THE LONG-TERM VIABILITY OF A ZERO-VALENT IRON PERMEABLE REACTIVE BARRIER STUART COWBURN

Transcription

1 THE LONG-TERM VIABILITY OF A ZERO-VALENT IRON PERMEABLE REACTIVE BARRIER by STUART COWBURN A thesis submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE in GEOLOGY Portland State University 2000

2 THESIS APPROVAL The abstract and thesis of Stuart Cowburn for the Master of Science in Geology presented May 10, 2000, and accepted by the thesis committee and the department. COMMITTEE APPROVALS: Andrew G. Fountain, Chair Carl D. Palmer Sherry L. Cady William Fish Representative of the Office of Graduate Studies DEPARTMENTAL APPROVAL: Ansel G. Johnson, Chair Department of Geology i

3 Abstract An abstract of the thesis of Stuart Cowburn for the Master of Science in Geology presented 5/10/2000. Title: The long-term viability of a zero-valent iron permeable reactive barrier. Zero-valent iron permeable reactive barriers (PRB s) hold significant potential as a tool for groundwater remediation. Uncertainties remain, however, as to the effective lifetime of Fe 0 barrier technology under full-scale operational conditions. Potential limits on barrier lifetime include reductions in permeability due to clogging by precipitates and oxidation of all Fe 0 prior to the exhaustion of the contaminant source. A 46 m long, 0.6 m wide, and 7.3 m deep Fe 0 PRB was installed at the US Coast Guard Support Center, Elizabeth City, NC, in June The barrier was designed to remediate groundwater principally contaminated with Cr(VI) and trichloroethelyne. Extraction tests were performed on materials retrieved from the Elizabeth City site to allow characterization of the geochemical environment associated with the barrier and to estimate the rate of precipitate build-up within the barrier. Mass balance calculations were performed using extraction test and i

4 groundwater monitoring data to estimate the rate of oxidation of barrier iron and place an upper limit on barrier lifetime. Reduction of SO 2-4 and NO - 3 accounts for 96 to 98% of total Fe 0 oxidized in the barrier. Secondary phases are precipitating within the barrier and in the region up to 12cm downgradient. Estimates of porosity loss due to precipitation indicate decreases in barrier porosity of 1.0% to 2.5% per year. If whole-barrier porosity loss continues at the calculated rate(s) barrier clogging could occur 6 to 23 years after installation. Mass balance calculations using data from extraction tests and downgradient groundwater monitoring indicate complete oxidation of the barrier Fe 0 in 13 to 78 years. Barrier failure, defined as breakthrough of contaminants at levels above drinking water standards, could occur in 10 to 59 years. Mass balance calculations utilizing influent oxidant concentrations and the reported mass of Fe 0 in the barrier (280 tons) indicate complete oxidation of barrier Fe 0 in 100 to 2.2x10 5 years. Barrier failure could occur in to 77 to 1.7x10 5 years. The variability in the results is directly attributable to the range of hydraulic conductivity values and influent oxidant concentrations used in the calculations. A best estimate for the upper limit of barrier lifetime is 490 years. ii

5 Acknowledgments Thanks to my advisor Dr. Carl Palmer for his continued support, advice and patience during the completion of this thesis. This work also benefited from the comments and suggestions provided by the other members of my thesis committee, Dr. Andrew Fountain, Dr. Sherry Cady and Dr. William Fish. Thanks to Ben Perkins for helping with the XRD analyses, putting up with my mess in the lab, and for answering many dumb chemistry questions. Finally, thanks to the all the other faculty, staff and students of Portland State University Geology Department who made my time here both educational and enjoyable. Funding for this project was generously supplied through the University of Waterloo, Ontario, Canada. i

6 Table of Contents Acknowledgments...i Table of Contents... ii List of Tables...iv List of Figures... vii Chapter 1: Introduction...1 Chapter 2: Study Site...4 Location...4 Site geology and hydrology...5 Nature and extent of contamination...7 The barrier...8 Chapter 3: Background...9 Permeable reactive barriers...9 Zero-valent iron as a reactive material...11 Chlorinated solvents...12 Chromium...13 Potential limitations of Fe 0 permeable reactive barriers...15 Reduction of barrier reactivity due to precipitate build-up...16 Reduction of barrier permeability due to precipitate build-up...18 Chapter 4: Methods...20 Sampling...20 Iron grain morphology...25 Mineralogical analyses...25 Extraction tests...25 Sequential extraction tests...26 Cation exchange...26 Manganese extraction tests...27 Acid extraction tests...28 Porosity tests...28 Chapter 5: Results and Discussion...31 Iron grain morphology...31 Mineralogy of aquifer materials...31 Sequential extractions...34 Results...34 Upgradient samples...37 Barrier samples...37 ii

7 Downgradient samples...39 Discussion...44 Cation exchange...49 Results...49 Discussion...52 Manganese extractions...54 Results...54 Discussion...58 Acid extraction...61 Results...61 Upgradient samples...61 Barrier samples...63 Downgradient samples...64 Discussion...66 Porosity and permeability...68 Results...68 Discussion...71 Lifetime of the barrier based on hydraulic properties...73 Chapter 6: Mass Balance Estimations of Barrier Lifetime...79 Upgradient mass balance...79 Oxidants...80 Groundwater flow...83 Iron oxidation...86 Lifetime of the barrier based on upgradient mass balance...88 Downgradient mass balance...95 Groundwater data...95 Extraction test data...97 Lifetime of the barrier based on downgradient mass balance Chapter 7: Conclusions Barrier geochemistry Effective lifetime of the barrier References Appendices Appendix A: Lab Analysis Results (Sequential Extractions) Appendix B: Lab Analysis Results (Cation Exchange) Appendix C: Lab Analysis Results (Acid Extractions) Appendix D: Lab Analysis Results (Manganese Extractions) Appendix E: Core Descriptions iii

8 List of Tables Table 1 Sequential extraction results for Elizabeth City samples. Each extraction test preferentially targets different compounds of the selected element(s) e.g. water-soluble phases, adsorbed phases (phosphate), amorphous phases (oxalate), and crystalline phases (DCB) Table 2 Exchangeable cations (meq/100 g soil) for upgradient and downgradient samples, Elizabeth City PRB site Table 3 Mn concentrations from DTPA, ER and HH extraction tests for upgradient and downgradient aquifer samples from Elizabeth City PRB site...55 Table 4 Mass of Cr(III) potentially oxidized per liter of groundwater passing through aquifer sediments at the Elizabeth City PRB site based on the results of Mn extraction tests. Values assume that all extractable Mn occurs as MnO 2 and that all Mn is used as an oxidant for Cr(III) Table 5 Results of acid extraction tests on materials from Elizabeth City PRB site...62 Table 6 Barrier porosity calculated from gravimetric and volumetric measurements. Samples A, B and C are all from core FW Table 7 Estimated porosity loss per year based on precipitate build-up in the barrier. Rate 1 and Rate 2 reflect low and high estimates (respectively) of precipitate build-up per year calculated from extraction tests data (see Mass Balance Downgradient section)...70 Table 8 Barrier permeability (k) and hydraulic conductivity (K) calculated using the Carman-Kozeny equation (eq. 13) and an initial porosity (φ) of Table 9 Major oxidants entering permeable reactive barrier at Elizabeth City site. Data collected in wells MW11, MW21 and MW31 by the University of Waterloo and US EPA between November 1996 and June High value equals highest single concentration recorded in all three wells over the complete sampling period. Average value equals the mean for data recorded between 4.5 and 6.5 m depth in all three wells over the complete sampling period...81 Table 10 Hydraulic parameters used in calculating oxidative dissolution of Fe 0 from Elizabeth City barrier. Hydraulic conductivity and hydraulic gradient data based on field estimates reported by (Bennett, 1997; Parsons Engineering- Science Inc, 1993; Puls et al., 1995)...83 Table 11 Moles of Fe 0 required per mole oxidant for reduction or complete reductive dechlorination. Reduced species are H 2 S, NH 4 +, Cr(III), OH -, ethane (for TCE, iv

9 DCE and VC), and H 2 respectively. Total mole Fe 0 required per liter water based on reduction of aqueous influent oxidant concentrations shown in Table Table 12 Calculated time for oxidation of Fe 0 in Elizabeth City PRB. Low and High flow estimates reflect the extremes for groundwater flow parameters recorded at the Elizabeth City site Table 13 Calculated time for oxidation of Fe 0 in Elizabeth City PRB based on estimated intermediate groundwater flow parameters Table 14 Effective lifetime of the Elizabeth City PRB based on upgradient mass balance. Calculations based on estimated time for VC to breakthrough barrier at levels above MCL (2 µg/l) assuming that PRB is 60 cm thick and oxidation is instaneous and complete. Estimates reflect the range of groundwater flow parameters recorded at the site...90 Table 15 Average residual concentrations of oxidant species at the downgradient edge of Elizabeth City PRB. Averages calculated from data collected by the University of Waterloo and U.S. EPA in monitoring wells ML14, ML24 and ML34 between November 1996 and June 1997 (data from Bennett(1997) Table 16 Time for contaminant breakthrough at the Elizabeth City site including adjustment to account for the incomplete reduction of TCE, cdce, VC and SO Compare to Table 14. Calculations based on estimated time for VC to breakthrough barrier at levels above MCL (2 µg/l) assuming that PRB is 60 cm thick. Estimates reflect the range of groundwater flow parameters recorded at the site...93 Table 17 Moles of iron removed from barrier between June 1996 and June 1997 as aqueous Fe 2+ based on average downgradient aqueous Fe 2+ concentration Table 18 Mass of Fe precipitated downgradient of the barrier. Thickness of the downgradient zone equals range covered by samples GP6/20C3(2) cm and cm. Soil dry density from lab estimate. Fe extractable per mass soil from combined DCB and oxalate extraction results...98 Table 19 Moles of Fe precipitated in the barrier per year. Estimates based on combined results of background-corrected oxalate and DCB extraction tests Table 20 Rate of precipitate build-up within the barrier based on combined background-corrected DCB and oxalate extraction results from FW and GP sample sets. Calculated rates reflect possible interpretations of the extraction data as either time or location dependent v

10 Table 21 Time to oxidize barrier Fe 0 based on downgradient mass balance considerations vi

11 List of Figures Figure 1 Location of U.S. Coast Guard Support Center, Elizabeth City, North Carolina...4 Figure 2 Representative (a) geologic and (b) soil profiles for the plating shop site, USCG Support Center, Elizabeth City, NC. Modified from Puls et al (1994)...6 Figure 3 Two configurations of permeable reactive barrier. a) A continuous trench reactive barrier. The permeable reactive medium in the trench provides a preferential conduit for contaminated ground water flow. b) A funnel and gate system where impermeable funnel walls direct the contaminant plume through a reactive gate Figure 4 Reductive dechlorination of TCE to ethene and ethane (modified from Puls and Powell, 1997) Figure 5 Map of USCG Support Station, Elizabeth City, showing sample core (in red) and monitoring well locations relative to granular iron barrier Figure 6 Schematic cross-section of the barrier showing relative position of core sections received from USEPA and composition of core subsamples used in extraction tests Figure 7 Photographs of core sections FW1 5-7 and FW1 7-9 B containing background aquifer material. Red cap indicates top of the core. Subsamples for extraction tests were removed from roughly to top 10 cm of each core...23 Figure 8 Photographs of core sections FW , (barrier iron), FW3 Top Section (barrier iron), and GP 6/20 C3(2) (downgradient interface). Subsamples for extraction tests were collected from cm and cm in cores FW and FW3 Top section respectively. Subsamples in GP 6/20 C3(2) were collected at 4-8 cm, cm, cm, cm, cm, and cm. The transition from barrier to downgradient aquifer material occurs over an angled contact between roughly cm. Note that core GP 6/20 C3(2) was collected June 1997 not June 1996 as shown in the photograph Figure 9 Grains of Peerless iron used as reactive material in the Elizabeth City PRB as seen in a stereo microscope Figure 10 Sequential extraction results for upgradient samples FW15-7 (top) and FW1 7-9 (bottom). Blank columns indicate test results below detection limits. Note that concentration is plotted on a log scale vii

12 Figure 11 Sequential extraction results for barrier samples FW3 Top Section (top) and FW (bottom). Blank columns indicate test results below detection limits. Note that concentration is on a log scale...38 Figure 12 Sequential extraction results for Fe from Elizabeth City samples. Missing data points indicate test results below detection limits. Each extraction was performed on three separate batches of each sample. Error bars reflect variation in analytical results between batches (mean ± 1 standard deviation) Figure 13 Sequential extraction results for Mn from Elizabeth City samples. Missing data points indicate test results below detection limits. Each extraction was performed on three separate batches of each sample. Error bars reflect variation in analytical results between batches (mean ± 1 standard deviation) Figure 14 Sequential extraction results for Cr from Elizabeth City samples. Missing data points indicate test results below detection limits. Each extraction was performed on three separate batches of each sample. Error bars reflect variation in analytical results between batches (mean ± 1 standard deviation) Figure 15 Sequential extraction results for Al from Elizabeth City samples. Missing data points indicate test results below detection limits. Each extraction was performed on three separate batches of each sample. Error bars reflect variation in analytical results between batches (mean ± 1 standard deviation) Figure 16 Total exchangeable cations (meq/100 g soil) in upgradient (background) samples and downgradient samples, Elizabeth City PRB site...50 Figure 17 Exchangeable cations as a percentage of total exchangeable cations in upgradient and downgradient samples, Elizabeth City PRB site...51 Figure 18 Concentration of Mn in upgradient and downgradient samples from Elizabeth City PRB site Figure 19 Potential mass of Cr(III) oxidized per volume water for Elizabeth City PRB site based on measured Mn concentrations in aquifer sediments. Results assume that all Mn measured in extraction tests occurs as Mn0 2 and serves as a proton donor, facilitating the oxidation of Cr(III) to Cr(VI) Figure 20 Acid extraction results for upgradient aquifer materials, Elizabeth City PRB site...61 viii

13 Figure 21 Acid extraction results for barrier samples, Elizabeth City PRB site. Note that concentration is on a log scale Figure 22 Acid extraction results for downgradient samples, Elizabeth City PRB site. Sample GP6/20 C3(2) 25-31cm is adjacent to the downgradient edge of the barrier. Sample GP6/20 C3(2) 4-8cm is the most distal from the barrier...64 Figure 23 Acid extraction results for all samples from the Elizabeth City PRB site. Note that Fe concentration is plotted on a separate scale Figure 24 Decline of hydraulic conductivity with time for the Elizabeth City PRB. K o = initial hydraulic conductivity of the barrier. Hydraulic failure occurs when K barrier = K aquifer...77 Figure 25 Concentrations of major oxidants entering Elizabeth City PRB. Data collected in wells MW11, MW21 and MW31 by the University of Waterloo and US EPA between November 1996 and June High value equals highest single concentration recorded in all three wells over the complete sampling period. Average value equals the mean for data recorded between 4.5 and 6.5 m depth in all three wells over the complete sampling period. Note that concentration is plotted in log scale...81 Figure 26 Percent of total Fe 0 needed for reduction of each of the major oxidized species present at the Elizabeth City site. a) High oxidant concentrations, b) Average oxidant concentrations. In both cases, if reduction of all oxidized species is complete, dissolved SO 4 2- and NO 3 - combined account for more than 96 % of the Fe 0 oxidized per liter ground water. The two major contaminants at the site, Cr(VI) and TCE, account for less than 3% of Fe 0 oxidized...85 Figure 27 Background corrected extractable Fe concentrations in downgradient sediment samples from the Elizabeth City site. Data corrected for background Fe by subtracting average extractable Fe obtained from tests on upgradient samples. Missing data points indicate results below detection limits...97 Figure 28 Background corrected extractable Fe concentrations in barrier samples. Data was corrected for background by subtracting concentrations of extractable Fe obtained from tests on virgin Peerless iron ix

14 Chapter 1: Introduction In response to the expense and often-poor results associated with conventional pump-and-treat approaches to groundwater cleanup (National Resource Council, 1994), various alternative remediation technologies have emerged in recent years. One innovative technique that may hold significant potential utilizes permeable reactive barriers incorporating zero-valent iron (Fe 0 ) as the reactive material (Blowes et al., 1995; Gillham et al., 1994; Gillham and O'Hannesin, 1994; Matheson and Tratnyek, 1994; Puls et al., 1995). A permeable reactive barrier is a subsurface structure emplaced in the in the path of a moving plume of contaminated groundwater. The barrier is engineered to be more permeable than surrounding aquifer in order to provide a preferential conduit for groundwater flow. The barrier takes advantage of chemical or biological-chemical reactions between the barrier-fill and contaminants to transform contaminants into non-toxic or immobilized chemical forms (Blowes et al., 1995; Puls et al., 1995). Metallic iron is an attractive choice for reactive material due to its apparent effectiveness in treating a wide variety of contaminants. Examples include many common chlorinated hydrocarbons (Gillham et al., 1994; Gillham and O'Hannesin, 1994; Matheson and Tratnyek, 1994; Puls et al., 1995), various nitro-aromatic compounds (Agrawal, 1995; Agrawal and Tratnyek, 1996), and inorganic contaminants such as chromium (Blowes et al., 1995; Puls et al., 1995), technetium and uranium (Cantrell et al., 1995; Clausen et al., 1995; Del Cul et al., 1993; Del Cul and Bostick, 1995). The results of a recent field study also indicate that groundwater 1

15 contaminated by a mixture of both organic and inorganic compounds may be successfully remediated via the use of zero-valent iron (Puls et al., 1995). In addition, Fe 0 is readily available, relatively inexpensive, and environmentally benign (Gillham and O'Hannesin, 1994). Perhaps one of the most appealing features of Fe 0 reactive barrier technology, however, is that it has the potential to provide a cost-effective means of treating groundwater fouled by enduring contaminant sources. Since treatment takes place in situ, and typically utilizes natural hydraulic gradients to move water through the reactive zone, the remediation technique requires no continuous energy input and only limited maintenance following installation (Puls and Powell, 1997). When dealing with contaminants that may persist for decades to centuries, such advantages are considerable. At present, however, the majority of data relating to barrier performance has been gleaned from relatively short-lived laboratory or small-scale field tests (Blowes et al., 1995; Gillham et al., 1994; Gillham and O'Hannesin, 1994; Matheson and Tratnyek, 1994; Puls et al., 1995). Consequently, field-monitoring data from fullscale operations are scarce and key questions pertaining to the long-term viability of the technique remain unanswered. The purpose of this study is to investigate various physical and chemical processes associated with one of the few full-scale remediation operations currently employing Fe 0 reactive barrier technology. The main objective is to understand the factors controlling the long-term performance of a Fe 0 reactive barrier. The results of 2

16 the study allow the development of a geochemical mass-balance model that predicts the effective lifetime of a full-scale Fe 0 reactive barrier under operational conditions. 3

17 Chapter 2: Study Site Location The study site is located at the U.S. Coast Guard (USCG) Support Center, situated on the southern bank of the Pasquotank River approximately 5 km southeast of Elizabeth City, North Carolina (Figure 1). Figure 1 Location of U.S. Coast Guard Support Center, Elizabeth City, North Carolina. 4

18 Within the base, Hangar 79 contains a chrome plating shop which discharged acidic chromium wastes and associated chlorinated solvents through a hole in the concrete floor for over 30 years prior to its closure in 1984 (Puls et al., 1994). Hangar 79 is located roughly 60 m south of the Pasquotank River. The wastes have infiltrated the soils and underlying aquifer immediately below the shop s foundation, creating a mixed-contaminant plume in the groundwater beneath and to the north of the plating shop (Puls et al., 1994). Site geology and hydrology The site geology in the vicinity of the chrome plating shop was described by Puls et al (1995) as consisting of typical Atlantic coastal plain sediments, characterized by a complex and variable sequence of surficial sands, silts and clays. Representative shallow stratigraphic profiles for the site are shown in Figure 2 a and b. The uppermost soils are silty clay, overlying a thin sandy layer at about 2 m, which in turn overlays sands and silty fine sands, with occasional pockets and lenses of silt and clay. Puls et al. (1994) report that in some locations a dense gray clay layer substitutes for the sandy clay at 2 m. Fine to medium sands are dominant between 4 and 20 m, and a dense gray clay unit persists at a depth of roughly 20 m, dipping gently from north to south (Puls et al., 1994). Groundwater at the plating shop site occurs at approximately 1.4 to 2 m below land surface and is generally unconfined (Parsons Engineering-Science Inc, 1993). Water level measurements indicate that shallow groundwater movement is mainly northward 5

19 Figure 2 Representative (a) geologic and (b) soil profiles for the plating shop site, USCG Support Center, Elizabeth City, NC. Modified from Puls et al (1994). 6

20 toward the Pasquotank River with a horizontal groundwater gradient of between and (Puls et al., 1995). Data collected from deep monitoring wells at the site indicate that the vertical flow of groundwater fluctuates with time between downward and upward flow. Further investigations are underway to evaluate the systematics of vertical and horizontal groundwater flow at the site (Parsons Engineering-Science Inc, 1993). A highly conductive layer at roughly 4.5 to 6.5 m below the ground surface is coincident with the highest aqueous concentrations of chromate and chlorinated organics (Puls et al., 1995). Hydraulic conductivites measured in aquifer materials at the site range between 0.1 and 26 m/day (Bennett, 1997). Nature and extent of contamination Data collected by Parsons Engineering-Science Inc. indicate that over the period November 1990 through June 1993 the chromium contaminated groundwater plume was limited to an area approximately 30 m wide that extended roughly 37 m from the former plating shop northward toward the Pasquotank River (Parsons Engineering-Science Inc, 1993). Aqueous chromate concentrations in excess of 10 mg/l have been recorded within the plume (Puls et al., 1994). The predominant organic contaminant at the site is trichloroethylene (TCE), although associated breakdown products such as tetrachloroethylene (PCE), dichloroethelyene (DCE), and vinyl chloride (VC) have also been detected along with the petroleum related compounds benzene and toluene (Parsons Engineering-Science Inc, 1993). Dissolved organic contaminants extend further in the shallow groundwater than does chromium, 7

21 and are believed to be discharging into the Pasquotank River (Parsons Engineering- Science Inc, 1993). The barrier A preliminary evaluation of Fe 0 reactive barrier technology for remediation of contaminated groundwater at the site was performed during 1994 and provided encouraging results (Puls et al., 1999; Puls et al., 1995). Both chromate and chlorinated organic solvent concentrations significantly decreased downgradient from a small-scale test barrier comprised of 21 cylinders or fence posts, installed in three rows and filled with metallic iron/sand mixtures (Puls et al., 1999; Puls et al., 1995). A full-scale cleanup operation utilizing reactive barrier technology was subsequently implemented leading to the installation of a continuous trench reactive barrier in June, Approximately 280 tons of iron was placed into an excavated trench oriented in an east-west direction, perpendicular to the direction of groundwater flow. The 46 m long, 7.3 m deep and 0.4 to 0.6 m wide trench was filled with granular iron from 7.3 m to roughly 2 m below ground surface. The average emplaced density of the iron was estimated at 1.69 g/cm 3 (Bennett, 1997). The mass of iron used equates to 5.01E+06 moles of Fe 0. The amount of iron utilized was significantly less than the 450 tons originally anticipated. The original estimate was based on the iron having an emplaced density of 2.72 g/cm 3, similar to the measured bulk density of granular iron used in laboratory column experiments. The lower emplaced density of the trench fill indicates that the iron may not occupy the entire volume of the trench (Bennett, 1997). 8

22 Chapter 3: Background Permeable reactive barriers Subsurface barriers have typically been utilized in groundwater remediation programs to restrict the movement of contaminant plumes rather than to actually treat polluted groundwater. Traditionally, such barriers are constructed of impermeable materials such as grouts, slurries, or sheet pilings, and are emplaced with the goal of preventing contact between a contaminant plume and sensitive receptors such as drinking water wells or discharge into surface waters (Puls and Powell, 1997). In contrast, permeable reactive barriers are designed not only to intercept a contaminant plume, but also to provide a preferential flow path for polluted groundwater. Contaminated water is remediated as it passes through the reactive substrate of the barrier, resulting in the discharge of environmentally acceptable treatment product downgradient of the barrier (Figure 3). Two different barrier designs are currently being used in full-scale remediation operations (Puls and Powell, 1997). The continuous trench barrier (Figure 3a) is simply a trench that is excavated and simultaneously back-filled with the reactive material. Polluted groundwater passes through the trench barrier under its natural gradient. The second design, a funnel and gate system (Figure 3b) utilizes an impermeable funnel, typically consisting of interlocking sheet pilings or slurry walls, to direct the flow of contaminated water to a permeable gate or gates containing the reactive media. Concentrating groundwater flow through a narrow treatment zone results in higher groundwater velocities within the barrier than those found in surrounding aquifer materials (Puls and Powell, 1997). 9

23 a. Groundwater flow Trench containing permeable reactive medium Contaminant Plume (up-gradient) Plume of treatment products (down-gradient) b. Impermeable funnel walls Groundwater flow Contaminant Plume (up-gradient) Permeable gate containing reactive medium Plume of treatment products (down-gradient) Figure 3 Two configurations of permeable reactive barrier. a) A continuous trench reactive barrier. The permeable reactive medium in the trench provides a preferential conduit for contaminated ground water flow. b) A funnel and gate system where impermeable funnel walls direct the contaminant plume through a reactive gate. 10

24 The thickness of the reactive zone is determined by the residence time needed to achieve remedial goals. Factors to be considered include contaminant type and concentration, contaminant degradation rate in the presence of the reactive media, reactive material stability under in situ aquifer conditions, and the rate of groundwater flow through the barrier (Blowes et al., 1995; Puls and Powell, 1997). Zero-valent iron as a reactive material In the presence of water, zero-valent iron metal, Fe 0, is readily oxidized to ferrous iron, Fe(II), resulting in rapid oxidative dissolution (corrosion) of the metal. The primary reaction responsible for the corrosion of Fe 0 in aerobic aqueous systems is represented by equation 1. Available O 2 is further consumed by oxidation of Fe 2+ to Fe 3+ (equation 2). Other species typically present in groundwater that may be reduced by corrosion reactions with Fe 0 are that the reduction of 1997). Nitrite, NO 3 and 2 SO 4. Laboratory experiments indicate NO 3 by Fe 0 is rapid (Cheng et al., 1997; Rahman and Agrawal, NO 3, is formed as an intermediate product and ammonia, NH 3, as the end product (equation 3). Reduction of sulfate by iron (equation 4), although typically quite slow in natural systems unless bacterially or microbially catalyzed (Appelo and Postma, 1994), may be somewhat quicker in the highly reducing environment associated with an iron barrier. Ferrous sulfides have been detected as coatings on mineral surfaces and in cores retrieved from the Elizabeth City site during a pilot study involving Fe 0 (Puls et al., 1999) suggesting that sulfate reduction is occurring. Under anaerobic conditions where there are no other strong oxidants available, water 11

25 alone may serve as the oxidant allowing corrosion to proceed slowly by the reduction of H + or H 2 O to H 2 (equation 5) (Johnson and Tratnyek, 1994). 2Fe O 2 + 2H 2O 2Fe + 4OH (eq.1) 4Fe O 2 + 2H 4Fe + 2OH (eq.2) Fe + NO3 + 10H 4Fe + NH 4 + 3H 2O Fe + SO H 4Fe + H 2S + 4H 2O Fe H2O 2Fe + H + 2OH (eq.3) (eq.4) (eq.5) Chlorinated solvents One of the two primary contaminants at the Elizabeth City site is the chlorinated solvent TCE. Chlorinated solvents such as TCE may also be reduced via corrosion reactions with zero-valent iron (Gillham and O'Hannesin, 1994; Matheson and Tratnyek, 1994) with the contaminant substituting for oxygen as the dominant oxidant. The reaction (equation 6) results in the dechlorination of the solvent (RCl) and the oxidative dissolution of Fe 0. The results of such a reaction are generally either less harmful or more amenable to further degradation by other processes (Tratnyek, 1996). Fe RCl + H Fe + RH + Cl (eq.6) Figure 4 illustrates the reductive dechlorination of TCE to ethene and ethane both of which are rapidly biodegradable (Puls and Powell, 1997). The complete reduction of the contaminant to an environmentally acceptable form may occur via either of the two competing pathways shown, sequential hydrogenolysis (a) and 12

26 reductive β-elimination (b). Each pathway involves the formation of several intermediate products, such as cis- 1, 2-DCE and chloroacetylene. Reactive barrier design must provide sufficient contaminant residence time to allow for the degradation of such intermediate products to the desired end product (Puls and Powell, 1997). [TCE] 2 3 a. [cis-1,2-dce] [VC] [ethene] [ethane] e + H Cl 2e + H Cl 2e + H Cl 2e + 2H C2H2Cl2 C2H3Cl C2H4 C 2H6 C HCl b. [TCE] C HCl 2 3 [chloroacetylene] [acetylene] [ethene] [ethane] e 2Cl 2e + H Cl 2e + 2H 2e + 2H C2HCl C2H2 C2H4 C2H6 Figure 4 Reductive dechlorination of TCE to ethene and ethane (modified from Puls and Powell, 1997). Chromium Although oxidation states of chromium range from -2 to +6, only the +3 and +6 states are prevalent in soil (Palmer and Puls, 1994). Of greatest concern from a contamination standpoint is the hexavalent form, Cr(VI), which is both more toxic and generally more mobile than trivalent chromium (Powell et al., 1995a). Cr(VI) comprises the second principle contaminant at the USCG Support Center, Elizabeth City. Under oxic conditions, hexavalent chromium can exist as Cr(VI) oxyanions, HCrO 4 (bichromate ion) at ph < 6.5 and CrO 4 2 (chromate ion) at ph > 6.5. Under acidic conditions and for total concentrations of Cr(VI) greater than 10 mm, HCrO 4 2 polymerizes to form dichromate, Cr (Palmer and Wittbrodt, 1991). When 2 O 7 13

27 appropriate electron donors are present, however, such oxyanions are reduced to trivalent chromium forms which readily precipitate under alkaline to slightly acidic conditions as Cr (OH) 3. In the presence of dissolved Fe 2+, Cr (III) may precipitate as the amorphous Fe x Cr 1-x (OH) 3 solid solution (Eary and Rai, 1988; Rai et al., 1987; Sass and Rai, 1987). This latter case may be seen as being particularly important for groundwater remediation since the mixed Cr(III)-Fe(III) hydroxide phase has a lower solution equilibrium activity than either pure solid-phase hydroxide (Powell et al., 1995a). Based on experimental water chemistry data, equation 7 was proposed by Eary and Rai (1988) to describe the precipitation reaction of the mixed Cr(III) Fe(III) hydroxide phase. A stoichiometry of (Cr 0.25 Fe 0.75 )(OH) 3(s) was calculated for the solid phase (Eary and Rai, 1988). [ ( )]( ) ( ) x 3+ x 3+ + Cr + (1 ) Fe + 3H 2O Cr Fe OH H x (eq.7) x 3 s The reduction of Cr(VI) by Fe 0 can be described by the overall reaction shown in equation 8. However, Gould (1981) notes that the reduction may actually proceed via the oxidation of ferrous iron to ferric iron (equation 9). Fe(0) + Cr(VI) Fe(III) + Cr(III) (eq.8) 3Fe(II) + Cr(VI) 3Fe(III) + Cr(III) (eq.9) A full evaluation of the effectiveness of permeable reactive barrier technology for the remediation of Cr(VI) must also consider the potential oxidation of Cr(III) to Cr(VI) downgradient of the barrier. As noted by Palmer and Puls (1994), only two 14

28 constituents in the environment are known to transform Cr(III) to Cr(VI): dissolved oxygen and manganese dioxide (MnO 2 ). Of these two potential oxidants, Mn dioxide is believed to be the most likely cause of Cr(III) oxidation in soil environments (Eary and Rai, 1987; Palmer and Puls, 1994; Richard and Bourg, 1991). The presence of manganese dioxide in site materials may thus potentially limit the efficiency of a reactive barrier. Oxidation of Cr(III) to Cr(VI) downgradient of the reactive zone would in effect short circuit the barrier and reverse the remedial process. Experimental results indicate that the oxidation of Cr(III) by MnO 2 follows the reaction (Amacher and Baker, 1982) CrOH + 1.5δ MnO 2 HCrO Mn (eq.10) Potential limitations of Fe 0 permeable reactive barriers Various factors may limit the effective lifetime of Fe 0 permeable barriers or reduce their expected performance under operational conditions. Such factors range from geotechnical design and barrier-stability issues to problems arising from geochemical processes associated with Fe 0 barrier environments (Puls and Powell, 1997). On a fundamental level, it is obvious that for effective ongoing remediation a barrier must incorporate sufficient reactive substrate to accommodate the contaminant source. To calculate the mass of Fe 0 required for full remediation, data pertaining to both the mass of contaminant(s) at a given site, and to the types of reaction(s) taking place within a barrier must be available. In reality, however, a precise assessment of 15

29 contaminant mass is often unavailable. Nonetheless, by knowing both the mass of iron in a given barrier and the concentrations of oxidant species in groundwater entering the barrier, one may estimate the time it will take to exhaust the barrier iron. Such calculations establish an upper limit for barrier lifetime and provide an order-ofmagnitude estimate of the barrier s remedial capabilities. A more specific problem that has been identified by many workers as potentially significant in limiting the utility of reactive barrier technology involves the formation of secondary precipitates resulting from of iron corrosion (Gillham and O Hannesin, 1994; Blowes et al., 1995; Ejkholt and Sivavec, 1995; Tratnyek, 1996; Puls and Powell, 1997). Lifetime issues related to precipitation within the barrier may be divided into two areas. First, the chemical activity of the iron may diminish as iron grains become coated with precipitating phases. Second, the hydraulic properties of the barrier may alter over time as a result of precipitate build-up. These ideas are discussed in more detail below. Reduction of barrier reactivity due to precipitate build-up [ P] d = ksaa ρ = k d t [ P] ρ [ P] s m SA a (eq. 11) Equation 11 (Johnson et al., 1996), describes the rate of halogenated organic reduction by iron metal, where k SA is the specific reaction rate constant (L h -1 m -2 ), a s is the specific surface area of Fe 0 (m 2 g -1 ), ρ m is the mass concentration of Fe 0 (g L -1 of 16

30 solution), ρ a is the surface area concentration of Fe 0 (m 2 L -1 of solution), and P represents the reacting halocarbon. As shown in equation 11, the rate of reaction is directly influenced by the ratio of Fe 0 surface area to solution volume. However, the aqueous corrosion of iron is typically accompanied by the formation of secondary reaction products that precipitate on the metal surface (Gillham et al., 1994; Johnson and Tratnyek, 1994; Blowes et al., 1995). If secondary precipitation leads to significant accumulations of passive phases on reactive iron surfaces, reaction rates may subsequently decrease and hence the long-term utility of a reactive barrier be adversely affected (Gillham et al., 1994; Johnson and Tratnyek, 1994; Blowes et al., 1995). Precipitation could result in contaminant breakthrough if the residence time of contaminants within a barrier remains constant whilst reaction rates diminish. Alternatively, precipitation of secondary phases may have little or no adverse effects on barrier performance, and may rather promote the desired results via the formation of new sites for adsorption, reaction, or catalysis. The possible effects of solid-phase precipitation on the reactive properties of iron metal are currently under investigation by several groups using mixed-batch and homogeneous column systems (Johnson and Tratnyek, 1994). The reduction of hexavalent chromium by metallic iron is also dependent in part upon available Fe 0 surface area (Gould, 1982). However, since aqueous Fe 2+ produced during corrosion is also utilized in chromate reduction, the impact of precipitate build-up on metallic iron surfaces may be offset to some degree (Gould, 1981). 17

31 The types of precipitates forming in Fe 0 barrier environments may be anticipated to vary with the specific geochemical conditions prevalent at a given site. Precipitates that may form include Fe(OH) 2 (Gillham and O'Hannesin, 1994; Johnson and Tratnyek, 1994), goethite (Blowes et al., 1995; Gillham et al., 1994; Pratt et al., 1997), iron carbonates (e.g. siderite), or sulfides (Johnson and Tratnyek, 1994), and the intermediate anaerobic iron corrosion product green rust (Refait and Genin, 1993). Green rusts are mixed Fe 2+ -Fe 3+ hydroxides that can incorporate anions such as CO 2-3, Cl -, and SO 2-4 in interlayer regions between octahedral sheets of divalent and trivalent cations coordinated with OH (Schwertmann and Taylor, 1989). Reduction of barrier permeability due to precipitate build-up Barrier fouling due to secondary phase build-up may also potentially limit the effectiveness of Fe 0 reactive barrier technology (Gillham et al., 1994; Johnson and Tratnyek, 1994; Blowes et al., 1995). Permeable reactive barriers are designed to provide preferential conduits for groundwater flow, thus ensuring that contaminated water moves through rather than around, under or over the treatment zone. Clogging a barrier with precipitates will reduce the porosity and hence the hydraulic conductivity of the wall, and could potentially lead to the redirection of a contaminant plume when barrier hydraulic conductivity falls below that of the surrounding aquifer. Such a scenario may be viewed as the hydraulic failure of a barrier. Although various physical and chemical methods have been devised to remove precipitates forming during laboratory trials with Fe 0 reactive mediums (Matheson and Tratnyek, 1994; Agrawal and Tratnyek, 1996), the suitability of such techniques is 18

32 questionable for full-scale field applications (Tratnyek, 1996). More importantly, since there is limited data available pertaining to the rate and scale of precipitate build-up that may be expected in a Fe 0 barrier, it is presently unclear if and how long it would take for barrier hydraulic conductivity to be adversely affected by precipitation. 19

33 Chapter 4: Methods Sampling Core samples collected in November 1996 and June 1997 at the Elizabeth City site were obtained from Dr. Robert Puls of Robert S. Kerr Environmental Research Laboratory, Ada, OK. Angled bores were driven at 30 from vertical roughly 2-3 m up- and downgradient of the barrier using a Geoprobe (Figure 5, Figure 6). The core sections retrieved from the bores contained samples of aquifer material and barrier fill. Cores were frozen in the field using liquid nitrogen and transported to the Robert S. Kerr Environmental Research Laboratory, Ada, OK. Portions of selected cores were later sent to Portland State University for analysis. In the lab, core samples were described and photographed while still frozen within their plastic sleeves (Figure 7, Figure 8). The cores were then thawed and sectioned in an anaerobic chamber to reduce the potential for oxidation. Water was removed from sectioned samples by washing with methanol, and the samples were air dried within the anaerobic chamber. Subsamples of roughly 60 g each were removed from selected core sections, homogenized and divided to provide three replicate batches for analyses. Depending upon the test to be performed, the appropriate amount of sample was then weighed, transferred to a clean receiving vessel, and the required reagents added. The receiving vessel was then capped and removed from the anaerobic chamber for heating, mixing and centrifuging as required. 20

34 21 Road 0 50 ft 0 10 m Figure 5 Map of USCG Support Station, Elizabeth City, showing sample core (in red) and monitoring well locations relative to granular iron barrier.

35 Figure 6 Schematic cross-section of the barrier showing relative position of core sections received from USEPA and composition of core subsamples used in extraction tests. 22

36 Figure 7 Photographs of core sections FW1 5-7 and FW1 7-9 B containing background aquifer material. Red cap indicates top of the core. Subsamples for extraction tests were removed from roughly to top 10 cm of each core In total, ten subsamples from five core sections were prepared for extraction (Figure 7, Figure 8). Each subsample was treated in three replicate batches. The subsamples were chosen to include a variety of sampling locations both within and around the reactive barrier, and to provide data on both small-scale (cm) and largescale (m) variations in geochemistry associated with the barrier. 23

37 Figure 8 Photographs of core sections FW , (barrier iron), FW3 Top Section (barrier iron), and GP 6/20 C3(2) (downgradient interface). Subsamples for extraction tests were collected from cm and cm in cores FW and FW3 Top section respectively. Subsamples in GP 6/20 C3(2) were collected at 4-8 cm, cm, cm, cm, cm, and cm. The transition from barrier to downgradient aquifer material occurs over an angled contact between roughly cm. Note that core GP 6/20 C3(2) was collected June 1997 not June 1996 as shown in the photograph. 24

38 Iron grain morphology Individual iron grains from within the permeable reactive barrier were examined and photographed in a stereo microscope to provide visual data on iron grain morphology. Mineralogical analyses Mineralogical analyses of aquifer materials from the Elizabeth City site were performed using of X-Ray Diffraction (XRD) and light microscopy. Grain mounts of aquifer material from the upgradient side of the barrier were prepared for light microscopy. X-ray diffraction spectra were obtained on a Phillips XRG 3000 at Portland State University using CuKα radiation (KV40, Ma 20). Powder mounts of bulk aquifer materials and samples of the <2µm clay fraction were analyzed. The clay fraction samples were mounted using the porous ceramic plate method (Moore and Reynolds, 1997) and analyzed in various states e.g. air-dried, ethylene glycol saturated, and oven-dried at 550 C. XRD analyses of iron grains from the barrier to identify secondary precipitates were beyond the scope of this study but are the subject of ongoing investigation. Extraction tests Various extraction tests were performed on aquifer and barrier materials to provide information about the permeable reactive barrier and its environs. All extraction tests utilized reagent grade chemicals and ultra-pure water (R >10 megaohm-cm). The majority of analyses of extractants were performed by atomic absorption spectrometry (AAS) using a Perkin Elmer AAnalyst 300 flame AA with a 25

39 HGA 800 graphite furnace. Cr(VI) concentrations from water and phosphate extractions were determined by colorimetry using the Diphenylcarbazide (DPC) method (Bartlett and James, 1988; Bartlett and Kimble, 1976). The following extraction tests were performed. Sequential extraction tests. Sequential extractions of water (Palmer and Wittbrodt, 1990), phosphate (Bartlett and James, 1988; Bartlett and Kimble, 1976), oxalate (Borggaard, 1988; Ku et al., 1978), and dithionate-citrate-bicarbonate (DCB) (Borggaard, 1988; Ku et al., 1978) were used to assess the amounts of Fe, Cr, Mn and Al in the samples. Each extraction test preferentially targets different compounds of the selected element(s) e.g. water-soluble, adsorbed, amorphous, and crystalline phases (respectively). The results were used to estimate the mass of precipitates forming on barrier and aquifer materials. The general procedure for each test involves mixing 5 g of sample with the reagent in a 50 ml glass centrifuge tube, shaking for a given period of time, centrifuging at 1500 x g for 20 minutes, and filtering the supernatant through 0.2-µm filters to minimize the inclusion of colloidal particles. After each filtering step and prior to addition of the next batch of reagent, used filters were added to the centrifuge tube containing the sample in order to reduce loss of sample mass. Analysis proceeds by either AAS or colorimetry. Cation exchange Cation exchange tests were performed on aquifer materials retrieved from up- and downgradient of the barrier. The ammonium acetate method of Thomas (1982) 26

40 was utilized, where 5 g of air-dried sample is combined with an 25 ml of 1 N NH 4 Oac at ph7 and shaken at 20 RPM for 30 minutes. The supernatant is decanted to a 50 ml volumetric flask, the process repeated with an additional 25 ml of 1 N NH 4 Oac solution, and then brought up to the final volume with 1 N NH 4 Oac solution. Analyses for Ca 2+, Mg 2+, Na +, K +, Fe 2+, Mn 2+ and Sr 2+ were performed by AAS. Manganese extraction tests Tests conducted were for Diethylenetriaminepentaacetic acid (DTPA) extractable Mn, easily reducible (ER) Mn, and for hydroxylamine hydrochloride (HH) extractable Mn. All three methods are presented by Gambrell and Patrick (1982). Extractants were analyzed by AAS. The DTPA extraction involves the addition of 10 ml DTPA solution to 5 g of soil. The mixture was shaken for 2 hours at 20 RPM then centrifuged for 20 minutes at 1500 x g and filtered through 0.2 µm filters. The ER extraction utilized 2.5 g soil and 25 ml of Ammonium acetate containing 0.2% hydroquinone at ph The mixture is shaken for 30 minutes at 20 RPM then intermittently for 6 hours prior to centrifuging and filtering as above. The HH extraction used 0.5 g of soil and 25 ml of hydroxylamine hydrochloride solution, shaken for 30 minutes at 20 RPM and centrifuged and filtered as above. The hydroxylamine hydrochloride extraction is reported to selectively dissolve hydrous Mn oxides in soils and sediments with minimal attack on co-existing Fe oxides (Gambrell and Patrick, 1982). As such, the HH test was expected to provide the best idea of Mn concentration as Mn oxides in site aquifer materials. The other two 27

41 tests were performed to help constrain the data. The DTPA test is considered somewhat more aggressive as it may also attack coexisting Fe oxides and thus liberate Mn incorporated into the structure of such minerals. The ER Mn test targets reducible Mn precipitated as sparing soluble hydrous oxides (Gambrell and Patrick, 1982). Data obtained from the Mn extractions was used to estimate the potential for Cr(III) oxidation downgradient of the barrier by aquifer sediments. Acid extraction tests Acid extraction tests allow determination of the acid-soluble or nonexchangable fraction of Fe, Cr, Al, Mn, Ca, Mg, Na, K and Sr in samples. The method utilized was a modification of the method presented by Lanyon and Heald (Lanyon and Heald, 1982), where 2 g of air-dried sample is combined and boiled for 15 minutes with 40 ml 1 N HCl. The suspension is filtered and washed with four 15 ml portions of 0.1 N HCl, cooled, diluted to 100 ml, and analyzed by AAS. Porosity tests The porosity of barrier samples was estimated both gravimetrically and volumetrically and the results used in calculations to estimate the effects of secondary precipitation on barrier permeability and hydraulic conductivity. The gravimetric determination of porosity relies on the assumption that core samples were fully saturated when recovered and that no draining occurred prior to analysis. Porosity was established via calculation of the saturated water content of known sample volumes. Frozen subsections of barrier cores were measured and 28

42 weighed, removed from their plastic sleeve and oven dried at 105 C until constant weight was achieved. Total porosity (φ ) was estimated using m = ρ m wet dry φ (eq. 12) w V t where m wet is the wet sample weight, m dry is the dry sample weight, ρ w is the density of water and V t is the total volume of the bulk sample. Volumetric analysis was performed by immersing the oven-dried samples in a known volume of water and measuring displacement to give the volume of the solids. Porosity is estimated using V = 1 V s φ (eq. 13) t where V s is the volume of the solids and V t is the total volume of the bulk sample. It was assumed a priori that neither porosity test would be sensitive enough to directly measure the anticipated change in porosity of barrier materials collected over time intervals of months. The tests were rather used to establish a baseline value for the porosity of the barrier fill. Changes in porosity over time were then estimated by calculating the additional volume contributed to the barrier fill by phases precipitating on the iron surfaces. Permeability and hydraulic conductivity within the barrier were estimated using the Carmen-Kozeny model (Carman, 1937) that relates the permeability of a media (k) to its porosity (φ ) and specific surface area based on a solids volume (S o ) by 29

43 3 Co( φ) k = (eq. 14) 2 ( 1 φ) S 2 o where C o is a constant whose value is 0.2 (Carman, 1937). 30

44 Chapter 5: Results and Discussion Iron grain morphology Individual iron grains from the barrier were viewed and photographed in a stereo microscope (Figure 9). The sizes of the Fe-grains were generally on the order of a few millimeters in length and have various shapes. Several of the grains had deep grooves over the length of the grain that increase the surface area of the metallic iron. Mineralogy of aquifer materials Binocular microscope examination of aquifer material revealed mostly quartz grains with minor feldspars, mica and trace amounts of ilmenite/magnetite. The observations were supported by powder XRD analysis of bulk aquifer sediment in which quartz and orthoclase were detected (R.B. Perkins, personal communication). XRD analyses of the <2 µm clay fraction detected the presence of quartz, feldspars, mica (illite), chlorite and vermiculite. Disappearance of the chlorite 001 reflection upon heating to 550 C was observed, indicating that the chlorite is somewhat weathered and potentially altering to vermiculite (G. Grathoff, personal communication). The chlorite 002 reflection (0.715-nm) detected in air-dried samples remained essentially unchanged in intensity upon heating to 550 C indicating the absence of kaolinite in the samples examined. 31

45 Figure 9 Grains of Peerless iron used as reactive material in the Elizabeth City PRB as seen in a stereo microscope. 32