SPEA UNDERGRADUATE HONORS THESIS
Geochemical Processes in a Constructed Wetland Receiving Outflow from a
Sulfate-Reducing Bioreactor Used to Treat Acid Mine Drainage
Mary Nicholas
Spring 2014
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Abstract
A common legacy of coal mining prior to the enactment of the Surface Mining Control and Reclamation Act of 1977 is acid mine drainage (AMD), which forms when mining waste, containing metal sulfides (e.g. pyrite), are exposed to infiltration of oxygenated water. AMD is characterized by a low pH (<4) and high concentrations of sulfate and dissolved metals, such as iron and aluminum. Over the past several decades many treatment options have been developed ranging from simple dilution to bioremediation facilitated by constructed wetlands and bioreactors. Past experience has indicated that using more than one method of treatment at an abandoned mine land site helps achieve more efficient, complete remediation. In a remediation effort at an abandoned mine lands site in Pike County, IN, a sulfate-reducing bioreactor (SRB) was constructed to treat AMD with average pH of 3 and concentrations of 3200 mg/L sulfate, 90 mg/L iron, and 130 mg/L aluminum. The SRB outflow averages pH 6.4, has a temperature increase of 6°C relative to the inflow, and 4 mg/L dissolved oxygen. The average outflow concentration of sulfate has been reduced to 2500 mg/L, and the concentrations of both iron and aluminum have decreased to 1 mg/L. A wetland system was constructed to further treat and equilibrate the discharging SRB water. Samples of water collected at the wetland outlet indicated a further increase of pH to as high as 7.2, a temperature increase to only 3°C greater than that of the raw AMD, increased dissolved oxygen to 9 mg/L, and further reduction in the concentration of sulfate to 2200 mg/L. These documented improvements in overall water quality are presumed to be a result of mineral precipitation due to oversaturation and subsequent sequestration as solids within the sediments of the wetland. In order to test this hypothesis, water was collected from four sampling stations within the wetland in the beginning and end of the growing season. The water samples were analyzed in the field for basic water chemistry and in the laboratory for sulfate, iron, aluminum, and other metals. Results of the analyses were subjected to geochemical modeling using PHREEQC to develop saturation indices for various mineral phases that account for the significant improvements in water chemistry. The results of the modeling indicated that aluminum hydroxide, alunite, aragonite, calcite, dolomite, gibbsite, goethite, gypsum, hematite, and rhodochrosite have the potential to precipitate out in the wetland system, which would explain the decrease in the concentration of sulfate, aluminum, iron, and calcium.
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Introduction
Coal is an important energy source that has been actively mined in the Illinois Basin for nearly two
centuries. In the mid-twentieth century, demand for coal increased rapidly, but there was little
regulation, so surface mines were left abandoned and pyrite spoils were exposed to weathering
processes that resulted in acid mine drainage (AMD). After the creation of the Surface Mining Control
and Reclamation Act of 1977, the coal mining industry became more regulated and the environmental
impacts were decreased. Yet, the abandoned mine lands from before 1977 still needed to be
remediated.
AMD forms when mining waste and tailings, which contain metal sulfides (e.g. pyrite) are
exposed to oxygen and water (Robb and Robinson, 1995; Treacy and Timpson, 1999; Michalková et al.,
2013). AMD is characterized by a low pH (1.5-3.5) and high concentrations of sulfate and dissolved
metals, such as iron, aluminum, manganese, lead, zinc, copper, nickel, and cadmium (Drury, 1999; Steed
et al., 2000). There are many methods to treat AMD, including various combinations biological, abiotic,
active, and passive systems. Factors such as location, climate, water chemistry, and hydrologic budget,
need to be assessed in order to determine which treatment options are best for a given abandoned
mine land site. Often it is beneficial to employ multiple systems aimed at optimally treating the AMD to
improve water quality.
AMD has three main treatment options: temporary remediation, active treatment, and passive
treatment (Robb and Robinson, 1995; Ford, 2003; Johnson and Hallberg, 2005). Passive treatment
methods are preferable because of their cost-efficiency and potential for long term performance
(Johnson and Hallberg, 2005; Sheridan et al., 2013). Two of the most popular passive treatment systems
are constructed wetlands and bioreactors. Constructed wetlands are artificial wetlands that are created
to mimic their natural counterparts by combining physical, chemical, and biological processes to remove
contaminants in the water and neutralize pH. Physical processes include gravitation, substrate
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adsorption, and flocculation; chemical processes include oxidation of metals and metalloids; biological
processes include nitrogen fixation, carbon fixation, iron reduction, and sulfate reduction. Bioreactors
are subterranean systems that utilize organic matter to maintain anaerobic zones that promote
reductive processes (Johnson and Hallberg, 2005).
The focus of this research is an abandoned mine lands site where an artificial wetland was
constructed immediately downstream of a sulfate-reducing bioreactor. The water entering the
Blackfoot bioreactor is highly acidic (average pH of 3), and contains high concentrations of sulfate (3200
mg/L), iron (90 mg/L), and aluminum (130 mg/L). After treatment by the bioreactor, the water is near-
neutral (pH ~6.5), with an average temperature increase of 6°C and lowered dissolved oxygen (4 mg/L),
and decreased concentrations of sulfate (~2500 mg/L), iron and aluminum (both 1 mg/L or less).
However, this pretreated water is still not ideal and requires more treatment in the wetlands, where it
mixes with untreated AMD before ultimately leaving the site. The wetland was only constructed for the
purpose of polishing and equilibrating the water leaving the bioreactor, though currently it is also
expected to neutralize the untreated AMD seep. The purpose of this research is to document the
changes in water chemistry through the bioreactor and constructed wetland at the beginning and end of
the growing season in order to evaluate the overall performance of the coupled treatment system
during its most productive time.
Conceptual Model and Hypotheses
Wetlands can be classified as either aerobic or anaerobic based on pH and their treatment methods
differ based on the amount of alkalinity present. Water that is deemed net alkaline should have at least
1.8 mg/L alkalinity per 1 mg/L dissolved iron and should be treated with an aerobic wetland that
encourages natural oxidation (Robb and Robinson, 1995; Smith et al., 2001). The Blackfoot wetland has
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an inflow of 40 mg/L alkalinity per 1 mg/L dissolved iron which makes it net alkaline, necessitating the
construction of a shallow aerobic wetland containing aquatic vegetation and surface flow.
Aerobic wetlands use a combination of chemical, physical, and biological processes to remove
contaminants from the water and neutralize the pH. The main chemical treatment process involved in
aerobic wetlands are precipitation of metals as metal hydroxides through hydrolysis (Robb and
Robinson, 1995). The hydrolysis reaction releases H+ ions, which increases acidity and lowers pH; thus,
decreasing pH can be an indication that hydrolysis is causing precipitation of metal hydroxides. Physical
treatment includes trace metal removal through various means including co-precipitation, adsorption,
complex formation with organic compounds, sorption to cell walls of plants, ion exchange, uptake by
organisms, and binding to ligands (Smith et al., 2001). All of these processes allow metals that are
present in the AMD in minute amounts to be effectively removed from solution without undergoing any
major precipitation reactions of their own. Direct precipitation of trace metals seldom occurs because
concentrations are very low and far below their saturation point. Aquatic plants can contribute to
physical removal in aerobic wetlands by slowing water flow, thereby allowing for longer residence times,
and thus more time for precipitation reactions to occur (Brix, 1997; Cheng et al., 2002). Plants also
contribute to biological processes by increasing soil hydraulic conductivity, up-taking nutrients, and
increasing dissolved oxygen (Brix, 1997).
All of the processes discussed above are likely to occur when an aerobic wetland is treating pure
AMD. However, the Blackfoot wetland is treating a combination of AMD and bioreactor outflow. The
acid seep may alter the alkalinity of the wetland, preventing aerobic treatment and pushing the wetland
into an anaerobic state that might require biological treatment by sulfate-reducing bacteria. Also, the
bioreactor has already significantly reduced the amount of iron and aluminum entering the wetland,
making it unlikely for major chemical precipitation through hydrolysis. It is more likely that only some
hydrolysis will occur and co-precipitation will be responsible for removing the minor amounts of residual
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trace metals. It is likely that plants will be of critical importance in this system because of their ability to
slow water flow, increase soil hydraulic conductivity, uptake nutrients, and increase dissolved oxygen.
On the other hand, increased acidity due to AMD seeps or decreased bioreactor performance can cause
previously sequestered minerals to become remobilized.
I hypothesize that only minor additional metal removal will occur in the study wetland at the
Blackfoot site because most metals are in trace concentrations by the time water leaves the bioreactor.
However, precipitation-dissolution reactions should still play major roles in mineral equilibrium. I also
anticipate that the acid seep will alter the balance of the minerals found in the wetland causing them to
fluctuate between solid and dissolved form. A preliminary test of these hypotheses is provided by
collecting and analyzing water samples from the wetland at its inlet, outlet, and in the vicinity of the acid
seep and mixing zone, during the two seasons of greatest functionality (spring and summer).
Methods
Study Area
The Blackfoot Abandoned Mine Lands (AML) Reclamation Site is located 3 km south of Augusta, Indiana
(Figure 1). The land was surface mined for coal from 1928 to 1950 and left in a derelict state prior to the
enactment of SMCRA. Several reclamation and remediation projects have been implemented on the
site since 1988, each reducing the negative impacts of past coal mining activities (Stacy, 2012). The
most recent treatment project uses passive treatment systems to accomplish remediation of relic AMD.
Untreated AMD enters the system (Figure 2) through a sulfate-reducing bioreactor (SRB) on its southern
end (SRB In). The discharging water from the bioreactor (SRB Out) then enters a system of wetland cells
that are intended to polish the water before it flows into a tributary of the Patoka River at the northern
end of the AML site. In cell 1 of the wetland, untreated AMD (C1 Seep) mixes with the treatment water
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from the bioreactor. The combined water flows throughout the remaining cells until it reaches the
wetland Outlet, where it enters the ditch that drains into a local tributary of the Patoka River.
Field Methods
Water samples were collected in the field and filtered using 0.45 μm filters before field analysis or
storage in Nalgene bottles. The bottled samples were kept in a cooler until ready for laboratory
analysis. Field analyses included basic water quality parameters, sulfide, and alkalinity. Water quality
parameters including temperature, conductivity, dissolved oxygen, pH, and ORP were measured by a
Sonde 600 XL multi-parameter quality monitor (YSI, Inc.). Sonde data was collected per USGS protocol
for collecting field parameters for water samples (Wagner et al., 2006). Sulfide concentration was
analyzed with the Hach spectrophotometer model DR/2010 and Standard Method 8131. Alkalinity was
determined using the Hach digital titrator Model 16900 and Standard Method 8201.
Laboratory Methods
Laboratory analyses included sulfate, ferrous iron, acidity, and metals. Laboratory determinations of
sulfate were achieved using a Hach spectrophotometer model DR 2700 and Standard Method 8051.
Ferrous iron was determined using two methods depending on the sample concentration: for low
concentration samples, I used the Hach spectrophotometer model DR 2700 and Standard Method 8146;
for the remaining samples, I used a potentiometric titration using a Cerium (IV) solution as outlined in an
analytical chemistry text book (Peters et al., 1974). Acidity titrations were run on an automated titrator
with sample changer as described by the U.S. Geological Survey (Rounds, 2012). The metals Ca, Mg, Fe,
Mn, Al, K and Na were all analyzed by EPA method 200.7 using an ICP-AES instrument at a commercial
laboratory in Indianapolis after samples were filtered, preserved with nitric acid, and delivered to the lab
(US EPA, 1994).
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Computer Methods
Using basic water quality parameters and chemical concentrations for sulfide, alkalinity, sulfate, iron,
and other metals, I was able to determine saturation indices using the aqueous geochemical modeling
program PHREEQC. One of the functions of the program is to model speciation of inorganic constituents
for saturation-index calculations (Parkhurst and Appelo, 2013). Saturation indices were displayed by the
program as either a positive (supersaturated) or negative (undersaturated) with respect to the solid
phase of given minerals.
Saturation index (SI) is calculated as a function of the ion activity product (IAP) and the
equilibrium constant (K) in the equation
SI = log (IAP
K)
(Schwartz and Zhang, 2003). When the mineral is in equilibrium with the system, the SI is zero.
Supersaturation is indicated by a positive SI and undersaturation is indicated by a negative SI. A mineral
in equilibrium has balanced precipitation and dissolution reactions so that the concentration of the
mineral in the dissolved and solid phases does not change with time. A supersaturated mineral has the
potential to precipitate, but has no capacity to dissolve. An undersaturated mineral has the potential to
dissolve, but has no capacity to precipitate.
Results
Basic Water Quality Parameter Results
Results of the water analyses are presented in Tables 1 and 2. The August samples (Table 2) had higher
temperatures (~2°C warmer), lower dissolved oxygen concentrations (~4 mg/L lower), higher pH (~0.4
units higher), more alkalinity (~130 mg/L more), and lower dissolved iron concentrations (~7 mg/L
lower). Overall treatment of sulfate by the entire system resulted in a reduction of concentration from
3100-3300 mg/L to 2200 mg/L in May and August. However, the bioreactor alone showed higher
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treatment potential for sulfate in August (decreasing sulfate by 1,000 mg/L compared to 600 mg/L in
May).
Though some differences were found between the two sampling periods, the averages found for
the two seasons together show similar results (Table 3). Untreated AMD (SRB In) had an average pH of
3 and concentrations of 3200 mg/L sulfate, 94.3 mg/L iron, 134 mg/L aluminum, and 7.5 mg/L dissolved
oxygen. Treatment by the bioreactor (SRB Out) increased pH to an average of 6.4 and reduced
concentrations of dissolved solids to 2425 mg/L sulfate, 0.9 mg/L iron, and 0.9 mg/L aluminum. The
concentration of dissolved oxygen also decreased to 3.7 mg/L and temperature increased by 6°C. In the
wetland, the bioreactor outflow mixes with an AMD C1 Seep and the mixture flows into the remaining
cells at C1 Out. Even with the inflow of the acid seepage, the wetland pH still remained constant,
though alkalinity decreased by about 300 mg/L. Some concentrations increased to 5.5 mg/L iron, 1.8
mg/L aluminum, and 5.4 mg/L dissolved oxygen, though sulfate concentration actually decreased from
2425 mg/L to 2225 mg/L. The resulting outflow from the combined treatment (Outlet) showed a further
increase in pH to 6.8, a temperature increase to only 3°C greater than that of the raw AMD, increased
dissolved oxygen to 9 mg/L, and further reduction in the concentration of sulfate to 2200 mg/L.
The results show decrease in concentration of harmful dissolved species, but they do not show if
the values are within the range of acceptable drinking and surface water standards. Table 4 compiles
the results from this study with standards set by the EPA for drinking water (EPA SMCL) and surface
water (EPA CCC and CMC). EPA Secondary Maximum Containment Level (SMCL) for drinking water is
the recommended maximum level a contaminant should be found at (US EPA, 2013). These are not
enforceable, but are a good goal. The overall treatment by the Blackfoot coupled treatment system is
only within range for pH, whereas the concentrations of dissolved aluminum, sulfate, iron, manganese,
and alkalinity are still too high. Such results should not be of too great of concern though because
humans will not be directly consuming the system outflow water. Thus, the EPA Criteria Maximum
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Concentration (CMC) and Criterion Continuous Concentration (CCC) for surface waters may be better
indicators for adequate treatment (US EPA, 2014). The CMC is the highest concentration that an aquatic
community can be exposed to briefly without harm, whereas the CCC is the highest concentration they
can be exposed to permanently without harm. For these standards, the Blackfoot system is within range
for pH and aluminum, though the concentration of iron and alkalinity are still too high.
Saturation Index Results
The calculated saturation indices (Tables 5 and 6) showed that throughout the entire system,
amorphous Al(OH)3, alunite, aragonite, calcite, amorphous Fe(OH)3, gibbsite, goethite, gypsum,
hematite, rhodochrosite, and siderite have positive SI values and thus have the potential to precipitate
and will not dissolve. However, only hematite consistently has a positive SI. All other minerals
evaluated have SI values that fluctuate between positive and negative, indicating that they may be
moving between the solid and dissolved forms as they try to establish mineral equilibrium. When
looking at the wetland alone, both goethite and hematite consistently have positive SI values. Both of
these minerals include iron in their mineral structure and thus indicate sequestration of iron within the
wetland.
When ignoring the AMD seep (due to the acidity preventing precipitation of many minerals), the
wetland has even more potential for precipitation of the minerals alunite, gibbsite, goethite, and
gypsum, all of which show positive SI in every part of the wetland except the acid seep. These minerals
would permit the sequestration of aluminum, sulfate, iron, and calcium. The wetland also shows
greater potential for precipitation in the summer (8/14) than in the spring (5/23). In addition to the
minerals previously mentioned, aragonite, calcite, dolomite, and rhodochrosite all consistently have
positive SI values in the wetland during the summer. This indicates sequestration of additional calcium,
as well as manganese and magnesium.
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Discussion
I hypothesized that because the concentration of most metals are trace by the end of treatment by the
bioreactor, only minor additional metal removal can occur in the study wetland. This hypothesis held
true, based on my interpretations of the data presented in Tables 1-3. The concentration of both
aluminum and iron were reduced to an average of less than 1 mg/L. The wetland was able to only
remove on average an additional 0.4 mg/L aluminum and 0.8 mg/L iron, though the iron removal only
occurred in August, while May showed increases in iron concentrations.
I also predicted that precipitation-dissolution reactions would play major roles in mineral
equilibrium. This is definitely the case as indicated by the calculated saturation index values
summarized in Tables 5 and 6. Within the wetland, several minerals are not at chemical equilibrium and
are even fluctuating between positive and negative SI values, indicating that their dissolved
concentration is fluctuating. This could also mean that the wetland is trying to establish mineral
equilibrium. Several minerals are also found to have only negative or only positive SI values, indicating
that they are not participating as actively in precipitation-dissolution reactions to establish equilibrium.
The goal of the study wetland is to polish the bioreactor outflow by continuing to remove contaminants
while also establishing mineral equilibrium to ensure that the water entering the tributary of the Patoka
River is more stable and will not experience dramatic precipitation-dissolution reactions along its flow
path.
My third hypothesis was that the acid seep would alter the balance of the minerals found in the
wetland causing them to fluctuate between solid and dissolved form. This hypothesis was not found to
hold true, as the results did not indicate that the untreated AMD seep entering near the inflow of the
wetland was altering acidity downstream much at all. In fact, treatment by the wetland seemed entirely
unaffected by the acid seep. Removal of sulfate, metals, and acidity continued within the wetland from
the inflow to the outflow, even within the mixing zone of the seep and wetland inflow. These results
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could mean that the wetland inflow (downstream of the bioreactor outflow) is the main source of the
wetland’s water. The acid seep may be entirely contained within the first cell of the wetland, where it
was suspected that the inflow (Inlet) mixed with the untreated AMD (C1 Seep) before exiting at C1 Out.
Another possibility is that the wetland is just as efficient at treating raw AMD as the bioreactor is.
Conclusion
The results of this study show that the Blackfoot coupled treatment system is able to perform efficient
treatment of the inflowing AMD. The system significantly reduces the concentration of dissolved
sulfate, iron, and aluminum, while also increasing pH into an acceptable range. The bioreactor alone
shows significant treatment, while the wetland is more of a polishing step for sulfate, iron, and
aluminum. However, the wetland also serves to return parameters like temperature, dissolved oxygen,
and elements like magnesium and manganese to similar values found in the untreated AMD.
The resulting concentrations of dissolved constituents may not all be within a range that is
acceptable to the EPA, but they are much closer than the raw AMD. Without the treatment by the
bioreactor and wetland, pure AMD would be entering the tributary of the Patoka River. All that acidity
mixed with extremely high concentrations of sulfate, iron, and aluminum would clog waterways with
precipitates and be highly toxic to aquatic communities. The Blackfoot coupled treatment system is
therefore necessary and sufficient to prevent many negative effects of derelict mine waste in the
watershed of the Patoka River.
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Acknowledgements
This research project was conducted through the Center for Geospatial Data Analysis in the Indiana
Geological Survey. Field and laboratory research was made possible through funding provided by the
Abandoned Mine Land Program of the Indiana Department of Natural Resources Division of
Reclamation. I am thankful for all the training, advising, and support over the past two years from my
faculty mentors Greg Olyphant and Tracy Branam.
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Tables and Figures
Figure 1. Google maps aerial image of the Blackfoot Abandoned Mine Lands Reclamation Site located in Pike County just south of Augusta, Indiana, and Hwy 64.
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Figure 2. Closer view of the study area: To the south is the bioreactor and to the north is the wetland. Sampling locations are
labeled. Water flows both underground and on the surface from south to north.
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Table 1. Water chemistry data for the Blackfoot coupled treatment system on May 23, 2013. SRB In is untreated AMD; SRB Out is after treatment by the bioreactor; Inlet is the inflow to the wetland; C1 Out is the mixing zone of the inlet and C1 Seep; C1
Seep is an untreated AMD flow into the wetland; and Outlet is the outflow of the wetland.
Sample ID SRB In SRB Out Inlet C1 Out C1 Seep Outlet
Date 5/23/2013 5/23/2013 5/23/2013 5/23/2013 5/23/2013 5/23/2013
Temp (°C) 16.7 24.2 21.86 21.49 19.41 21.68
SpCond (µS/cm) 3900 3706 3528 3346 2156 2763
DO Conc (mg/L) 10.2 3.8 7.50 7.60 9.19 10.00
pH 3.2 6.3 6.63 6.13 2.89 6.18
Eh vs SHE (mV) 540 -51 147.70 169.70 634.70 285.70
Alkalinity (mg/L CaCO3) 0 420 269.0 48.0 0.0 46.0
Acidity (mg/L CaCO3) 1090 72 54.1 57.8 440.4 56.8
Sulfide (mg/L) 0.006 0.002 0.000 0.002
SO4 (mg/L) 3100 2500 2450 2200 1500 2200
Ca (mg/L) 483 856 787 635 316 654
Mg (mg/L) 207 257 238 185 82 198
Fe(tot) (mg/L) 113 1.2 6.9 7.3 9.1 5.0
Fe(II) (mg/L) 74 1.1 7.32 7.92 2.68 5.44
Mn (mg/L) 32 35 35.4 29.3 12.8 32.4
Al (mg/L) 126 0.9 0.456 1.77 57.6 0.452
K (mg/L) <10.0 3.4 3.71 2.89 <10 3.42
Na (mg/L) <10.0 10.3 10.3 7.66 <10 8.35
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Table 2. Water chemistry data for the Blackfoot coupled treatment system on August 14, 2013. SRB In is untreated AMD; SRB Out is after treatment by the bioreactor; Inlet is the inflow to the wetland; C1 Out is the mixing zone of the inlet and C1 Seep; C1
Seep is an untreated AMD flow into the wetland; and Outlet is the outflow of the wetland.
Sample ID SRB In SRB Out Inlet C1 Out C1 Seep Outlet
Date 8/14/2013 8/14/2013 8/14/2013 8/14/2013 8/14/2013 8/14/2013
Temp (°C) 21.3 26.6 23.41 23.55 21.08 23.64
SpCond (µS/cm) 4058 3643 3466 3419 3138 3148
DO Conc (mg/L) 4.8 3.7 3.98 3.14 2.66 7.21
pH 2.9 6.5 6.99 6.94 2.89 7.38
Eh vs SHE (mV) 656 -148 -2.36 65.64 690.64 330.64
Alkalinity (mg/L CaCO3) 0 612 425.0 339.0 0.0 170.5
Acidity (mg/L CaCO3) 1210 109 58.0 51.8 723.8 29.7
Sulfide (mg/L)
SO4 (mg/L) 3300 2350 2400 2250 2400 2200
Ca (mg/L) 463 729 713 715 373 599
Mg (mg/L) 222 258 240 239 122 215
Fe(tot) (mg/L) 75.6 0.49 10.6 3.7 33.4 <0.1
Fe(II) (mg/L) 41 0.57 9.96 0.08 4.68 0.08
Mn (mg/L) 28.3 28.9 27.5 28.5 18.3 24.6
Al (mg/L) 142 <0.5 <0.5 <0.5 71.6 <0.5
K (mg/L) <10.0 <10.0 <10 <10 <10 <10
Na (mg/L) <10.0 12.1 13.6 12.4 <10 12.7
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Table 3. Averages for basic water chemistry parameters for the Blackfoot Coupled Treatment System during Spring (5/23) and Summer (8/14) of the year 2013. SRB In is the raw AMD; SRB Out is after treatment by the bioreactor; Inlet is the inflow of the
wetland; C1 Out is the mixing zone of the Inlet and C1 Seep; C1 Seep is an untreated AMD Seep; and Outlet is the wetland outflow after treatment by the entire system.
Blackfoot Coupled Treatment System: Spring and Summer Averages
SRB In SRB Out Inlet C1 Out C1 Seep Outlet
Temp (°C) 19.0 25.4 22.6 22.5 20.2 22.7
SpCond (µS/cm) 3979 3675 3497 3383 2647 2956
DO Conc (mg/L) 7.5 3.7 5.7 5.4 5.9 8.6
pH 3.0 6.4 6.8 6.5 2.9 6.8
Eh vs SHE (mV) 597.7 -99.8 72.7 117.7 662.7 308.2
Alkalinity (mg/L CaCO3) 0.0 516.0 347.0 193.5 0.0 108.3
Acidity (mg/L CaCO3) 1150.1 90.8 56.0 54.8 582.1 43.3
Sulfide (mg/L) 0.006 0.002 0.000 0.002
SO4 (mg/L) 3200 2425 2425 2225 1950 2200
Ca (mg/L) 473.0 792.5 750.0 675.0 344.5 626.5
Mg (mg/L) 214.5 257.5 239.0 212.0 102.0 206.5
Fe(tot) (mg/L) 94.3 0.9 8.8 5.5 21.2 5.0
Fe(II) (mg/L) 57.6 0.8 8.6 4.0 3.7 2.8
Mn (mg/L) 29.9 32.0 31.5 28.9 15.6 28.5
Al (mg/L) 134.0 0.9 0.5 1.8 64.6 0.5
K (mg/L) <10.0 3.4 3.7 2.9 <10 3.4
Na (mg/L) <10.0 11.2 12.0 10.0 <10 10.5
Table 4. Chemical concentrations of major ions, pH, and alkalinity of the Blackfoot coupled treatment system before treatment (Untreated AMD), after bioreactor treatment (Bioreactor Outlet), and after wetland treatment (Wetland outlet) compared to the EPA Secondary Maximum Containment Level (SMCL) for drinking water and the EPA Criterion Continuous Concentration
(CCC) or Criteria Maximum Concentration (CMC) for surface waters.
Contaminant Raw AMD SRB Out Wetland Outlet EPA SMCL EPA CCC (CMC)
Al (mg/L) 134.0 0.9 0.5 0.05-0.2 0.087 (0.75)
SO42- (mg/L) 3233.3 2466.7 2150 250 N/A
Fe (mg/L) 94.3 0.9 5.0 0.3 1
Mn (mg/L) 29.9 32.0 28.5 0.05 N/A
pH 3.0 6.4 6.8 6.5-8.5 6.5-9
Alkalinity (mg/L) 0 516 108 N/A 20
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Table 5. Calculated saturation indices for samples of water collected from the Blackfoot coupled treatment system on May 23, 2013. SRB In is untreated AMD; SRB Out is after treatment by the bioreactor; Inlet is the inflow to the wetland; C1 Out is the
mixing zone of the inlet and C1 Seep; C1 Seep is an untreated AMD flow into the wetland; and Outlet is the outflow of the wetland. Red boxes are for positive SI values, indicating potential for active precipitation.
Mineral Formula SRB In SRB Out Inlet C1 Out C1 Seep Outlet
Al(OH)3(a) Al(OH)3 -5.67 0.64 0.35 0.86 -6.52 0.31
Alunite KAl3(SO4)2(OH)6 -0.13 8.28 6.65 9.59 -2.37 7.84
Anhydrite CaSO4 -0.26 -0.09 -0.11 -0.19 -0.55 -0.18
Aragonite CaCO3 -0.16 -0.09 -1.46 -1.37
Calcite CaCO3 -0.02 0.06 -1.31 -1.22
CO2(g) CO2 -0.71 -1.25 -1.54 -1.56
Dolomite CaMg(CO3)2 -0.23 -0.1 -2.86 -2.66
Fe(OH)3(a) Fe(OH)3 -2.34 -5.21 -0.08 -1.1 -2.75 0.88
Gibbsite Al(OH)3 -2.9 3.34 3.07 3.58 -3.78 3.03
Goethite FeOOH 3.25 0.65 5.69 4.66 2.94 6.65
Gypsum CaSO4:2H2O -0.02 0.14 0.12 0.05 -0.31 0.05
H2(g) H2 -25.2 -10.8 -18.26 -18.06 -27.58 -22.16
H2O(g) H2O -1.73 -1.53 -1.59 -1.6 -1.66 -1.6
Hausmannite Mn3O4 -30.03 -24.04 -15.08 -18.43 -29.69 -13.85
Hematite Fe2O3 8.46 3.3 13.38 11.31 7.85 15.3
Jarosite-K KFe3(SO4)2(OH)6 -1.34 -19.32 -5.05 -6.74 -1.83 -0.86
Manganite MnOOH -10.1 -11.14 -6.72 -7.84 -9.83 -5.64
Melanterite FeSO4:7H2O -3.03 -5.28 -4.44 -4.35 -4.33 -4.53
O2(g) O2 -35.68 -61.86 -47.74 -48.27 -29.96 -40.01
Pyrochroite Mn(OH)2 -12.56 -6.4 -5.71 -6.73 -13.48 -6.58
Pyrolusite MnO2 -14.91 -21.91 -14.14 -15.41 -12.99 -11.14
Rhodochrosite MnCO3 1.06 1.21 -0.11 0.01
Siderite FeCO3 -0.66 0.24 -0.97 -1.06
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Table 6. Calculated saturation indices for samples of water collected from the Blackfoot coupled treatment system on August 14, 2013. SRB In is untreated AMD; SRB Out is after treatment by the bioreactor; Inlet is the inflow to the wetland; C1 Out is the
mixing zone of the inlet and C1 Seep; C1 Seep is an untreated AMD flow into the wetland; and Outlet is the outflow of the wetland. Red boxes are for positive SI values, indicating potential for active precipitation.
Mineral Formula SRB In SRB Out Inlet C1 Out C1 Seep Outlet
Al(OH)3(a) Al(OH)3 -6.23 0.31 0.07 0.11 -6.51 -0.29
Alunite KAl3(SO4)2(OH)6 -1.27 6.95 5.06 5.26 -2.13 2.76
Anhydrite CaSO4 -0.28 -0.16 -0.15 -0.17 -0.38 -0.22
Aragonite CaCO3 0.17 0.45 0.32 0.39
Calcite CaCO3 0.32 0.59 0.46 0.53
CO2(g) CO2 -0.72 -1.4 -1.44 -2.19
Dolomite CaMg(CO3)2 0.53 1.03 0.77 0.93
Fe(OH)3(a) Fe(OH)3 -1.68 -6.58 -1.32 -0.79 -1.7 1.84
Gibbsite Al(OH)3 -3.51 2.98 2.78 2.82 -3.78 2.41
Goethite FeOOH 4.08 -0.63 4.52 5.05 4.05 7.68
Gypsum CaSO4:2H2O -0.05 0.05 0.07 0.06 -0.15 0.01
H2(g) H2 -28.2 -8 -13.98 -16.08 -29.38 -25.96
H2O(g) H2O -1.61 -1.47 -1.55 -1.55 -1.61 -1.55
Hausmannite Mn3O4 -27.86 -25.42 -17.28 -15.31 -27.24 -2.88
Hematite Fe2O3 10.15 0.75 11.03 12.09 10.1 17.37
Jarosite-K KFe3(SO4)2(OH)6 1.92 -23.4 -9.27 -7.6 1.8 -0.96
Manganite MnOOH -9.27 -12.27 -8.3 -7.3 -8.85 -1.52
Melanterite FeSO4:7H2O -3.44 -5.74 -4.31 -4.77 -4.06 -7.94
O2(g) O2 -28.06 -66.65 -55.77 -51.52 -25.77 -31.73
Pyrochroite Mn(OH)2 -13.23 -6.13 -5.15 -5.2 -13.4 -4.36
Pyrolusite MnO2 -11.81 -24.05 -17.6 -15.53 -10.84 -4.8
Rhodochrosite MnCO3 1.34 1.63 1.53 1.63
Siderite FeCO3 -0.68 0.97 0.4 -2.65
