1 EVALUATION OF NATURAL AND IN-SITU REMEDIATION ...

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

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 onehalf 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.

1

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

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 DArea 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.

2

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

Figure 1.

Close Up Map of D-Area with 1996 Aerial Photograph

3

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

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.

4

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

Figure 2.

Beryllium Plume and Treatability Study Sampling Locations

5

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

Table I.

DCPRB Groundwater Geochemistry Considering Sulfate Reduction Average DCB-8 Results

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

Optimal SRB Condition Relative to the Parameter

Drinking Water Standard

pH Eh (mV)

4.9 NA

2.8 NA (461.5 1)

5.5 to 9 0 to –150

6.5 to 8.5 2 -

Dissolved Oxygen (mg/L)

NA

NA (4.6 1)

0-trace

-

0.379

1.933

6300 for Lactate

-

956

856

10,000 3

1,873

1,353,000

Aluminum (µg/L) Beryllium (µg/L) Cadmium (µg/L) Calcium (µg/L) Chromium (µg/L) Copper (µg/L) Iron (µg/L) Ferric Iron Fe(III)

121 114->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 12

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

Table III. Location

Comparison of soil and porewater results for D-Area aquifer samples Sample depth (feet)

DCP211 DCP211 DCP211

8-12 18-20 24.526.5

DCP211 DCP211 DCP168 DCP168 DCP168 DCP168 DCP168 DCP170 DCP170 DCP170 DCP170 DCP170

32-34 34-36 15-18 20-22 28-31 31-33 37-40 9-12 14-16 17-20 20-22 30.533.5 37-39 11-14 21-24 24-26 32-35

DCP170 DCP171 DCP171 DCP171 DCP171

Porewater pH

Porewater Eh(mV)

Porewater [S] (mg/L)

Porewater Cr (mg/L)

Soil Cr3050b mg/kg

Porewater Ni (mg/L)

16.8 17.9 4.89

259.7

260