Article pubs.acs.org/est
Cite This: Environ. Sci. Technol. 2018, 52, 14321−14330
Diffusion-Coupled Degradation of Chlorinated Ethenes in Sandstone: An Intact Core Microcosm Study Rong Yu,† Richard G. Andrachek,‡ Leo G. Lehmicke,§ Amanda A. Pierce,∥ Beth L. Parker,∥ John A. Cherry,∥ and David L. Freedman*,† †
Department of Environmental Engineering and Earth Sciences, Clemson University, Clemson, South Carolina 29634, United States Stantec, 1340 Treat Boulevard, Suite 300, Walnut Creek, California 94597, United States § CO2&Water, 295 Kenilworth Drive, Akron, Ohio 44313, United States ∥ 360 G Institute for Groundwater Research, University of Guelph, Guelph, Ontario N1G 2W1, Canada
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‡
S Supporting Information *
ABSTRACT: Matrix diffusion must be considered when assessing natural attenuation and remediation of chlorinated ethenes in fractured porous bedrock aquifers. In this study, intact sandstone rock and groundwater from a trichloroethene (TCE)-contaminated site were used in microcosms (maintained for approximately 600 days) to simulate a single fracture-matrix system with a chamber at the top of the core allowing advection to represent fracture flow. Diffusioncoupled degradation with and without biostimulation were evaluated and compared to crushed-rock, batch microcosms. In the diffusion-transport microcosms, lactate stimulated reductive dechlorination of TCE to cis-1,2-dichloroethene (cDCE) and sulfate reduction. Reduction of TCE to cDCE led to a higher rate of chlorinated ethene removal from the cores, likely due to higher concentration gradients, along with lower sorption and a higher diffusion coefficient for cDCE relative to TCE. Reduction of cDCE to vinyl chloride or ethene did not occur as in crushed rock microcosms, inferring an absence of Dehalococcoides in the intact cores. Abiotic transformation was evident in the core microcosms based on the appearance of acetylene and enrichment in δ13C-TCE and δ13C-cDCE. Core microcosms permit a more realistic representation of the behavior of chlorinated ethenes in water-saturated fractured porous rock by incorporating the combined influence of fracture flow and matrix diffusion on transport and transformation.
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INTRODUCTION Contaminant migration and plume behavior in fractured bedrock aquifers are commonly controlled by diffusion in the low-permeability rock matrix, while advection and dispersion occur in fractures. Permeable fractures act as preferred flowpaths, and less permeable porous rock matrix blocks between the fractures serve as primary storage for contaminants.1 Delivery of fluid amendments for contaminant destruction is challenging because flow is strongly preferential in fractures but is limited by diffusion transport in lowpermeability zones.2 Natural attenuation of halogenated contaminants, including trichloroethene (TCE), is favored by reducing conditions in the rock matrix, where solid-phase organic matter and reduced minerals are found and the surface area for abiotic transformation is largest. Microbes in the rock matrix may degrade aqueous and sorbed mass,3,4 or a biofilm may grow on the fracture surface to destroy contaminant mass.5−7 Natural attenuation can be enhanced through biostimulation, which may also facilitate transformation of TCE and cis-1,2dichloroethene (cDCE) via abiotic pathways.8,9 © 2018 American Chemical Society
Estimation of degradation rates is important for design of in situ remedies. Unlike unconsolidated aquifers, it is not feasible to determine transformation rates within the rock matrix based on concentration changes over a distance in contaminant plumes due to the strong influence of physical processes like diffusion and sorption into/out of the low-permeability matrix and the difficulty in estimating the magnitude of this effect due to complexity and uncertainty in the fracture network conditions. Therefore, laboratory methods must be used where the fracture surface area and matrix conditions are known. Rate determinations for fractured sedimentary rock with low-permeability matrix usually involves bench-scale testing in microcosms where rock samples are crushed and homogenized; however, it is unclear how crushing affects degradation processes and rates.2,10,11 The transformation rates in crushed rock relative to intact sedimentary rock matrix Received: Revised: Accepted: Published: 14321
July 26, 2018 November 5, 2018 November 12, 2018 November 12, 2018 DOI: 10.1021/acs.est.8b04144 Environ. Sci. Technol. 2018, 52, 14321−14330
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Figure 1. Schematic of the intact rock core microcosms and processing of samples. Rock cores were encased in Teflon tape and heat-shrinkable tubing and then inserted into a stainless steel pipe that was welded to stainless steel end caps. Groundwater in the upper chamber was used to simulate flow over a fracture surface.
soluble compounds observed in the laboratory are likely transformation pathways in the field. Lactate was shown to be an effective electron donor for enhancing reductive dechlorination.27 Field evidence exists for abiotic and biotic transformation pathways with TCE transformation to cDCE dominated by reductive dechlorination and further degradation of cDCE to ethene with minor VC occurring in a spatially variable manner based on water quality and isotopic analysis.25 The objectives of this study were to evaluate transformation of TCE in intact sandstone core microcosms capable of closely representing in situ conditions with diffusion in and out of the core by flushing the sampling chamber to simulate advection in a fracture and to compare the results to crushed rock microcosms performed in batch mode. The novelty of the intact core, porous medium diffusion microcosms used in this study is highlighted by comparison to previous experimental designs (Table S1 and S2). Flow of clean groundwater over a simulated fracture surface mimicked the role of active flow in fractures. Measures were taken to represent in situ conditions, including use of site-specific groundwater and carefully preserved rock samples cored without drilling muds, use of inert materials (e.g., Teflon and stainless steel [SS]) to construct the microcosms, and maintaining anaerobic conditions. In addition to evaluating diffusion and abiotic transformation as in previous studies, this system evaluated naturally occurring reductive dechlorination, the effect of biostimulation, and the impact of sulfate reduction. The chemical gradient from the fracture surface to inside the rock matrix was evaluated at the end of incubation by analyzing sliced core sections.
blocks may be different. Crushing opens pores and creates small particles with increased exposure to primary mineral surfaces, thereby increasing the surface area available for reactions. Minerals such as magnetite and iron may contribute significantly to abiotic processes. Particles and surface coatings of solid-phase organic matter are also common in sedimentary rock. During crushing, the surface area and accessibility of these minerals and organic matter may be altered, causing an increase in reactivity, or transformation rates and capacity may be decreased by exposure to air. Furthermore, batch microcosms using crushed rock cannot represent the role of diffusion between the rock matrix and fractures as they occur in the field. Microcosms using pieces of intact rock provide a more realistic environment to evaluate transformation processes, but their application has been limited. Barone and colleagues12−14 conducted tests on intact shale and mudstone to determine chloride diffusion and rock tortuosity. Blocks of rock from fractured limestone or sandstone have been used to evaluate dissolution of nonaqueous phase tetrachloroethene,15,16 to study the effect of bioaugmentation on dissolution,17 and to evaluate diffusion and reaction of permanganate.18 Boving and Grathwohl19 developed a diffusion cell with minimally disturbed sedimentary rock to study iodide diffusion. A similar configuration was employed by Schaefer et al.20,21 to determine iodide diffusion and core orientations to assess effects of anistropy and to evaluate abiotic reaction of TCE under anaerobic and diffusion conditions. Intact rock core samples have also been used to evaluate thermal remediation.22 However, prior studies with intact rock that incorporate matrix diffusion did not include flow in a fracture in contact with the rock matrix. This study used rock core samples from a fractured interbedded sandstone and siltstone site for assessment of degradation processes in a diffusion-transport microcosm that better simulates field conditions. Results from groundwater and rock core samples and laboratory crushed rock microcosms at this site show good consistency,23−26 indicating that reductive dechlorination of TCE to cDCE and abiotic transformation of TCE and cDCE to acetylene, CO2, and
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MATERIAL AND METHODS Core Sample Collection. Samples of core were obtained using diamond drilling, an HQ2 split core barrel, and cooled with minimal amounts of water to avoid the use of polymer or bentonite drilling fluids. Cores for the microcosms were collected at depths of 288, 298, 310, 312, 351, and 421 m below ground surface (bgs) and cut into 7.62 cm lengths selected to appear homogeneous and represent the primary
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biostimulated with lactate (L). One of the unamended controls (U4) broke during preparation, leaving five unamended microcosms. Three microcosms without core were filled with groundwater containing TCE, bromide, and resazurin (no lactate) to serve as chamber/equipment controls to evaluate potential abiotic reaction of TCE with the 304 SS. The core microcosms were incubated quiescently and upright at room temperature (22−24 °C; Figure S4a). To simulate flow through the fracture, site groundwater (∼2 mL) containing no detectable TCE or bromide was injected each week into the chamber through one of the Mininert valves; the displaced groundwater was collected through the other valve (Figure S4b). Groundwater added to one-half of the microcosms was amended with lactate (2−6 mM). The 2 mL samples were used to measure VOCs, organic acids, anions, and pH. δ13C measurements were conducted at approximately 3 month intervals. The mass of each microcosm was recorded before and after sampling. When incubation ended (∼21 months), the core microcosms were sent on ice to the University of Guelph where they were immediately frozen (−80 °C), removed from the SS casing, and sliced into three equally sized cylinders (Figure 1). Each section was then cut into quarters for analysis of VOCs (University of Guelph), molecular microbiology and organic acids (Clemson University), and anions (University of Ottawa) (Figures S5 and S6). Crushed Rock Microcosms. Crushed rock microcosms were constructed with rock samples from the same core hole to aid in making comparisons to the intact rock core results. Two sets of 12 crushed rock microcosms were prepared; one set received ∼14 mg/L TCE to mimic the average starting concentration in the intact core microcosms. The other set received ∼0.7 mg/L, closer to what was used in previous crushed rock microcosms.27 Among each set, one-half of the bottles received lactate and the other half did not. Analytical Methods and CSIA. The amount of TCE, cDCE, VC, ethene, ethane, acetylene, and methane present in the top reservoir of the microcosms was monitored by gas chromatographic (GC) analysis of 0.5 mL headspace samples.29 Bromide and sulfate were measured by ion chromatography (Dionex AS9-HC anion exchange column, 4 mm × 250 mm; 9 mM Na2CO3 eluant, 1 mL/min). Organic acids were analyzed by high-performance liquid chromatography.27,28 Samples preserved with 10 M NaOH were analyzed for δ13C-TCE and δ13C-cDCE at the University of Waterloo Environmental Isotope Laboratory. Genetic Analyses. At the end of incubation, DNA was extracted from the groundwater in the chamber and crushed rock (∼1 g) from each core section using MOBIO PowerSoil DNA Isolation Kit following the manufacturer’s protocol. Dehalococcoides, bvcA, vcrA, and tceA were quantified using the TaqMan probe-based qPCR method. Other bacteria, including Dehalobacter, Desulfitobacterium, Sulfurospirillum, Desulf uromonas, Geobacter, and Geobacter lovleyi, were quantified with SYBR Green dye-based qPCR.27,28
medium-grained sandstone lithology of the Chatsworth Formation. Samples were also collected from the same corehole at depths of 277, 295, and 297 m bgs and crushed. All samples were placed in vacuum-sealed bags, purged with high-purity N2, and shipped overnight on ice to Clemson University. Groundwater was collected at a monitoring well in the source zone where TCE as a dense, nonaqueous liquid entered the subsurface decades ago and groundwater contamination persists. Additional information about the corehole and sampling methods are available.28 Chemicals. The chemicals used (purity, source) were TCE (99%, Alfa Aesar), cDCE (99%, TCI America), vinyl chloride (VC; 99.5%, Fluka), polymer grade ethene (99.9%, Airgas), ethane (99.95%, Matheson), methane (99%, Matheson), and acetylene (99%, Matheson). Sodium lactate syrup was from EM Science (58.8 to 61.2% sodium lactate). L-(+)-Lactic acid in water (90%) was from Acros Organics. All other chemicals were reagent grade. Intact Core Microcosms. The objective of the intact microcosms was to simulate what happens in the watersaturated rock matrix in combination with water flow and transport in a fracture to which the rock matrix and the fracture experience one-dimensional diffusion-driven mass transfer of dissolved constituents. The microcosms consisted of 7.62 cm (length) × 6.03 cm (diameter) sandstone core and end caps made of 316-SS (Figure 1). The rock matrix volume is cylindrical, so that core samples from the field site can be used without alteration except for two cuts across the core ends to achieve the desired length. Cores were wrapped in Teflon tape and heat-shrinkable Teflon tubing. Initially, cores were wrapped in self-fusing rubber tape confined by hose clamps to permit pumping of groundwater (containing 20 mg/L TCE, 1 mM bromide, and 1 mg/L resazurin to serve as a redox indicator) through the cores using a bladder accumulator to achieve saturation and near homogeneous pore fluid composition initially. Hose clamps and rubber tape were then removed to reveal the Teflon sleeve. The ends of the Teflon sleeve were shortened by 0.635 cm to expose part of the SS end caps. Each core was then inserted into smooth-bore seamless 304-SS tubing (6.35 cm outside diameter × 0.165 cm wall thickness) with the inner wall custom machined to fit each core snuggly. Tungsten inert gas welding sealed the rock core tubes with 304-SS end caps, forming a leak-proof seal. The welding was done at a sufficiently low temperature to minimize disruption of the core or loss of TCE. One of the end caps was machined to create a 0.635 cm deep head chamber next to the end of the core. The core represented the rock matrix, and the chamber mimicked a fracture. Once assembly of the cores was completed, the chamber was flushed with groundwater containing resazurin but no TCE or bromide to simulate initially uncontaminated conditions in the fracture and invoke a reverse concentration gradient for TCE from the rock core matrix. Two Mininert valves were then installed on the top cap, corresponding to the chamber, to allow circulation of water through the chamber representing the fracture with flow. A SS plug was used to seal off the hole in the bottom cap, which is the no-flow boundary furthest from the chamber representing the fracture. Figures S1−S3 in Supporting Information (SI) provide details. Twelve core microcosms were prepared, divided into six pairs. Each pair was constructed with adjacent core segments. The intent was for each pair to have similar characteristics, with one serving as an unamended control (U) and the other
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RESULTS AND DISCUSSION VOCs, Ions, and Organic Acids. Lactate stimulation resulted in reductive dechlorination of TCE to cDCE in five of the six lactate-amended microcosms (L3 was the exception), while U2 was the only one out of five microcosms without lactate stimulation that exhibited TCE reduction to cDCE (Table S4). Responses of U5 (Figure 2) and L5 (Figure 3)
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Figure 2. Results for (a) VOCs and (b) inorganic ions in unamended intact core microcosm U5. Color bar below the legend in panel a indicates the color of the resazurin in weekly groundwater samples (blue and pink represent an oxidation−reduction potential above −110 mV; no color indicates a level below −110 mV).
concentration gradients formed between the groundwater in the chamber and the core pore water, which contained a high initial concentration of TCE. Decreasing concentrations in the chamber occurred shortly after monitoring began, most reasonably attributed to decreasing concentration gradients and, hence, smaller diffusive flux. There was no reductive dechlorination of TCE in U5 during 594 days of monitoring. The trend for bromide was similar to TCE (Figure 2b), confirming that changes in TCE were primarily caused by diffusion rather than degradation. The lack of reductive dechlorination is consistent with the redox potential remaining above −110 mV (as indicated by the resazurin color); methane remained below 0.07 mg/L. Sulfate was relatively constant, suggesting the concentrations in the rock matrix and the fracture were similar, thereby yielding no measurable diffusive exchange. These results also indicate an absence of sulfate reduction, consistent with deficient electron donor. Data for the paired lactate-amended core microcosm (L5) are presented in the same manner as U5, i.e., there are two data points for each day (Figure 3). Lactate is an exception, as the measured value is lower than the calculated value that follows it. This occurred because lactate was always present at a higher concentration in the groundwater being added compared to the sample being displaced. TCE started at 0.47 mg/L and increased to 2.70 mg/L, followed by a slow decrease. Results for the first 126 days were similar to U5. Thereafter, reductive dechlorination of TCE to cDCE began; by day 200, TCE was
microcosms were mostly representative of the trends in the other microcosms (Figures S7−S10). In the head chamber of unamended core microcosm U5, TCE and bromide increased rapidly (within 6 days) followed by a steady and slow decrease. Sulfate remained almost unchanged (Figure 2). Organic acids were undetectable in unamended microcosms at the beginning of the incubation and were not monitored thereafter. There were low levels of TCE (0.71 mg/L) and bromide (0.04 mM) at the beginning (Figure 2, insets). This is because the first sample was taken approximately 1 h after the head chamber was flushed with uncontaminated groundwater, which allowed a small amount of TCE and bromide to diffuse into the chamber. The next data point on the same day was calculated based on removal of 2 mL from the 14.5 mL chamber and replacing it with groundwater that was free of TCE and bromide, hence, the vertical decrease. The data for VOC and inorganic ions are plotted in this same “measured-to-calculated” manner, giving rise to the “sawtooth” pattern. For example, on day 34, the measured TCE level was 2.46 mg/L in U5 (Figure 2a). By replacing 2 mL in the head chamber with groundwater containing no TCE, the new concentration in the head chamber was calculated to be 2.08 mg/L; this resulted in the vertical drop in Figure 2a on that day. After 1 week of incubation, while the volumetric exchange rate for groundwater remained nearly constant, TCE and bromide increased to 2.16 mg/L and 0.26 mM, respectively (Figure 2, insets). These increases are attributed to 14324
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Figure 3. Results for (a) VOCs, (b) inorganic ions, and (c) organic acids in lactate-amended intact core microcosm L5. Color bar below the legend in panel a indicates the color of the resazurin in weekly groundwater samples (blue and pink represent an oxidation−reduction potential above −110 mV; no color indicates a level below −110 mV).
∼0.01 mg/L. cDCE peaked at 3.34 mg/L (∼day 223), followed by a gradual decline (Figure 3a). The onset of reductive dechlorination in this core coincided with a change in resazurin color from pink to clear (∼day 50), indicating redox conditions below −110 mV. VC and ethene were not detected. Bromide in L5 behaved similarly to U5, indicative of similar rates of diffusion from the paired rock samples. Unlike U5, sulfate consumption started in L5 soon after operation began and rapidly dropped below detection; weekly increases were attributable to the addition of site groundwater with ∼1.5 mM sulfate, which was repeatedly consumed, concurrent with reductive dechlorination of TCE to cDCE. Lactate consumption started immediately. Concentrations in the head chamber decreased until lactate was close to or below detection (∼day 40), and further additions were consumed within 1 week (Figure 3c). After day 20, acetate accumulated but soon leveled off (∼0.7 mM). In an attempt to stimulate cDCE reduction to VC, lactate in the groundwater was increased (2 to 4 mM) on day 196. This led to a small residual lactate level after each week, coupled with increasing acetate. The lactate dose was increased again (5 mM) on day 307. Acetate continued to accumulate and reached a maximum on day 524 (4.6 mM). Propionate was detected starting on day 377; prior analyses for propionate were not available.
Propionate mostly increased during the period of highest lactate addition, but its maximum level (0.34 mM) was more than an order of magnitude lower than the peak for acetate. Accumulation of propionate is indicative of excess hydrogen from fermentation of lactate.30 This suggests that the lack of reductive dechlorination of cDCE to VC or ethene was not a consequence of inadequate electron donor. Reductive dechlorination of TCE to cDCE in the intact core microcosm (Table S4) was consistent with results for crushed rock microcosms, in which TCE reduction to cDCE occasionally occurred in unamended microcosms but was stimulated by lactate.27 Unlike lactate-amended crushed rock microcosms, there was no further reduction to VC or ethene in intact core microcosms.27 One potential explanation is the heterogeneous distribution of halorespiring microbes, especially Dehalococcoides, at this site. Since all core samples were collected from a single corehole at depths different from those where samples were collected for a previous crushed rock microcosm study,27 it is likely that samples for the intact core microcosms were located in zones without indigenous Dehalococcoides and thus may explain the sparsity of complete reductive dechlorination. The heterogeneous distribution of microbes involved in TCE and cDCE degradation processes was evaluated in a sedimentary rock by Lima et al.4 and is expected at this site based on the large variability of TCE and 14325
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Figure 4. δ13C enrichment results for intact core microcosms based on (a) δ13C-TCE levels in individual microcosms without or prior to the onset of reductive dechlorination and (b) δ13C-cDCE levels in individual microcosms with cDCE production. WC stands for water controls. SS stands for stainless steel container controls, which were only tested twice at the end.
Enrichment of δ13C. δ13C-TCE results are shown in Figure 4a for microcosms with no appreciable reduction of TCE to cDCE (U3, U5, U6, and L3); data were not collected for microcosms that underwent biotic reduction of TCE to cDCE. U1 was not tested because its TCE concentration was too low. Also shown are the results for triplicate water controls (WC), consisting of the same groundwater that was used on a weekly basis for exchange of water in the head chamber of the microcosms, plus freshly added TCE saturated groundwater prior to each analysis, and triplicate container controls, consisting of groundwater incubated within SS-cylinders with the same dimensions as the microcosms. Average δ13C-TCE results for all of the water controls were used in place of a time zero measurement for δ13C-TCE (Figure 4a). There was no statistically significant change in the WC values or the SS container controls (p ≫ 0.05), while a statistically significant increase in δ13C-TCE (up to 5‰) occurred in the four microcosms (0.002−0.006 ‰/day). δ13C-cDCE results are shown in Figure 4b for microcosms that underwent reductive dechlorination of TCE to cDCE (U2, L2, L4, L5, and L6). L1 was not tested because its cDCE concentration was too low. Enrichment in δ13C-cDCE occurred in all of the microcosms (0.002−0.011 ‰/day), consistent with rates in prior crushed rock microcosms.27 In a few of the microcosms, the increases in δ13C-TCE or δ13CcDCE over the early incubation period were followed by a plateau (Figure 4). This may have been caused by a depletion of reducing capacity of iron-bearing minerals, as observed in
degradation products along with hydrochemistry conditions observed in rock core porewater.27 Methane did not accumulate in any of the unamended microcosms (Figure S11), consistent with the presumption that these microcosms were deficient in electron donor. Methane accumulation occurred in L1, L3, and L4 but only after >300 days of incubation when the lactate dose was increased. In two of the microcosms with active reductive dechlorination (L1 and L4), methane accumulated when TCE was close to the detection level. Acetylene accumulated (Figure S12) but at levels too low to be visible in Figures 2 and 3. Acetylene remained below 0.02 mg/L in the unamended microcosms. Levels were consistently higher in the lactate-amended microcosms, suggesting lowering the redox potential enhances abiotic transformation. Notably, acetylene levels were highest in the microcosms with limited (L1) or no (L3) reductive dechlorination. Acetylene levels started to increase in these microcosms around the same time methane increased. The chemical oxygen demand (COD) associated with acetate production and sulfate consumption was less than onehalf of the lactate COD consumed. Methane and propionate production and reductive dechlorination of TCE contributed only a minor amount to lactate consumption, suggesting that a significant fraction of the COD diffused into the rock matrix. This is consistent with the end of incubation results for acetate, shown below. 14326
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Figure 5. Results for end-of-incubation analyses of (a) water to rock mass ratio, (b) bromide, and (c) acetate distribution. Three or four bars for each microcosm represent (from left to right) the concentrations in the reservoir inside the head chamber (C) and inside the top (T), middle (M), and bottom (B) rock sections. Water-to-rock mass ratio is not relevant for the chamber.
previous crushed rock microcosms.31 Nevertheless, the overall trend toward enrichment was statistically significant in all cores shown. Prior results for crushed rock microcosms suggested that accumulation of 14CO2 and 14C-soluble, nonstrippable residue (NSR) correlated with enrichment in δ13C-TCE and δ13CcDCE.27 By extension, enrichment in δ13C-TCE and δ13CcDCE in the intact core microcosms from this study was likely a result of abiotic transformation processes that produced CO2 and NSR as products. Since 14C-labeled compounds were not added to the intact core microcosms, a degradation rate based on accumulation of 14CO2 plus 14C-NSR was not calculated.27 The VOC and δ13C results cannot be used directly for rate estimation because they represent the combined effect of biotic/abiotic degradation, diffusion, and sorption. pH and Mass Change. Average pH ranged from 7.73 to 7.90 in unamended intact core microcosms and from 7.51 to 7.73 in lactate-amended microcosms. The lower mean pH in lactate-amended microcosms was likely due to the greater CO2 production from metabolism of lactate and acetate as well as the use of lactic acid for biostimulation. The pH in all microcosms remained in a range that is conducive to biotic
reductive dechlorination (Table S5). The site mineralogy provides a significant level of buffering.32 The total mass of the microcosms was recorded before and after weekly exchange of groundwater to ensure that the sample collected was properly displaced by injected groundwater and to reveal if leaks developed. There was an increase over the incubation period, ranging from 1.3 to 5.4 g per core. The increases primarily occurred during several of the sampling events, i.e., when less than 2 mL of groundwater was displaced from the chamber after 2 mL was injected. This was likely a consequence of displacing gas bubbles in the head chamber. The mass increases were compared to the mass of water that should have been present if the cores were uniformly saturated (28 g), indicating that an average of 89.2 ± 3.5% (95% confidence interval) of each core was saturated (Table S5). End of Incubation. At the end of incubation, several core samples were crushed, dried, and rehydrated to extract nonvolatile compounds. A water-to-rock mass ratio specific to each rock section was calculated (Figure 5a). Comparison of the three core sections from each microcosm suggested uniform water distribution from top to bottom. The cores in each pair exhibited similar water-to-rock ratios, indicating that 14327
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Figure 6. Mass balance for (a) bromide and (b) chlorinated ethenes at the end of the incubation period from each microcosm.
total bacteria were higher in the lactate-amended microcosms than in unamended microcosms (p < 0.05, one-tail Student’s ttest), but no difference was found for the core sections.28 Results for the core sections were more variable, possibly due to the heterogeneity of microbial distribution at very low populations,4 the sample size used (∼1 g) relative to the entire core section, or decay of microbes by the time the cores were measured. Mass Balances. The amount of bromide removed during exchange of groundwater plus the amount remaining at the end of incubation are shown in Figure 6a. Assuming a starting amount of 28.3 μmol/microcosm, recoveries ranged from 66% to 102% for pairs 2−6 and 37% to 47% for pair 1. An overestimation of porosity likely caused the lower recovery in pair 1, as suggested generally by their lower water-to-rock mass ratio (Figure 5a). These bromide recovery variations are consistent with heterogeneous distribution of porosity within the rock matrix.25 TCE and cDCE removed from the microcosms plus the amount remaining at the end of incubation is shown in Figure 6b. Assuming a starting amount of 4.3 μmol/microcosm, recoveries averaged 104 ± 26% for microcosms in which TCE underwent reduction to cDCE and 65 ± 20% in those that did not experience significant reductive dechlorination. Higher recoveries from microcosms that underwent extensive dechlorination may be a consequence of the higher diffusion rate and lower adsorption for cDCE than TCE, allowing more
the porosities and saturation levels were similar between the pairs. Using the ratio and an average dry rock bulk density for sandstone at this site (2290 kg/m3),33 the porosity was 8.2− 16.2%, with an average of 13.2%. This is close to the average value for sandstone reported at the site (13.6%).33 Microcosm pair U1 and L1 had significantly lower water-to-rock mass ratios compared to the others, indicative of a lower rock porosity from this depth or incomplete saturation. At the start of each microcosm, no bromide was added to the chamber water. Therefore, the concentration gradient from the core to the chamber (as observed in the concentration gradients at the end of the experiments; Figure 5b) caused continual diffusive loss of bromide. The differences in bromide at the top and bottom of the core and in the chamber (Figure 5b) are slightly variable, indicating different effective diffusion coefficients among samples. Acetate was the major organic acid identified in the sacrificed cores (Figure 5c); lactate and formate were also present but at 4−10-fold lower concentrations.28 As expected, acetate decreased from the head chamber to the bottom of the core in lactate-amended microcosms because acetate was generated from lactate added to the head chamber, which subsequently diffused into the core. Geobacter and total bacteria were quantifiable in the sacrificed cores, while Dehalococcoides, vcrA, bvcA, tceA, and Geobacter lovleyi were below detection.28 When comparing the gene copy concentrations in the head chambers, Geobacter and 14328
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transport microcosms are more representative of field conditions over batch tests. Results from this study demonstrate the advantages of diffusion-transport microcosms for laboratory evaluation of sites with chlorinated ethene contamination in fractured sedimentary bedrock. Due to the existence of multiple, interacting processes with differential rates for reactions and diffusion, numerical simulations of these physical, biogeochemical processes are needed to compute degradation rate constants. This study provides insights on rate-limiting and varying conditions in both ambient (natural) and biostimulated diffusion-controlled zones that can inform prospects for abiotic and biological degradation in these dual porosity and permeability systems. Lactate was effective in significantly increasing the rate of TCE reduction to cDCE, which then diffused out of the cores at a higher rate than TCE. This may not necessarily be beneficial if further transformation of cDCE to nonhazardous end products cannot be achieved either biologically or abiotically. Limitations in using intact core microcosms include (1) the core may not adequately represent spatial heterogeneity at the site scale unless a representative number of cores are tested, (2) the extent of natural transformation may be underestimated if reduced minerals are depleted during the lengthy monitoring period, and (3) they are more time consuming and costly to construct and maintain than crushed rock microcosms. Nevertheless, intact core microcosms can prove extremely insightful as to dominant active processes and their quantification in situ, thus providing additional support for any restorative actions to be considered at the site scale.
mass to enter the head chamber. Microcosms with higher VOC levels at the end of incubation may have experienced proportionately more loss when the cores were processed, contributing to lower recoveries. Recoveries were lowest in pair 1, also likely due to an overestimation of their porosity. The results indicate that biostimulation could contribute to enhanced diffusion of cDCE, thereby reducing the longevity of VOCs in the rock matrix. Core Saturation. Several lines of evidence suggest the cores were close to saturation following preparation of the microcosms: (1) The mass ratio of water-to-rock (Figure 5a) indicated a uniform distribution of water and the calculated porosity was close to reported values for the site. (2) The mass balance for bromide (Figure 6a) in 9 of the 11 microcosms was 84 ± 8%. Bromide recovery in U1 and L1 was lower (37− 47%), likely due to an overestimation of their porosity and consistent with their lower water-to-rock mass ratio. (3) The mass balance for chlorinated ethenes (Figure 6b) in the five cores that exhibited TCE reduction to cDCE was 104 ± 26%. Recoveries were lower in microcosms without reductive dechlorination. (4) The mass gain in the microcosms during operation (Table S5) permitted an estimate of an average water saturation of 89 ± 4%. The percentage of void pore volume (∼11%) is similar to the deficit in the bromide mass balance (∼16%). Taken together, these metrics suggest that groundwater forced through the microcosms during preparation reached 84−89% saturation of the pores in the cores, consistent with variations in microcosm results considering effects of water-filled porosity on diffusion. Crushed Rock Microcosms. Crushed rock microcosms were prepared at the same time as the diffusion-transport core microcosms using rock from adjoining parts of the same corehole. TCE underwent a relatively slow rate of reductive dechlorination to cDCE in 2 out of 6 microcosms with low levels of TCE (∼0.010 μM/d) and 1 out of 6 with high levels of TCE (∼0.15 μM/d) for the unamended treatment and approximately 10-fold faster reduction to cDCE in 6 out of 6 microcosms with low levels of TCE (∼0.12 μM/d) and 3 out of 6 with high levels (∼1.4 μM/d) for the lactate-amended microcosms (Figure S13). The onset of reductive dechlorination was faster in bottles that were started with a lower initial TCE concentration. Only one bottle with 0.7 mg/L TCE exhibited complete reduction to ethene, a lower frequency than what was observed in prior crushed rock microcosms.27 These results were consistent with the lack of cDCE reductive dechlorination in the diffusion-transport rock core microcosms and the lack of Dehalococcoides in the rock samples. None of the bottles with high TCE concentrations (∼14 mg/L) showed reductive dechlorination beyond cDCE, suggesting the concentration of TCE infused into the rock may be inhibitory to the indigenous Dehalococcoides. The effect of elevated TCE concentrations on Dehalococcoides varies depending on the source of the culture.34 Both abiotic and biotic processes were observed in intact core microcosms as in crushed rock microcosms performed in this study and others.27 However, the extent and rate of degradation appear to be lower in the intact core microcosms, as indicated by a lack of complete reductive dechlorination to ethene, and less enrichment of δ13C compared to those in previous crushed rock microcosms.27 Intact cores may be affected more by the heterogeneous distribution of dechlorinating bacteria and/or reduced iron minerals. Also, transport of substrate or chemicals is more limiting, yet diffusion-
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.8b04144.
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Intact rock core methodology comparison; experimental design, preparation and monitoring of the intact core microcosms; end of incubation analysis method; VOCs results for intact core microcosms; methane and acetylene results for intact core microcosm; pH results and mass increase; crushed rock microcosms (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
David L. Freedman: 0000-0001-6778-3706 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by MWH Global (now Stantec). The authors are grateful to Dr. Ramon Aravena and the Environmental Isotope Laboratory at the University of Waterloo for their assistance with VOC CSIA. We want to thank Drs. John Wilson and Ramon Aravena for their advice on interpreting the CSIA results in the context of the experiment and site conditions. Maria Gorecka, Rashmi Jadeja, Ryan Kroeker, and Carla Rose supported rock core sampling, preservation, and shipping to various laboratories for analyses post experiment and performed VOC analyses. We also thank 14329
DOI: 10.1021/acs.est.8b04144 Environ. Sci. Technol. 2018, 52, 14321−14330
Article
Environmental Science & Technology
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Tom Al and Nimal De Silva at the University of Ottawa for their help with anion analyses.
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