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WM ’03 Conference, February 23-27, 2003, Tucson AZ

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EVALUATION OF NATURAL AND IN-SITU REMEDIATION TECHNOLOGIES FOR A COAL-RELATED METALS PLUME

Jeffrey A. Ross, Cassandra L. Bayer, Ronald P. Socha, Cynthia S. Sochor Bechtel Savannah River Inc., Environmental Restoration Division

Aiken, SC 29808

Carl B. Fliermans, Pamela C. McKinsey, Margaret R. Millings, Mark A. Phifer, Kimberly R. Powell, Steven M. Serkiz, Frank C. Sappington, and Charles E. Turick

Westinghouse Savannah River Company, Savannah River Technology Center Aiken, SC 29808

ABSTRACT

Metals contamination exceeding drinking water standards (MCLs) is associated with acidic leachate generated from a coal pile runoff basin at the Savannah River Site (SRS) in Aiken, South Carolina. The metals plume extends over 100 acres with its’ distal boundary about one-half mile from the Savannah River. Based on the large plume extent and high dissolved iron and aluminum concentrations, conventional treatment technologies are likely to be ineffective and cost prohibitive. In-situ bioremediation using existing groundwater microbes is being evaluated as a promising alternative technology for effective treatment, along with consideration of natural attenuation of the lower concentration portions of the plume to meet remedial goals.

Treatment of the high concentration portion of the groundwater plume by sulfate-reducing bacteria (SRB) is being evaluated through laboratory microcosm testing and a field-scale demonstration. Organic substrates are added to promote SRB growth. These bacteria use dissolved sulfate as an electron acceptor and ultimately precipitate dissolved metals as metal sulfides. Laboratory microcosm testing indicate SRB are present in groundwater despite low pH conditions, and that their growth can be stimulated by soybean oil and sodium lactate. The field demonstration consists of substrate injection into a 30-foot deep by 240-foot long permeable trench. Microbial activity is demonstrated by an increase in pH from 3 to 6 within the trench. Downgradient monitoring will be used to evaluate the effectiveness of SRB in reducing metal concentrations.

Natural attenuation (NA) is being evaluated for the low concentration portion of the plume. A decrease in metal mobility can occur through a variety of abiotically and/or biotically mediated mechanisms. Quantification of these mechanisms is necessary to more accurately predict contaminant attenuation using groundwater transport models that have historically relied on simplified conservative assumptions. Results from matched soil/porewater samples indicate higher soil/water partition coefficients (Kds) with increasing distance from the source. In addition, site-specific metals availability is being assessed using sequential extraction techniques, which more accurately represent environmental conditions as compared to default EPA extraction methods. Due to elevated sulfate levels in the plume, SRB are most likely to be the dominant biotic contributor to NA processes.


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INTRODUCTION

The D-Area powerhouse was placed in operation in 1951 to provide steam and electricity for Savannah River Site (SRS) industrial facilities. D-Area is located about one mile from the Savannah River. Coal for the powerhouse is staged in uncovered piles over an area of up to nine acres. Runoff from the D-Area coal pile collects in drainage ditches that flow to the D-Area Coal Pile Runoff Basin (DCPRB). The DCPRB was built in 1978 as a sedimentation/seepage basin to remove suspended solids (i.e., coal and coal fines) and to minimize the direct discharge of coal pile runoff to Beaver Dam Creek. DCPRB is a permitted wastewater treatment facility with a surface area of 12.5-acres and a maximum volume of 14.5 million gallons (1). The powerhouse, coal pile, and coal pile runoff basin are currently operational and are expected to continue operating until at least 2015. The location of these features are shown in Figure 1.

Chemical and biological oxidation of the sulfur compounds (primarily pyrite) associated with coal produces significant concentrations of sulfate and acidity, which leach metals from the D-Area coal pile and subsequently runs off into the DCPRB. This oxidation and leaching process continues in the DCPRB, due to the presence of coal and coal fines within the DCPRB. From the basin, the low pH/metals/sulfate contaminated water seeps into the water table aquifer. High concentrations of dissolved iron, aluminum, and sulfate are present in the contaminant plume. Metals exceeding drinking water standards (MCLs) include beryllium, cadmium, chromium and uranium; metals significantly exceeding background concentrations include manganese, copper, nickel, and zinc. The plume is located within an approximately 50-foot thick water table aquifer, which consists of a series of interbedded sand, silt, and clay layers. The water table intersects the bottom of the basin and typically ranges from 5 to 15 feet below grade downgradient of the basin. The aquifer discharges to the Savannah River and adjacent wetland, located about one mile downgradient of the basin. Elevated levels of sulfate have been detected in wetland piezometers, and the extent of metals contamination above MCLs is about 3000 feet (2). Figure 2 shows the extent of the beryllium plume, which is one of the more mobile dissolved metals.

Conventional ex-situ treatment methods for low pH and metal contaminated groundwater are generally inefficient and often do not meet remediation goals (i.e. MCLs) for toxic metals due to the presence of high concentrations of dissolved iron and aluminum. Extraction of metals contaminated groundwater is plagued by iron fouling, and chemical treatment of water with high quantities of dissolved aluminum leads to the production of large quantities of aluminum hydroxide sludge that requires waste management. Thus, in-situ technologies are preferable to address this problem. This paper discusses the approach to and preliminary results of in-situ bioremediation using existing groundwater microbes, in conjunction with natural attenuation of the lower concentration portions of the plume to meet remedial goals.


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Figure 1. Close Up Map of D-Area with 1996 Aerial Photograph


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DISCUSSION

The following sections discuss the approach used to evaluate the potential for in-situ bioremediation and natural attenuation as viable remedial alternatives to reduce contamination to acceptable levels. In-situ Bioremediation Approach

Sulfate reduction remediation typically consists of modifying the geochemical conditions of a contaminated aquifer in order to sustain and enhance the growth of existing sulfate reducing bacteria (SRB) populations and to promote subsequent sulfate reduction. Sulfate reduction remediation may also involve bioaugmentation (i.e. addition of SRB), if SRB are not naturally present or are present in very low numbers. SRB mediated sulfate reduction results in the oxidization of a carbon substrate such as lactate (CH3CHOHCOO-), reduction of sulfate (SO4

-2) and hydrogen ions (H+), the generation of bicarbonate (HCO3

-) and hydrogen sulfide (H2S), an increase in pH, and the subsequent inorganic precipitation of metal sulfides, hydroxides and carbonates (3).

SRB grow best and are most numerous at a pH range of 5.5 to 9.0; and at Ehs from 0 to –150 mV; with sufficient available organic carbon substrates that supply carbon and energy; with sufficient micronutrients; under anaerobic conditions; with minimal nitrate, manganese (IV), and ferric iron (FeIII); and with an abundance of sulfate (3, 4, 5, 6, 7, 8, 9, 10). Table I provides a comparison of these optimal sulfate reduction conditions versus the existing DCPRB groundwater geochemistry Monitoring well DCB-8C is upgradient of the DCPRB, and well DCB-19 and –21 are downgradient of the DCPRB in the most contaminated portion of the plume. Table I also provides metal concentrations and compares them to drinking water criteria. The major microbial competitors to SRB include aerobes, nitrate reducers, manganese reducers, and iron reducers. However, SRB should out compete these microbial competitors for carbon substrate and micronutrients (nitrogen and phosphate) if the sulfate concentration is significantly greater than the concentrations of the electron acceptors needed by these microbial competitors (i.e. O2, NO3

-, Mn+4, and Fe+3, respectively) (7). As observed in Table I, the sulfate concentrations are significantly greater than the other electron acceptors. However, the aerobic, oxidizing, and low pH condition of the groundwater are not optimal. Additionally, very little organic carbon substrate is available as indicated by the low TOC value. In addition to providing a carbon source, the addition of an appropriate organic carbon substrate(s) will help to increase the pH and decrease the Eh. In order to support SRB growth, a base amendment and micronutrients may also need to be added (3, 10).

To address these questions, a two phase treatability study (10) was prepared that consisted of laboratory testing and field application.

Phase 1 laboratory testing was designed to 1) determine if SRB are present in the most contaminated portion of the plume, and if so, at what concentrations, 2) determine what carbon source(s) are optimal for promoting sulfate reduction, and 3) evaluate whether pH adjustment is needed for sulfate reduction to efficiently occur. The results of this work would then be used to direct the field application.


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Figure 2. Beryllium Plume and Treatability Study Sampling Locations


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Table I. DCPRB Groundwater Geochemistry Considering Sulfate Reduction

Parameter

Average DCB-8 Results

Average

DCB-19A, DCB-21A, & DCB-21B Results

Optimal SRB

Condition Relative to the Parameter

Drinking

Water Standard

pH 4.9 2.8 5.5 to 9 6.5 to 8.5 2

Eh (mV) NA NA (461.5 1) 0 to –150 -

Dissolved Oxygen (mg/L) NA NA (4.6 1) 0-trace -

Total Organic Carbon (mg/L) 0.379 1.933 6300 for Lactate -

Nitrate as Nitrogen (µg/L) 956 856 Small fraction of SO4 concentration

10,000 3

Sulfate (µg/L) 1,873 1,353,000 Significant SO4 concentrations

250,000 2

Aluminum (µg/L) 121 114,167 - 50 to 200 2 Beryllium (µg/L) <5 20 - 4 3 Cadmium (µg/L) <5 7 - 5 3 Calcium (µg/L) 788 80,067 - - Chromium (µg/L) <5 46 - 100 3 Copper (µg/L) 54 280 - 1000 2 Iron (µg/L) 511 51,067 - 300 2 Ferric Iron Fe(III) NA NA (~1.0 1) Small fraction of SO4

concentration -

Magnesium (µg/L) 731 60,500 - - Manganese (µg/L) 3.4 7,603 Small fraction of SO4

concentration 880 4

Nickel (µg/L) <5 656 - - Zinc (total) 29 1,703 - 5000 2 Notes to Table II: 1) NA = Not analyzed 2) Sources for optimal SRB conditions: (4, 5, 6, 7, 8, 9) 1 Data shown in parentheses are from a nearby monitoring well - DCB-49 (11) 2 SS = Secondary Standard 3 MCL = Maximum Contaminant Level 4 Tap water risk based concentration


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The carbon substrates considered to have the greatest potential to promote SRB growth and subsequent sulfate reduction include sodium lactate, soybean oil, and Regenesis Hydrogen Release CompoundTM (HRC), which is a commercial time-release lactate-containing polymer product (3). The various biological and physical properties of sodium lactate, soybean oil, and HRCTM may make either their use alone or in combination beneficial. Lactate, which is denser than water and is highly soluble, is immediately available to SRB and can be effectively emplaced throughout the aquifer. HRCTM, which is denser than water and has a low solubility, and is immediately available to SRB, could be used to provide a long-term carbon source. Vegetable oil, which is lighter than water and has a low solubility, requires initial microbial degradation to be available to SRB. It could be used to provide a long-term carbon source in the most contaminated upper portion of the aquifer.

The laboratory testing consisted of the two types of tests. Microbial population analyses of groundwater and soil cores taken from background and contaminant locations adjacent to DCPRB were conducted to evaluate the existing total, aerobic heterotrophic, anaerobic heterotrophic, and sulfate reducing bacteria populations. These bacteria population counts were performed by microscopic direct counting methods, plate counts, and most probable number assays. Anaerobic microcosm testing utilizing groundwater and soil cores taken from background and contaminant locations adjacent to DCPRB will be conducted to evaluate the ability to promote sulfate reduction by the addition of the following organic carbon substrates: sodium lactate, soybean oil, and HRCTM. Microcosm sampling and analysis was conducted to determine microbial populations, soluble organic concentrations, pH, Eh, sulfate/sulfide concentrations, and micronutrient concentrations (12).

Phase 2 of the treatability study is the field application of the carbon source(s). This has been initiated utilizing an existing interceptor well, DIW-1, located just downgradient of the DCPRB and shown on figure 2. DIW-1 is partially penetrating and intercepts the upper most portion of the water table aquifer. It consists of an approximately 30-foot deep by 240-foot long vertical HDPE membrane with coarse gravel pack on either side of the membrane. Both horizontal and vertical screen zones (wells, piezometers, and horizontal drainage pipe) are located in the gravel pack on either side of the membrane (13). Figure 3 provides a longitudinal cross-sectional view of DIW-1. Because the interceptor well is not keyed into a low permeability unit, water can flow down the gravel pack in front of the membrane and underneath the well. This allows for excellent injection characteristics. Selected DIW-1 wells, piezometers, and horizontal drainage have been utilized as the injection points for the carbon sources. Natural Attenuation Approach

Monitored natural attenuation (MNA) is the “reliance on natural attenuation processes (within the context of a carefully controlled and monitored site cleanup approach) to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods” (14). The attenuation of inorganic contaminants resulting in decreases in metal toxicity and mobility occur primarily through sorption (including adsorption and absorption) and precipitation. These mechanisms may be abiotically or biotically mediated; for example, reduction of inorganic contaminants often reduces both metal toxicity (e.g.,


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chromium (Cr) VI to Cr III) and mobility primarily through precipitation (e.g., UO2 (VI) to U (IV)).

Two distinct areas exist within the low pH/metals/sulfate plume. In the area near the source (DCPRB), groundwater may be toxic to indigenous microbial populations and geochemical controls such as sorption will dominate attenuation processes. In the distal portion of the contaminant plume where the pH of the system is higher, biotic processes (e.g., microbially mediated removal of metals by sulfate reduction and precipitation as a metal sulfide) could contribute to the overall attenuation process. Groundwater transport models are capable of accounting for contaminant mass-reduction by processes such as radioactive decay and biodegradation. Partitioning between the groundwater and soil phases is most often represented by a single linear partitioning coefficient (Kd) that does not vary over the flow path of the model.

However, previous studies (15) show that the manner in which metal contaminants at D Area partition during groundwater transport is highly variable, but systematic, along the groundwater flow path. Once the main mechanisms have been identified, a groundwater transport model can be developed to incorporate these mechanisms to model the attenuation capacity of the aquifer as the plume migrates. Figure 3. DIW-1 Injection Well, Lactate and Soybean Oil Injection Points, and Oil

Distribution

90

100

110

120

130

0 50 100 150 200 250

Distance Along DIW-1 (ft)

Elev

atio

n (ft

-msl

)

DIW-P13Cluster

DIW-P11Cluster

DIW-P09Cluster

DIW

1-2

DIW-P03Cluster

DIW-P05Cluster

DIW-P07Cluster

Lateral #4

Central Sump

Extent of HDPE Barrier

Extent of FX-99Gravel Pack

Bentonite

PiezometerCluster Screens

Ground Surface

A

B

C

A

B

C

A

B

C

A

B

C

A

B

C

A

B

C

SouthNorth

Lateral #3

Lateral #1

Lateral #2

Water TablePotentiometric Surface

Soybean Oil in yellowWater Table Potentiometric Surface in blue Sodium Lactate Injection Locations in red

Soybean Oil(displaces groundwater)

18-Jul-02

Soybean Oil Injection locations in green A, DIW 1-2

DIW

1-1


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A treatability study (16) was prepared to (1) determine the availability of the contamination sources for transport into groundwater and for uptake by environmental receptors, and (2) provide data that can be used to model the attenuation processes and attenuation capacity of the aquifer system at D Area for metal contaminants.

The source availability study compares the standard USEPA extraction technique to a sequential extraction technique. USEPA Method 3050b, the hot nitric acid extraction technique, is typically used to define the concentration of metals available for transport at a waste sites. This technique is very aggressive and almost certainly overestimates the source term available for contaminant transport. The sequential extraction approach will generate a less conservative, although still environmentally relevant, measure of the available fraction of metals. This approach will entail a series of extraction steps designed to release metals from the soil in an operationally defined extraction sequence (17; Table II). The amount of metals released in each extraction step is summed to represent the total of all the metals available if the soil were perturbed under a range of environmental conditions.

The development of contaminant transport factors will consider an electrostatic thermodynamic approach known as surface complexation models (SCMs). Because of the widely variable geochemical conditions, the use of a single linear contaminant partitioning coefficient will not be adequate to account for natural attenuation associated with sorption processes at the site. The linear partitioning approach only depends on the total contaminant concentration. The SCM approach is capable of modeling adsorption of contaminants to reactive mineral surfaces as a function of pH, solute concentration, and ionic strength (18, 19, 20, 21, 22, 23).

Twelve locations were sampled, from the most contaminated to the distal low concentration portion of the plume. Matched soil/ pore water samples were collected at each location from two to three depths. To validate the SCM approach, soil samples with associated pore water were analyzed to determine in situ Kd values under various geochemical conditions. Additionally, batch sorption tests may also be performed with these soils for a range of pH and contaminant concentrations. From these field and laboratory tests, a mechanistic sorption model can be developed to support future transport analyses.

Microbial attenuation processes will be assessed through microbial analysis at a subset of these locations. The specific objectives of this characterization will be to:

• Determine whether direct monitoring of highly selected microbiological parameters can serve as a surrogate for defining the capacity of natural attenuation.

• Evaluate the contribution of naturally occurring microorganisms to the attenuation of metals concentrations and the low-pH groundwater at D-Area groundwater and surface water plumes.

• Determine the concentration of the bacterial components of the community present in D-Area groundwater plume and determine if the bacterial communities are associated with the porewater, sediments, or wetland habitats.


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Table II. Description of Sequential Extraction Procedure Steps Fraction Reagent Description Extraction

Conditions Targeted Phase

1. Distilled deionized water

Tumble for 16 hours at room temperature

easily soluble salts and ions already present in the soil

solution 2. 0.5 M calcium

nitrate neutral salt Tumble for 16 hours

at room temperature Easily exchangeable ions on

soil surfaces 3. 0.44 M acetic acid

& 0.1 M calcium

nitrate

weak acid w/ neutral salt

Tumble for 8 hours at room temperature

carbonate minerals, acid exchangeable metals on the

soil surfaces

4. 0.01 M hydroxylamine-

hydrochloride & 0.1 M nitric acid

weak reducing agent

Tumble for 0.5 hours at room temperature

manganese oxides

5. 0.1 M sodium pyrophosphate

complexing agent

Tumble 24 hours at room temperature

organic matter

6. 0.175 M ammonium oxalate & 0.1 M

oxalic acid

buffered mild reducing agent

Tumble 4 hours in darkness at room

temperature

amorphous iron oxides

7. 0.15 M sodium citrate, 0.05 M citric

acid, & 25 g/L sodium dithionate

buffered strong reducing agent

Shake for 0.5 hours in water bath at 50°C

crystalline iron oxides

8. 48% hydrofluoric acid & aqua regia

Strong corrosive

Microwave digestion all remaining solids

Total Digestion

48% hydrofluoric acid and aqua regia

Strong corrosive

Microwave digestion total digestion of untreated soil

Note: Adapted from (17)

• Characterize bacterial populations present and assess their activity with respect to D-Area plume.

• Investigate the correlation between presence, density and activity of identified bacteria and natural attenuation of D-Area plume using selected immunoprobes.


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RESULTS

The following sections will discuss the results of the in-situ bioremediation and natural attenuation treatability studies to date. In-situ Bioremediation results

Three carbon sources, soybean oil, sodium lactate and HRCTM were evaluated in microcosms for their effectiveness in stimulating SRB activity (12). Increased population densities of SRB occurred as a result of the addition of soybean oil to non-contaminated sediments and groundwater as well as sediments and groundwater receiving pH amendments. Hydrogen sulfide concentrations also increased in microcosms demonstrating increased SRB densities. Sodium lactate was shown to serve as a carbon and energy source for growth of SRB from D-area sediments; however, an inhibitory response was observed at lactate concentrations greater than or equal to 6.3 g/L. Therefore, the sodium lactate injected in the field was diluted to reduce its concentration to around 3.15 g/L lactate. Addition of HRCTM resulted in a decrease in pH and microbial activity in microcosms and was therefore removed from consideration as a carbon source for bioremediation at this site.

In July 2002, 440 gallons of soybean oil was injected into the upgradient side of both the south and north wings of the interceptor well (DIW-1) through seven different DIW-1 injection points. Fifteen gallons of 60 percent sodium lactate and 90 gallons of monitoring well DCB-8 groundwater were injected into the upgradient side of only the south wing of DIW-1 through four different DIW-1 injection points. The DCB-8 groundwater was injected in conjunction with the sodium lactate to flush the sodium lactate out of the injection points and to provide a source of additional SRB. Figure 3 shows the DIW-1 cross-section with the location of the soybean oil a few days after injection (denoted in yellow). The figure also shows the four sodium lactate injection points, located in the south wing and shown in red, and the seven soybean oil injection points, which include the upper (A) screen zone of all upgradient clusters and the DIW-1-2 screen. Lactate injections occurred weekly for 2 months, biweekly for 2 months, and then every 3 weeks for 2 months.

The comparison of two sampling events subsequent to the injection of the soybean oil and lactate to baseline conditions indicate that both substrates are being utilized by in-situ microbes as a carbon source. The pH and Eh (redox potential) results indicate that the pH is increasing significantly (greater than 3 orders of magnitude at the injection point) within DIW-1 and the Eh is decreasing as anticipated. Figure 4 presents the pH results across the south wing of DIW-1, which has received both soybean oil and lactate injection. The figure shows from left to right, results from upgradient to downgradient. Bulk Eh values, although decreasing, are not yet within the range for optimal sulfate reduction to occur.


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Figure 4. pH Across DIW-1

Natural Attenuation Results

The results from 12 matched soil/porewater samples (from 4 locations) are shown in Table III below. In general, these results indicate that the higher chromium and nickel concentrations in pore water are associated with the higher sulfur concentrations and lower pH values found in the more contaminated portion of the plume (DCP211). The sorbed fraction of these metals in the lower pH portions of the plume is low.

In situ Kd values were calculated using seven sets of “matched” pore water and soil samples, where Kd = (COC concentration in soil)/(COC concentration in pore water). Kd values measured in these studies demonstrate that Kd values can vary an order of magnitude even at one sampling location. Using the 3050b method, the ranges for four metals (Cr, Ni, As, and Se) measured in this study are, respectively, >5.9->386, 0.63->308, >114->3600, and >0.71->265 (note that the > [greater than] values are the result of pore water concentrations that are below the method detection limit). The lowest values were calculated from the samples taken closest to the source. A large part of the variability in reported Kd values is a result of the very low concentrations of COCs in the pore water. The results demonstrate one of the problems associated with the selection of a single Kd value even when using site-specific data (16).

Using preliminary results from the natural attenuation study and other site-specific Kds, sorption parameters were developed to model the transport of three metals: beryllium, nickel, and total

pH Across DIW-1 South Wing (Soybean Oil & Sodium Lactate) - Section D-D

0

1

2

3

4

5

6

7

DCB-8 DCB-21A DIW-P11A DIW-P11B DIW-P11C DCB-22C DCB-70B

Well / Piezometer

pH

6-24-02 9-9-02 11-5-02

Background Upgradient

Injection Point

Downgradient of DIW-1DIW-1

Downgradient of Injection Point

Initial Injection dates 7/15 and 7/16/02


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Table III. Comparison of soil and porewater results for D-Area aquifer samples

Location Sample depth (feet)

Pore-water

pH

Pore-water

Eh(mV)

Porewater [S]

(mg/L)

Pore-water

Cr (mg/L)

Soil Cr-3050b mg/kg

Pore-water

Ni (mg/L)

Soil Ni-3050b mg/kg

DCP211 8-12 16.8 3.9 DCP211 18-20 17.9 8.2 DCP211 24.5-

26.5 4.89 259.7 260 <0.05

0.23

DCP211 32-34 4.69 -83.3 492 0.34 1.56 DCP211 34-36 2.0 1.0 DCP168 15-18 5.61 338.4 54.8 <0.05 <0.062 DCP168 20-22 19.3 19.1 DCP168 28-31 6.31 288.7 3.49 <0.05 <0.062 DCP168 31-33 12.7 4.8 DCP168 37-40 6.57 228.6 13.6 <0.05 0.44 DCP170 9-12 4.59 442.3 36.2 <0.05 <0.062 DCP170 14-16 4.2 1.8 DCP170 17-20 5.83 266.8 36.0 <0.05 <0.062 DCP170 20-22 7.2 7.5 DCP170 30.5-

33.5 5.68 250.4 33.3 <0.05 <0.062

DCP170 37-39 5.75 236.3 36.7 <0.05 0.75 DCP171 11-14 5.71 275.9 6.06 <0.03 0.09 DCP171 21-24 6.13 217.0 28.5 <0.05 0.07 DCP171 24-26 3.0 1.9 DCP171 32-35 6.20 177.9 75.4 <0.03 0.41

uranium (24). A constant source term was estimated and run until a reasonable match to the current plumes was achieved. Then the model was used to predict metal concentrations into the future using the calibrated source term. This assumption is considered conservative since recent operational changes have been made to minimize the size of the coal pile and periodically remove coal fines from the DCPRB, which should reduce the amount of low pH runoff/leachate (source term). Since metal sorption is highly dependent on pH, a pH distribution and pH/Kd relationships (see Table IV below) were developed for use in the metal transport runs. Site-specific data were used to estimate sorption parameters using a Freundlich sorption isotherm for hydrogen ion (pH) (25). Sorption parameters (Kds) for U, Ni, and Be considered the site conceptual model and site-specific empirical data at a range of pH values (26). The model simulated metal contaminant transport until 2015, at which time powerhouse operation is expected to cease, and flow and source conditions are expected to change significantly.


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Table IV. pH – Kd Relationship

pH

[H+] ppb Beryllium Kd (L/kg)

Nickel Kd (L/kg)

Uranium Kd (L/kg)

7 0.1 110 389 1998 6 1 55 389 1946 5 10 1 155 530 4 100 .014 .002 8.2 3 1000 .00014 .001 1.1 2 10000 3E-05 .001 1

CONCLUSIONS

The initial results of both treatability studies support the consideration of in-situ bioremediation and natural attenuation as viable remedial alternatives for a large metals plume derived from the generation and infiltration of low pH leachate associated with coal and coal fines in a runoff basin. Future groundwater monitoring downgradient in the vicinity of the interceptor well will show if SRB in groundwater are contributing to metals sorption and precipitation. The natural buffering capacity of the aquifer in the distal portion of the plume, through increasing pH and potential microbial driven sulfate reduction, is likely to continue to prevent significant expansion of the plume into the wetland towards the Savannah River. Sequential extraction will allow for further refinement of the source term for the groundwater transport model for D-Area. Microbial characterization of the distal portion of the plume will help determine if SRB are contributing to the natural attenuation of the plume. REFERENCES

1. “RCRA Facility Investigation/Remedial Investigation Work Plan Addendum for the D-Area Expanded Operable Unit (U)”, WSRC-RP-99-4067, Rev. 0, Westinghouse Savannah River Company, Savannah River Site (2000)

2. “RCRA Facility Investigation/Remedial Investigation/Baseline Risk Assessment for the D-Area Expanded Operable Unit (U)”, WSRC-RP-2001-4062, Rev. 0, Westinghouse Savannah River Company, Savannah River Site (2002)

3. PHIFER, M. A., TURICK, C. E., and MILLINGS M.R., “D-Area Coal Pile Runoff Basin Sulfate Reduction Literature Review and Feasibility Report (U)”, WSRC-TR-2001-00371, Revision 0, Savannah River Site, (2001)

4. BENNER, S.G., D.W. BLOWES, W.D. GOULD. R.B. HERBERT, JR. AND C.J. PTACEK. “Geochemistry of a permeable reactive barrier for metals and acid mine drainage”. Environmental Science and Technology. 33: 2793-2799. (1999)

5. CHAPELLE, F. H., “Ground-water microbiology and geochemistry”, John Wiley & Sons, Inc., New York. (1993).


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6. “Microbial Processes Affecting Monitored Natural Attenuation of Contaminants in the Subsurface”, EPA/540/S-99/001, United States Environmental Protection Agency, (1999)

7. FAUQUE, G.D. “Ecology of sulfate reducing bacteria”. pp. 217-235. In. Sulfate-reducing bacteria. (ed) L.L.Barton. Plenum Press. N.Y, N.Y. (1995)

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10. “RCRA D-Area Coal Pile Runoff Basin Sulfate Reduction Remediation Treatability Study Work Plan (U)”, WSRC-RP-2001-00923, Rev. 0, Westinghouse Savannah River Company, Savannah River Site (2001)

11. WASHBURN, F. A., DENHAM, M. E., JONES, W. E., PHIFER, M. A., and SAPPINGTON, F. C., “Permeable Reactive Barrier/Geosiphon Treatment System for Metals Contaminated Groundwater Final Report”, WSRC-RP-99-01063, Revision 0, Savannah River Site, (1999)

12. TURICK, C. E., MCKINSEY P. C., PHIFER, M. A., SAPPINGTON, and F. C., MILLINGS M.R., “D-Area Sulfate Reduction Study Bacteria Population and Organic Selection Laboratory Testing (U)”, WSRC-TR-2002-00346, Revision 0, Savannah River Site, (2002)

13. PHIFER, M. A., SAPPINGTON, F. C., PEMBERTON, B. E., and NICHOLS, R. L., “ Interim Report D-Area Interceptor Well, DIW-1 Water Table Aquifer (U)”, WSRC-TR-99-00017, Revision 0, Savannah River Site, (1996)

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