were also higher in the hyporheic zone compared to the surface water (p = 0.003) (Figure 5d). The median Fe concentration in the surface water was 0.11 mg L -• (range = 0.040 to 0.422 mg L-•), compared to a median

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Figure 3. Calcium, a conservative element in the system, versus (a) magnesium, (b) manganese, (c) arsenic, and (d) iron in the ground water (crosses), hyporheic zone water (squares), and surface water (diamonds), from the 1995 data set. The degree of linearity indicates conservative versus reactive behavior of the solute.

of 55.7 (range - 0.32 to 140.3 mg L -•) in the hyporheic zone. Ground water Fe concentrations were highly variable, ranging from 0.008 to 51.31 mg L -•, with a median of 17 mg L -•. A statistically significant difference between ground water and hyporheic zone water was not found (p = 0.121). However, estimates of the contribution of Fe by the ground water to the hyporheic zone suggest that Fe in the hyporheic zone cannot be accounted for by ground water input alone. Multiplying the mean percent ground water in all 1998 hyporheic zone samples (13.8%) by the median ground water Fe concentration (17.4 mg L -•) yields 2.4 mg L -•, a value smaller than the median Fe concentration of the hyporheic zone samples (42.4 mg L-•). Even if the highest Fe concentration found in the ground water samples (51.3 mg L -•) is used in place of the median, the resulting value of 7.09 mg L -• is still smaller than the hyporheic zone median.

Mbdng ratios were calculated for the 1998 hyporheic zone samples in the same manner as described for the 1995 samples. Ground water and surface water samples used in the mixing equation were taken from the 1998 data set only. Using the mean (_+1 standard deviation) of the ground water sample concentrations of Ca and Mg, the mixing ratios indicated that 75% (_+5%) of the hyporheic zone samples contained at least 75% surface water.

4.2.2. Microcosms. The three microcosms increased in

total dissolved Fe and As concentration, as well as in the percentage of As(III), over the experimental period. The surface water mixed with the streambed sediments in the micro-

cosms initially had a total As concentration of 8-10 tag L -•, of which 20% was As(III). After 24 hours, the concentration doubled to 20 tag L -•, 9% of which was As(III). After 13 days, the As concentrations in the three microcosms had increased

to 76, 78, and 192 tag L -•, of which 8%, 31%, and 78% was As(III), respectively. Fe concentrations rose from 0.055 mg


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L -z at time zero to 0.180, 2.33, and 0.631 mg L -z in the three microcosms at the end of the 13 days. These concentrations, as well as the pH and D.O. levels in the microcosms, are similar to those found in the hyporheic zone. The pH dropped to 6.7-7.0 in the microcosms from the starting pH of 7.8. Dis- solved oxygen, originally at 11.5 mg L -z, dropped to 1.7-2.3 mg L-z in 24 hours, and to 0.0 mg L-z in all three microcosms at 13 days.

5. Discussion

Because both the surface water and the ground water samples had dissolved arsenic concentrations significantly lower (p value <0.01) than those in the hyporheic zone samples for both data sets, simple physical mixing between the waters cannot generate the relatively high arsenic concentrations found in the hyporheic zone. The elevated concentrations must originate

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Figure 5. Boxplots of concentrations in ground water, hyporheic zone, and surface water of dissolved (a) calcium, (b) manganese, (c) arsenic, and (d) iron collected in 1998.


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Figure 6. (a) Arsenic(III) and (b) percent As(III) of total As for concentrations in the ground water, hyporheic zone, and surface water (1998 data).

from chemical reactions within the hyporheic zone substrate involving a third source, which we propose to be the contaminated hyporheic zone sediments.

According to the pH and dissolved oxygen conditions in the hyporheic zone, it is expected that the distribution of metals and metalloids would be different from that in the oxic surface

water and in the generally anoxic, acidic ground water. The pH-controlled adsorption-desorption reactions strongly control the movement of arsenic and iron species between dissolved and solid phases [Ferguson and Gavis, 1972; Peterson and Carpenter, 1986]. Redox conditions also can strongly control the mobility of inorganic arsenic in aquatic systems [Cherry et al., 1979; Gulens et al., 1979; Farmer and Lovell, 1986; Moore et al., 1988; Cullen and Reimer, 1989].

In oxic surface water environments, negatively charged arsenate species are commonly sorbed by iron and manganese oxyhydroxides, whose stabilities are controlled by pH [Pierce and Moore, 1982; Rampe and RunnelIs, 1989; Moore, 1994; Nimick et al., 1998]. Analysis of precipitation bands on the bead columns confirms the close association between iron and

arsenic in the oxic environment of Silver Bow Creek. Sorption onto iron oxyhydroxides transfers arsenic from the dissolved phase to the particulate phase [Peterson and Carpenter, 1983; Goldberg, 1986; Brannon and Patn'ck, 1987; Belzile and Tessier, 1990; Fuller et al., 1993], and Mn-oxyhydroxides can catalyze redox reactions transforming solute As(III) to particulate As(V) [Oscarson et al., 1983; Moore et al., 1990]. In Silver Bow Creek, Mn and Fe-oxyhydroxides with bound arsenic are con- stantly supplied to the streambed by eroded floodplain sediments largely composed of oxidized mine tailings.

Under anoxic-sulfidic conditions As(III) sulfides (such as orpiment) readily form particulates [Peterson and Carpenter, 1986; Brannon and Patrick, 1987; Aggett and Kriegman, 1988; Moore et al., 1988] and pyrite can fix large amounts of arsenic in its crystal lattice [Belzile and Lebel, 1986; Morse, 1995]. This may explain why relatively low levels of arsenic and iron were found in the ground water, whereas these elements were highly concentrated in floodplain sediments. The relatively low median percentage of As(III) in the ground water (15%) also may be explained by these processes, which can deplete As(III) from solution.

At the interface between the surface water and ground water, a unique chemical environment is present that may explain the anomalous arsenic and iron concentrations. When Fe-

oxyhydroxides in the streambed sediments become buried and subject to reducing conditions, as found in the shallow hyporheic zone, arsenic sorbed or co-precipitated to them may be released owing to reductive dissolution of the Fe oxides, a process that increases total arsenic concentrations and fosters the reduction of As(V) to As(III) [Masscheleyn et al., 1991; Harrington et al., 1998]. The partitioning of iron species between the solute and particulate phases can be controlled by pH-induced changes in sorption equilibria, and because of the close association between Fe and As, shifts in Fe partitioning will influence As release and sorption as well [Peterson and Carpenter, 1983; Brannon and Patrick, 1987; Belzile and Tessier, 1990]. In the shallow hyporheic zone of Silver Bow Creek, the pH is between 6 and 7, which is lower than in surface water and is likely due to input from adjacent acidic ground water.

These hyporheic zone processes may be controlling arsenic concentrations and speciation in the surface water as well. In freshwater systems, arsenic has been found to occur most commonly as inorganic arsenite and arsenate [Braman and Fore- back, 1973; Anderson and Bruland, 1991]. Arsenate, generally thought to be less toxic of the two [Naqvi et al., 1994], is typically the predominant form in river water and has been documented to travel conservatively over tens to many hundreds of river kilometers [Stauffer et al., 1980; Nimick et al., 1998; Wilke and Hering, 1998]. Studies have shown that As(III) is expected to be low in river water, forming from <1% to 5%, because surface waters tend to be well oxygenated. Although As(III) can predominate in river water (upward from 60%) near inputs from geothermal springs rich in arsenic [Wilke and Hering, 1998; Stauffer et al., 1980], there are no geothermal inputs to Silver Bow Creek to explain the high proportion of total arsenic as As(III) in Silver Bow Creek's surface water. Arsenite is also common in reducing ground waters and makes up 70%-100% of the total arsenic found in some aquifers [Welch et al., 1988; Moore et al., 1988]. Yet in Silver Bow Creek, the presence of -20% of the total arsenic as As(III) in the surface water cannot be accounted for by ground water con- tributions alone. According to mixing calculations using Ca and Mg as conservative elements, no more than 30% of the surface water is derived from ground water input [Nagorski, 1997]. If 30% of the surface water is derived from ground water, where the median As(III) concentration is 2.6/•g L -x, then the maximum concentration of As(III) in surface water that could be contributed by ground water, assuming conser-


Table 1. Molar Ratios of Fe:As in the Surface Water, Hyporheic Zone Water, Bead Coatings (Precipitation Zones Only), and Fine-Grained Bed Sediments

Molar Ratio Fe:As

that solute arsenic may be generated simply by surface water mixing within the reducing conditions found in the hyporheic zone, and ground water is not a necessary component.

Mean Median Minimum Maximum

Surface water (1998 data) 25 16 6 57 Hyporheic zone water 523 402 21 1506

(1998 data) Beads (1995 data) 303 155 38 1161 Bed sediment (<63 gm) 136 134 82 176

(1995 data)

vative behavior, is 0.8 /•g L -•. However, the surface water concentrations of As(III) were between 1.6 and 2.0 /•g L -• (median = 1.8/•g L-•), suggesting that ground water contri- butions alone could not account for the As(III) concentrations in the surface water.

Because the hyporheic zone has higher concentrations of arsenic than the ground water and is generally dominated by surface water (according to the mixing calculations), a more likely explanation for the presence of As(III) in the surface water is that surface water exchanges with the hyporheic zone. When it enters the hyporheic zone, it incorporates the dissolved arsenic and iron generated by the reduction of Fe oxy- hydroxide minerals, and it flushes them into the surface water column. A portion of the dissolved iron oxidizes, precipitates, and sorbs arsenic upon contact with the oxic, slightly basic surface water, as evidenced by the precipitation bands on the bead columns. Yet we hypothesize that not all of the As(III) is immediately oxidized and scavenged by the Fe-oxyhydroxides, and instead a portion travels in the surface water channel for an undetermined amount of time before it oxidizes or flushes

through the hyporheic zone again. Examination of molar Fe:As ratios gives insight into the relative rates of Fe precipitation and As sorption across the streambed-surface water interface. On the basis of the 1998 data, which includes quan- tiffable arsenic concentrations, the median Fe:As ratio in the hyporheic zone water is 402, whereas it is only 16 in the surface water (Table 1). Because iron is far less enriched over arsenic in the surface water, it is evident that arsenic is not removed as rapidly from solution as is iron.

Other researchers have reported similar arsenic cycling characteristics at the sediment-water interface in marine envi-

ronments [Edenborn et al., 1986; Peterson and Carptenter, 1986; Sullivan and Aller, 1996] and in lake bottom sediments [Har- rington et al., 1998]. These researchers have documented enriched solute arsenic concentrations at shallow depths of arsenic-rich bottom sediments and have credited the processes to the redox controlled dissolution of associated iron oxides and

oxyhydroxides. However, these processes have not before been documented in river systems.

The arsenic cycling hypothesis at Silver Bow Creek is cor- roborated further by the sediment microcosm experiment, which showed that solute arsenic could be generated simply by applying reducing conditions to a mixture of surface water and bed sediments. Although the microcosms may not fully represent hyporheic zone conditions, the pH and D.O. levels did mimic field-measured values in the hyporheic zone, and these two parameters appear to be the main controllers of arsenic partitioning. The mixing ratios and the microcosms illustrate

6. Conclusions

The goal of this research was to determine the occurrence and behavior of solute arsenic in the shallow hyporheic zone of a contaminated stream. In Silver Bow Creek, hyporheic zone dissolved arsenic is present in quantity and form different from that found in the adjacent surface and ground water. Total dissolved arsenic and iron are enriched in the hyporheic zone of Silver Bow Creek, and a majority of the total arsenic was found to occur as As(III). We propose that the source for the enrichment is the streambed sediments, which release arsenic as a result of reductive dissolution of iron oxyhydroxides upon burial in the more acidic and reduced hyporheic zone. These chemical processes in the hyporheic zone also may be controlling arsenic occurrence and speciation in the surface water. Estimation of the flux from the hyporheic zone was not possi- ble with this data set because Fe and As removal by precipitation at the streambed-stream interface could not be ade-

quately quantified, although evidence for the process is presented here. These findings have potentially strong implications for environmental monitoring designs, for the health of aquatic organisms, and for our understanding of the geochemistry of hyporheic zones and its controls on surface water in general. In addition, they elucidate the need for inclusion of the hyporheic zone in studies that aim to identify the hydrological components that together control the solute chemistry of surface waters.

Acknowledgments. We gratefully acknowledge Temple McKinnon and Devin Shay for assistance with field work and William Woessner and Tom DeLuca for helpful critique of the project. We also thank C. C. Fuller and two anonymous reviewers, whose recommendations greatly improved the manuscript. The Murdock Charitable Trust pro- vided funding for laboratory equipment used in the 1998 portion of the study, and the Western Mineland Reclamation Center and Geological Society of America funded the 1995 portion.

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(Received January 27, 1999; revised June 28, 1999; accepted June 30, 1999.)

arsenic mobilization in the hyporheic zone of a contaminated stream

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