Environ. Sci. Techno/. 1995, 29, 1789-1795
Metal Behavior during Interaction, Silver Bow Creek, SHAWN G. BENNER,* ERIC W. SMART, AND JOHNNIE N. MOORE Geology Department, University of Montana, Missoula, Montana 59812
This work documents the geochemical character of the hyporheic zone in the bed sediment of a small creek underlain by acidic, metal-rich groundwater. Use of a unique in situ method of sampling the solidphase chemistry (elemental composition of coatings on installed ceramic beads) combined with water chemistry data allowed us to build a more complete picture of the geochemistry during surfacegroundwater interaction. A mixing ratio of 96:4 surface water (pH 7.9-9.1, [Fe] 0.2 mg/L) to contaminated groundwater (pH 4.2 - 4.9, [Fe] 400 mg/L) can explain the composition ofthe -1 m thick hyporheic zone. Accumulations of metals on the ceramic beads support nonconservative behavior predicted by mixing ratios and show partitioning between the solid and aqueous phases. The behavior of metal contaminants in this system is discussed.
Introduction The interface between streams and underlying shallow groundwater is increasingly recognized as a distinct biogeochemical environment. Important transformations occur within this zone that can impact the chemical makeup of both the overlying surface water and underlying groundwater systems. Ecologists have established the importance of this zone, called the hyporheic zone, not only as a biologically rich ecotone but also as an interface that is important to the entire river’s ecology (1). Extensive physical and chemical interaction in coarse-bedded stream channels, between stream water and underlying sediment pore water, results in a zone of mixing (1-6). Triska et al. (2)defined four zones within the bed sediment of a stream channel the channel zone containing surface water, the groundwater zone, and an area of mixing between the two zones composed of the “surface hyporheic” and the “interactive hyporheic” zones. In this subdivision, the surface hyporheic zone (SHZ) is composed of water of almost identical chemical composition to the surface water and the interactive hyporheic zone (IHZ) is a mixing zone composed of between 10% and 98% surface water. The mobility of many ions is altered at the interface between infiltrating surface water and groundwater (7-9). Bourg and Bertin (9)used a physical mixing model to examine the relative conservative behavior of metals during surfacegroundwater interaction. Much of this work has suggested that the interactions between the aqueous and solid phases play an important role in controlling solute behavior. However, fully understanding ion speciation is often hampered by a lack of knowledge of the solid phase present. In beds of streams with coarse-grained bed sediment, the collection of solid-phase data can be quite difficult. In addition, were it be possible to remove an intact core from the bed of a coarse-grained stream, the analysis of the sample can be problematic. Sediments often have a history of weathering and coating accumulation. Extraction and analysis of coatings on sediment grains will necessarily include not only the outermost layer of the coating present but all the underlying layers of material as well. These underlying layers are not necessarily in equilibrium with the surrounding water and may be the product of very different geochemical conditions. The complexity of the coating history is only limited by the age of the grain. The use of an artificial solid matrix has the potential to eliminate problems of collection and history because the design of the artificial solid matrix can allow easy removal from the bed and the solid matrix will only reflect the geochemical conditions during the period in which they are in the sediment. However, the artificial matrix itself does not exactly mimic the physical and chemical characteristics of the natural solid matrix. The specific mineralogy of the sediment can affect adsorption processes (10).It is also possible that existing microbial populations can affect the rate and nature of coating formation. Additionally, equilibration with the surrounding sediment * Present address: Waterloo Centre for Groundwater Research, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada. E-mail address:
[email protected]; Telephone: (519) 885-121 1 X5668; Fax: (519) 725-8720.
0013-936)(/95/0929-1789509.00/0 0 1995 American Chemical Society
VOL. 29, NO. 7, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
1789
may result in initially high rates of metal accumulation on the heads. Despite these limitations, it seems probable that coating accumulationson an artificial solid matrixwill approximate and reflect the solid phase in equilibriumwith the aqueous species present. Clearly, the collection of any solid-phase data can only improve our understanding of the geochemical system. We have combined a unique, in situ, solidphase samplingtechniquewithwatersampling and analysis in an attempt to provide a more complete picture of the behavior of chemical constituents in a stream-groundwater system contaminated with mine-related metals. The production of acidic, metal-rich waters from the oxidation of sulfide minerals found in mine tailings is well documented (11.12). Thesewatenoftencontainhighlevels of Fe and Mn in addition to elevated amounts of the trace elements As, Cd, Cu, Pb, andZn. FeandMncanprecipitate from these waters, forming oxide mineral phases. Trace metal contaminants are often associated with these oxides by the processes of coprecipitation and adsorption (1317). The oxidationof Fe and Mn to form oxide precipitates is induced by an increase in pH or pE. The toxicity of these waters is, in large part, directly related to the presence of the metals in the aqueous phase (18). Therefore, understandingthe processes controlling metal behavior in waters in natural settings is critical to any meaningful assessment of the impact and remediation of acid mine drainage problems. Study Site. Silver Bow Creek, located in the headwaters of the Clark Fork River drainage of western Montana, provides an opporNnity to study the geochemical processes at the interface of acidic, metal-rich groundwater and relatively uncontaminated surface water (Figure 1). Three hydrostratigraphic units are present within the flood plain along Silver Bow Creek. The base of the aquifer (2-4 m below ground surface) is bound by a clay-rich layer of undetermined thickness that is interpreted to be the weathered horizon overlying a volcanic tuff. Above the clayeylayer,the aquifer is -2 m thick and composed mostly of coarse-grained sand, gravel, and cobbles. The unsamated zone is 0-2 m thick and is composed of a complex stratigraphy of silt, fme to coarse-grained sand, and gravel. These flood plain deposits are interlayered and intermixed with mine tailings enriched in As, Cd, Cu, Fe, Ph, and Zn (19).
At the location of this study, the near-surface aquifer is locally contaminated with acidic, metal-rich water. This contaminationislikelytheresultofthe oxidationofsulfides in the vadose zone and the subsequent migration of acidic, metal-rich water downward into the aquifer. The groundwater has a pH of -4.5, alkalinity of zero, and dissolved oxygen less than 1 mg/L. Metal concentrations are high with elevated levels of Cd, Cu, Fe, Mn, and Zn. The gmundwater is significantlymore reduced and acidic than the surface water. The pH of the surface water is near-neutral to basic (pH = 7.9-9.1). Alkalinity and dissolved oxygen concentrations are high (1.4 mequiv/L and 0.13 mglL, respectively). Although metal concentrations in the surface water are low, as expected given the high pH and dissolved oxygen levels, short periods of highlevel contamination do occur. Rain storm runoff events, lasting on the order of minutes to hours, result in an influx of acidic, metal-rich water. Water table response to changes in stream stage and potentiometric maps of the water table show that the 1790 m ENVIRONMENTAL SCIENCE & TECHNOLOGY I VOL. 29, NO. 7.1995
Mlles Crossing
FIGURE 1. Location of the study site cross section and a potentiometric map of "semi-Ready-state" conditions on July 1. 1994. Groundwater contour intervals (dashed lines1 are 0.15 m.
groundwaterandsurface water systems are well connected. At the study site, groundwater is flowing nearly perpendicular to the flow direction ofthe creek. Though the flow direction within the bed sediment of the creek itself is not clearly defined, the flow field suggests that bed sediment pore water may receive input from the upgradient groundwater, the overlying surface water, or both.
Methods Field. Tubes filled with 2 mm diameter aluminum silicate ceramic beads were installed vertically into the creek bed to collect mineral coatings. Bead tubes were constructed with 1.75 m long polycarbonate tubing in. o.d., ' 1 4 in. i.d.)whichwasslotted(l mm) ontwosidesat l/zcmintewals using a band saw. Beads (2 mm average diameter) were theninsertedintotheslotted tubingwitheachheadsection (15 cm) separated by a solid piece of polycarbonate rod. The spacers provided divisions for sampling purposes and prevented water from flowing vertically within the head tube. The bead tube was then connected as a package with a nonslotted tube for additional structural support.
groundwater now dlrectlon
t
-
2
0 rneten
0= PH in water 0= Fe concentration in water (mg/U
FN = Flezometer Nest Bs = 6 Week bod %t
FIGURE 2. Cross-sectional profile of pH values and Fe concentrations in the water. Hash-marked area defines transition zone. Transition zone chemistry is intermediate between surface and groundwater. Vertical exaggeration used to show vertical differentiation.
This bead tube set was then acid cleaned in 50% reagentgrade HCl for 1 h and rinsed repeatedly with sterilized, deionized water until a pH of 5 was reached. The bead tube set was transported in a sterile container to the field site. The tube was installed by first pounding a 2 in. steel pipe, with a solid rod inside, into the sediment, removing the inner steel rod, inserting the bead tube into the now open pipe, and finally removing the pipe, allowing the sediment to collapse back around the bead tube set. One bead tube set was then left in the sediment for 6 weeks, a second for 15 weeks. When each bead tube set was removed, it was immediately rinsed with sterile, deionized water, cut into 15 cm sections, and stored for transport in either an oxic or anoxic chamber, depending on the chemical environment of the depth from which they were removed. In addition to the bead sites, groundwater access tubes were constructed of 3 / 8 in. (0.d.) polyethylene tubing with 5 cm of the tip slotted and covered with a fine mesh screen and installed as a nest. They were placed at 30 cm intervals into the creek bottom using the method described above for bead tube installation. During water sampling, a small sample volume was removed to avoid averaging and to delineate vertical variation as much as possible. Sampling was accomplished using a 60 mL syringe. One tube volume was purged and then a sample was taken. Samples were filtered through 0.45 ,urn filter and acidified with concentrated trace-metal grade HC1 to a pH less than 2 for cation analysis. Samples for anions were also filtered but not acidified. Alkalinity samples were collected unfiltered and unacidified. Dissolved oxygen and pH were measured immediately upon removal. Though the measured values for dissolved oxygen correlated well with data collected using an enclosed flow-throughcell, the sampling method requires that dissolved oxygen values be regarded only as a trend in relative oxygen concentrations. Nested wells were also installed -1 m away from the creek to sample groundwater approaching and leaving the creek (Figure2). These wells were also sampled as described above. Because of the need to take small sample volumes, it was not possible to purge until chemical parameters were stable or to collect duplicate samples. Laboratory. Upon arrival in the laboratory, the bead tube sections were dried at 70 "C for 24 h. The beads were
removed from each section and placed into a beaker. Any accumulated alga was removed by gently breaking it off and sieving to separate it. The coatings were extracted from the beads using aqua regia digest (3:1, HCl-HNOd. This solution was then analyzed for As, Ca, Cd, Cu, Fe, K, Mg, Mn, Na, Pb, and Zn using a Thermo Jarrel-AshAtomComp 800 ICP. These results are reported as micrograms of element per gram of beads. Water samples were analyzed for cations using standard techniques on the same ICP. Anions (nitrate, sulfate, chloride) were analyzed using standard techniques on a Dionex 2000 ion chromatograph (IC). Water chemistry results are reported in milligrams per liter. The precision of the ICP analysis ranged within &8%for all elements with the exception of Zn, Na, and K, which ranged within f22%. Spikes were recovered within &17%of actual value for all ions. The precision of the IC analysis ranged within f 2 % . Spikes were recovered within f 3 R of actual value. Bead blanks exhibited significant concentrations of Al and Si, and data for these metals on the beads have been deleted. Bead blanks exhibited insignificantlevels of contamination for all other elements. Fe speciation between Fez+ and Fe3+was determined by a ferrozine colorimetrictechnique (20).Total alkalinity was determined using a Hach colorimetric technique.
Results and Discussion The Hyporheic Zone. The pH profile of the water within the creek and downward in the sediment clearly defines the depth of the hyporheic zone. The profile appears to fit well with the profile proposed by Triska et al. (21, showing four distinct zones. The surface water has a pH of 7.8-9.1. The surface hyporheic zone, extending from 0 to - 15 cm, has a pH of 7.8-7.9. The interactive hyporheic zone, extending from -15 to -80 cm, has a pH of 6.2-6.9. The groundwater zone, below -80 cm, has a pH of 4.4-4.9 (Figure2). Pore water chemistry in the IHZ, to a depth of 80 cm in the stream bed, is clearly different from both surface and surrounding groundwater. This water, with a average pH = 6.5, average alkalinity of -70 mequivll, and average dissolved oxygen concentrations of -5 mglL, chemically falls in between surface and groundwater. The lateral extension of the hyporheic zone appears to be influenced by the regional groundwater flow field. Water from wells located 1 m from the upgradient side (relative VOL. 29, NO. 7,1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
1791
TABLE 1
Mixing Ratio Calculations between Surface Water and “Average” Groundwater To Achieve Average Interactive Hyporheic Zone (IHZ) Water mixing ratios AI
surface water (mgiL) avgroundwater (mg/L) av IHZ water (mg/L) % groundwatera % depletedb sw(0.94)
g~(0.06)~
0.14 33.17
0.00
n/ad -100 0.14 1.85
Ca
Cd
Cu
Fe
K
Mg
Mn
0.00 0.25 0.10 4.27 9.97 0.81 0.55 19.12 365.0 12.29 33.50 28.12 0.01 0.35 0.25 4.17 8.79 1.91 nia 1.54 1.28 n/a n/a 4.02 nia -22.8 -72.4 -70.2 -98.8 -11.7 -22.1 -18.3 42.96 0.00 0.10 0.24 4.03 9.41 0.76 7.87 0.03 1.07 20.37 0.69 1.87 1.57 45.50 141.0 39.25
Na
Si
23.30 42.63 22.65
14.00 38.93 11.90
Zn
0.74 54.05 2.36 nia 3.05 n/a -7.1 -22.7 -36.4 22.00 13.22 0.70 2.38 2.17 3.02
CI
NOn
SO.,
13.20 1.50 67.4 20.37 0.00 1487 13.60 1.30 81.8 5.58 13.33 1.01 0.0 -8.2 -44.2 12.46 1.42 63.6 1.14 0.00 83
a Amount of groundwater that would need to be mixed with the surface water to achieve the concentration of that constituent found in the IHZ. Percentage of each constituent that must be removed to achieve theconcentrationsfound undertheassumption of 5.58% groundwater contribution 4 ) ) groundwater (gw(0.06)),respectively, during conservative (based on CI mixing ratio). The amount contributed by the surface water ( ~ ~ ( 0 . 9 and mixing. n/a, not applicable.
to groundwater flow) shows no obvious chemical influences from the creek, while the wells located 1 m from the downgradient side of the creek do show influences of the creek (Figure2). The upgradient bank of the creek is coated with a bright orange Fe-rich crust at the water table, indicating that metal-rich groundwater is enteringthe creek on this side. A well located 10 m away from the creek on the downgradient side shows no measurable influence of the creek. These wells constrain the hyporheic zone to less than 1 m on the upgradient side and at least 1 but less than 10 m on the downgradient side. Mixing Model. The above profile suggests that the hyporheic zone water is the result of mixing between surface water and contaminated groundwater. Though it is not known with certainty which ions in this system behave conservatively, calculating mixing ratios from ion concentrations in the surface and groundwater can identify potential conservative constituents (Table 1). These calculations were done using the equation
where C,, is the concentration of constituent in surface water, C, is the concentration of constituent in the groundwater, ch is the concentration in interactive hyporheic zone water, and x is the percent of groundwater in the hyporheic zone water. Constituents with negative values in Table 1have concentrations in the hyporheic zone that are lower than the surface water. For these constituents, the mixing ratio was not calculated. Chloride has been used as a natural, conservative tracer in surfacegroundwater interaction ( 2 , 5 , 9 ) . Chloride does appear to behave the most conservatively of all ions measured. Mixing ratios based on chloride indicate that the interactive hyporheic zone water (-20 to -80 cm depth) is composed of about 6% groundwater and 94% surface water (Table 1). Mixing ratios calculated from Mn concentrations in the waters indicate 4% groundwater and 96% surface water, which is in good agreement with the chloride numbers, indicating that Mn is also behaving conservatively. McKnight et al. (21)also found Mn behaved conservatively in mixing between acidic and pristine surface waters. In contrast to the findings at Silver Bow Creek, these authors found that S042--behaved conservatively. It is possible to calculate the degree to which a given constituent is depleted in the hyporheic zone water, relative to the concentration expected, given only conservative mixing. These calculations were done using the equation 1792
ENVIRONMENTAL SCIENCE &TECHNOLOGY 1 VOL. 29. NO. 7.1995
where C,, is the concentration of constituent in surface water, ,C is the concentration of constituent in the groundwater, c h is the concentration of constituent in the interactive hyporheic zone water, and y is the percent the constituent is depleted relative to the concentration predicted by conservative mixing. This number reflects the degree of nonconservative behavior (precipitation or adsorption) for that constituent. For example, while Fe and Cu (-99% and -70%, respectively) show a high degree of nonconservative behavior, K and Na (-12% and -7%, respectively) exhibit depletion near the error of measurement and are considered to be acting conservatively. We also compared the amount contributed by the surface water (0.94(Cs,),Table 1)and groundwater (O.O6(C,,), Table 1) with the amount found in the IHZ. If the calculated amount contributed by the groundwater or surface water is greater than the total actually found in the IHZ water, metals are being depleted from that aqueous-phase portion. Calculated concentrations of Cd, Cu, Fe, Mn, and Zn in the groundwater portion are higher than those found in the IHZ, indicating that all these elements are being depleted from the groundwater portion. Calculations also indicate that Ca and Mg are undergoing removal from the surface water portion. Since the degree of mixing required to account for the IHZ water chemistry is very small, it seems possible that mixing may not be occurring. Other mechanisms do not, however, provide a more plausible explanation. The sequential consumption of electron acceptors by oxidizing organic matter in natural groundwater (7,8,22-24) could produce increased Fe and Mn concentrations by the partial reduction of oxides of Mn and Fe. This is unlikely because the reduction of Fe is not favored, given the oxygen and nitrate concentrations present in the water, and because the reduction of nitrate, manganese oxide, and iron oxide are acid-consuming reactions. The pH can be lowered by the production of C 0 2from the oxidation of organic matter, but this does not explain the decrease in alkalinity found in the hyporheic zone. The documented presence of oxiderimmed sulfide grains in the sediment of the Clark Fork (25) could provide a source of acidity and metals by slow oxidation, but this reaction sequence would result in the net production of S042-, which is depleted in the IHZ. Correlationwith the Solid Phase. Solid-phasedata from the bead coatings support the above mixing model and
0
Dissohred Oxygen (mfl) 1 2 3 4 5 6 7 8
9
Nitrate (mgiL) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
0
CI (mg/L) 10 15
5
20
25
Surface water - 1 1 1 1 - - - - 1 -
SHZ
1 1 1 1 1 1 1 1 1 1 1
IHZ
Goundwater
1
0
1
2
3
4
5
6
7
8
.
.
.
.
4
1
.
1
.
0 102030405060708090100
0
Alkalinity (meq/L)
PH
lo00
500
1500
2000
Sulfate (mg/L) Surface water 1 1 1 1 1 - 1
SHZ
1 - 1 1 - 1 1
IHZ 1 - 1 1 - 1 1
Goundwater
0.001 0.01 0.1
1
10
100 1000 1
10
100
1000 0.1
1
10
100
FIGURE 3. Vertical profiles of water chemistry within the bed sediment of Silver Bow Creek. Dashed lines show geochemically defined divisions between surface water, surface hyporheic zone (SHZ),interactive hyporheic zone (IHZ), and groundwater. Oxygen values are shown to represent trends only.
correlate well with the aqueous-phase data. Metal concentrations in bead coatings on the 6 week bead tube match the above zonation within the hyporheic zone. In general, concentrations of Ca, Cd, Cu, Fe, K, Mg, Mn, Na, and Zn in the bead coating are directly related to the pH and inversely related to the concentrations of metals in the water. In the groundwater, where metal concentrations are high in the aqueous phase and pH is low ([Fel e~300500 mglL, pH = 4.4-4.91, the concentrations on the beads are low ([Fel a: 5 pglg of bead). In the surface water and SHZ,which have low metal concentrations in the aqueous phase and high pH ([Fel 0-0.2 mglL, pH = 9.1-7.91, concentrations of metals on the beads are high ([Fel 60-160 pglg of bead). Within the IHZ, pH is 4.4-4.9 and metal concentrations are moderate ([Fe]= 5-60 pglg of bead). The relative degree of nonconservative behavior by all metals during mixing (Fe > Cd > Cu > Zn > Ca > Mg > Mn) correlates well with the relative amount accumulated on the beads (Fe > Cu > Zn > Ca > Mg > Mn > Cd). Only Cd does not correlate well, and this can be attributed to the error associated with the measurement of the neardetection level concentrations throughout the system. The apparent conservative behavior of Mn within the IHZ is supported by the low levels of accumulation on the beads relative to the aqueous-phase concentrations (lessthan 1.5 pglg of bead, aqueous-phase concentration of -2 mglL) (Figures 3 and 4). The 15 week bead set, which was installed -2 m away from the 6 week bead set and water samplingwells, exhibits
a profile similar to the 6 week bead set but indicates a smaller hyporheic ZOne(=60 cm). This suggests that there is a degree of local and or temporal variability within the stream bed resulting from changes in the hydrologic or geochemical regimes. Behavior of Inorganics. Relative concentrations of metals on the beads and in the aqueous phase appear to fit well with previously established theoretical and fieldrecorded behavior. Iron speciation data indicate that the soluble Fe in the IHZ and groundwater is F$+. The presence of extensive, orange, Fe-rich coatings on stream sediment and on the beads indicates that Fe is precipitating as iron oxides in the stream (25,261.High Fe concentrations and orange staining on the beads in the hyporheic zones indicate that precipitation of iron oxides is occurring to a depth of -80 cm (Figure 4). Below this point, Fe concentrations are high in the aqueous phase and low in the solid phase. This suggests that the boundary between the hyporheic zone and groundwater is also the geochemical boundary of iron oxide stability. This fits well with theoretical and field data on iron oxide precipitation (16,27). In the sediments of Silver Bow Creek, this phase change may be induced by a change in redox potential, pH, or a combination of the two, since both parameters decrease with depth. Manganese shows high levels of accumulation on the beads in the surface water and the SHZ but, unlike Fe, little or no accumulation on the beads in the IHZ (Figures 3 and 4). Studies of sediment in Silver Bow Creek have indicated that solid-phase Mn is primarily manganese oxides (25, 26). Aqueous manganese concentrations are high in the VOL. 29. NO. 7. 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
1793
0
50
pg/gm bead 100 150
200
20 ----(
0
Surface water SHZ
-20
-5
Y
(A)
-40
60
$ -80 CI
-----------
-1OO
-120
Goundwater
-140
see surle below
.1M)
0 pg/gm bead
0
50
100
0 5 10 1 5 2 0 2 5 3 0 3 5 4 0 45 50
pg/gm bead
150
200
IHZ
Goundwater
Fg/gm bead
)rg/gm bead
pg/gm bead
FIGURE 4. Vertical profile of bead coating chemistrywithin the bed sediment of Silver Bow Creek. Dashed lines show geochemically defined divisions between surface water, surface hyporheic zone (SHZ), interactive hyporheic zone (IHZ), and groundwater: (A) accumulations of the 6 week bead set; (B) accumulations on the 15 week bead set.
groundwater and the IHZ and low in the surface water and SHZ. This indicates that the manganese oxide stability boundary lies at the transition between the surface water and the hyporheic zone. Other researchers have documented a similar relationship between Fe and Mn with respect to stability of oxide phases under changing redox and pH conditions (28, 29). Cd, Cu, and Zn are mostly partitioned into the solid phase in the SHZ and IHZ and into the aqueous phase in the groundwater (Figures 3 and 4). Mixing ratios indicate that these elements are all being removed from solution within the IHZ. The removal of these elements may be the result of the precipitation of solid-phase minerals, or they may also be removed from the aqueous phase at the pH range found in the IHZ by adsorption on iron oxyhydroxides (15,30,31). Measurable concentrations ofAs and Pb were found on the beads (1-7 and 0.5-3.5 yglg of bead, respectively) in the surface water and the IHZ indicating that they are mobile in this system, despite the fact that these contaminant metals were not detected in the aqueous phase sampling. The profile of alkalinity concentrations in the water mimics the profile of pH (Figure3). It is likely that alkalinity is being consumed by acid from the mixing with acidic groundwater. The transition for alkalinity speciation appears to correlate with the lower boundary of the IHZ. Sulfate shows a high degree of depletion in the IHZ relative to conservative mixing (44%). There are no obvious mineral species that can account for the missing sulfate. Unfortunately, sulfate concentrations were not measured on the ceramic beads. 1794
ENVIRONMENTAL SCIENCE & TECHNOLOGY, VOL. 29, NO. 7 , 1995
The decrease with depth of oxygen and nitrate concentrations is likely being driven by the oxidation of Fe and/or organic matter (Figure 3). The consumption of oxygen and nitrate as surface water enters the groundwater system has been well documented (8,23). At the base of the bead tubes, metal concentration profiles exhibit a high for a number of metals. Given the lack of oxygen in the groundwater, it seems unlikely that oxides could be forming at this depth. These high metal concentrations on the beads may be the result of sulfide formation. Though the water chemistry does not indicate that metals are precipitating from the groundwater at this depth (Le., the soluble concentrations are not lower), a profile of bacterial populations shows the presence of sulfate-reducing bacteria at this depth (32). This suggests that sulfides are forming, perhaps at rates that do not significantly reduce soluble concentrations. The major redox couples are clearly not in equilibrium in the hyporheic zone (reduced Fe and Mn in the presence of oxygen and nitrate). This is not surprising considering that the hyporheic zone represents the boundary between two very distinct geochemical environments. For this reason, equilibrium geochemical modeling was not attempted. Flow beneath the Stream. Mixing calculations indicate that there is some component of downward flow into the bed sediment. The two-tiered model ofthe hyporheic zone proposed by Triska et al. (2)is supported by the aqueousand solid-phase data collected at Silver Bow Creek and suggests a conceptual model of the flow field composed of two scales of mixing. The larger scale, IHZ mixing, may be
due to “reach-scale” variations in hydraulic head differentials between surface and groundwater. Reach-scale variations have been documented by Harvey and Bencala (6‘). The smaller scale mixing in the surface-hyporheic zone may be the result of “cobble-scale” or “bed-formscale” pressure variations in the flow regime at the surface water-pore water interface. Pressure variations induced by bed forms have been documented by Thibodeaux et al. (33).
The boundary between the IHZ and flood plain groundwater is very distinct. Metal concentrations in the water vary by as much as 3 orders of magnitude over a distance of less than 20 cm. In addition, the bead sets exhibit high accumulation of Fe at this contact and orange staining on the beads indicates that this transition is less than 5 crn in width. It is difficult to picture producing the chemical profile of the lower boundary of the hyporheic zone by gradational dilution or mixing. Therefore, we propose that the reach-scale component of downward flow dissipates at the -80 cm depth. At that point, the water in the hyporheic zone is moving parallel to the plane of the regional groundwater flow system.
Conclusions The addition of solid-phase data collected on an artificial solid matrix has enhanced physical and geochemical conceptual models of the interface between the surface and groundwater systems along Silver Bow Creek. We believe that this system of solid-phase collection may be applicable to other settings where the collection of solidphase data would otherwise be unattainable. Mineralogical data from the bead coatings would greatly enhance their usefulness. While the geochemical setting at Silver Bow Creek is quite different from previous work, the profile zonation is remarkably similar to the profile found by Triska et ai. (21,supporting the applicability of their model. In this study we have shown the importance of the hyporheic zone in attenuating metal movement from the groundwater into the surface water system. The high levels of metal accumulation on the beads in this zone suggest that the hyporheic zone may be acting as a sink for metals and metal loading to the bed sediment of the creek may be significant. Despite the fact that the surface water has relatively low metal concentrations, riparian biota that are dependent of the hyporheic zone may be adversely impacted by the flux of acidic, metal-rich waters into this zone.
Acknowledgments We acknowledge Jim GaMOn and Bruce Wielinga for their contribution to the bead tube idea. We thank Lynn Biegelsen, for laboratory analysis, and Chris Brick, for iron speciation data and review and discussion of the manuscript. We also thank our reviewers for their thoughtful comments. This work was funded by The Western Mine Land Reclamation Center and the University of Montana.
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Received for review September 7, 1994. Revised manuscript received March 29, 1995. Accepted April IO, 1995.@
ES940560W @
Abstract published in Advance ACS Abstracts, May 15,1995.
VOL. 29, NO. 7, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
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