Letter pubs.acs.org/journal/estlcu
Biodegradable Organic Carbon in Sediments of an ArsenicContaminated Aquifer in Bangladesh Rebecca B. Neumann,*,† Lara E. Pracht,† Matthew L. Polizzotto,‡ A. Borhan M. Badruzzaman,§ and M. Ashraf Ali§ †
Department of Civil and Environmental Engineering, University of Washington, Seattle, Washington 98195, United States Department of Soil Science, North Carolina State University, Raleigh, North Carolina 27695, United States § Department of Civil Engineering, Bangladesh University of Engineering and Technology, Dhaka 1000, Bangladesh ‡
S Supporting Information *
ABSTRACT: Laboratory incubations of sediment collected from an arsenic-contaminated aquifer in Bangladesh revealed a hitherto undocumented pool of biodegradable sedimentary organic carbon. Sampling, homogenizing, handling, and/or experimentation with the sediment released organic carbon, causing dissolved organic carbon (DOC) concentrations to reach ∼150−250 mg/L when DOC was mixed with recharge water. The native sedimentary microbial community rapidly consumed the released carbon, producing methane, while no loss of DOC was observed in sterilized sediments. In both sterilized and native incubations, dissolved arsenic equilibrated with the sediment; arsenic concentrations initially dropped and then remained constant over the 180 day experiment. Collectively, these data suggest that in situ perturbations to the aquifer matrix could promote mobilization of bioavailable sedimentary organic carbon. Although this sedimentary organic carbon did not influence arsenic concentrations here, it represents a carbon source that could, in the presence of arsenic-bearing iron (hydr)oxides, fuel microbial reductive release of arsenic.
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INTRODUCTION
Inherent in much of the organic carbon debate is the assumption that the reactivity of sedimentary organic carbon decreases over time. The prevailing thought is that microbes utilize the most chemically labile portions of the organic carbon pool first and then progress to more chemically recalcitrant forms of organic carbon. Thus, on depositional time scales, the bioavailable organic carbon within the aquifer sediments is consumed, and the ability of the sedimentary organic carbon pool to fuel microbial respiration, and thus arsenic mobilization, is weakened. This view is supported by field work conducted in Vietnam demonstrating that organic carbon reactivity (equated to the rate of methanogenesis) and groundwater arsenic concentrations decrease with increasing sediment age.14 However, a body of work has established that organic carbon recalcitrance is not a chemical property but rather a result of external factors such as physical accessibility (e.g., sorption onto mineral surfaces or occlusion in aggregates) and availability of nutrients and/or energy for microbes to process the carbon.15,16 Within the framework of carbon stabilization, it is possible to envision a situation in which, even within old sediments where apparent carbon reactivity is low, physical and chemical perturbations to the subsurface could disturb the sediment matrix and release previously inaccessible bioavailable
For more than a decade, researchers have debated the source of organic carbon fueling microbial reactions responsible for mobilizing arsenic off aquifer sediments into groundwater within the deltaic environments of South and Southeast Asia. The debate centers on the involvement of either sedimentary organic carbon codeposited when the aquifers were formed or surface-derived carbon transported into the subsurface with recharging water. Identifying the source of organic carbon involved with arsenic mobilization is important because it clarifies if human activities (e.g., groundwater development and land-surface modifications) can affect the arsenic contamination problem. This information is needed for developing solutions to the contamination problem and for identifying the vulnerability of groundwater systems to future arsenic contamination. The involvement of sedimentary organic carbon is supported by spatial correlations between sedimentary organic carbon and dissolved arsenic concentrations, including the presence of organic-rich fine-grained or peaty deposits near arsenic hot spots.1−8 The involvement of surface-derived organic carbon is supported by radiocarbon-young microbial biomass and dissolved inorganic carbon (the byproduct of microbial respiration) in arsenic-contaminated groundwater9−11 and by using hydrologic measurements and chemical tracers to connect arsenic-contaminated groundwater back to recharge from organic-rich wetland and pond sediments.12,13 © 2014 American Chemical Society
Received: January 20, 2014 Accepted: March 25, 2014 Published: March 25, 2014 221
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Collection and Storage of Recharge Water. We collected aquifer recharge water from a shallow well installed 5 m beneath a rice field and from the sediments of a 200-yearold pond into glass BOD bottles. See section 2 of the Supporting Information for a description of water collection and preservation methods. Electrochemical properties and arsenic concentrations of collected recharge water are listed in Table S2 of the Supporting Information. Experimental Setup for Incubation. Inside an anaerobic chamber, we combined 8.2 g (dry weight) of sediment and 10 mL of recharge water together in 20 mL acid-washed and ashed crimp-top serum bottles sealed with autoclaved butyl rubber stoppers (Chemglass Inc.). We created four treatments by combining the two recharge waters with both the native and gamma-irradiated sediment. Because of the unexpectedly high rate of carbon consumption, we ended up creating three experimental sets that used identical source materials but were initiated at different times. We kept all sealed serum bottles outside of the anaerobic chamber at room temperature and gently inverted them roughly every 2 weeks to reduce the development of diffusive gradients. We found from previous efforts that vigorous shaking hindered microbial activity. Processing and Analysis of Samples. At given time points, we sacrificed one to two serum bottles from each of the four experimental treatments. We opened serum bottles inside the anaerobic chamber and measured the pH of the water. We vacuum-filtered the water through a 0.2 μm polyethersulfane membrane filter into acid-washed and ashed containers. Water was frozen at −18 °C until it was analyzed for organic carbon or acidified with trace-metal-grade nitric acid and then frozen until it was analyzed for arsenic. On a few bottles, before opening them, we used a two-way needle to collect headspace methane samples in evacuated glass vials (Labco Exetainer), and a needle pressure gauge to measure pressure in the bottles before and after headspace collection. We measured organic carbon on a GE Analytical Instruments Sievers 900 TOC analyzer (model TOC 900 Portable) in line with a Dionex UltiMate 3000 high-performance liquid chromatography (HPLC) system, arsenic on a Perkin-Elmer ELAN-DRCe ICP-MS instrument inline with a Perkin-Elmer Series 200 HPLC system, and methane on a SRI 8610C GC-FID instrument. In all instances, the HPLC system acted as an autosampler and pump, delivering water to the instruments.
sedimentary organic carbon that could fuel microbial reactions. Such a possibility is supported by field work, also conducted in Vietnam, at a site where groundwater pumping for the city of Hanoi has altered subsurface flow patterns and chemical conditions.17 At this site, allochthonous organic carbon is implicated in arsenic mobilization, but mass-balance calculations indicate that the inflowing DOC cannot solely sustain the observed extent of reaction, and DOC concentrations inexplicably increase in the aquifer area where active arsenic mobilization is occurring (see section 3 of the Supporting Information for further discussion). Within this context, we conducted incubation experiments to assess the bioavailability of sedimentary organic carbon in a Bangladeshi aquifer. We mixed sediment collected from an uncontaminated portion of the aquifer with porewater collected from recharge sources that contain different amounts of biodegradable dissolved organic carbon.13 We found the sediment contributed a significant amount of organic carbon to solution, overwhelming any contribution from the recharge waters, and the native sediment microbial community rapidly consumed the mobilized sedimentary organic carbon. However, consumption of this carbon was not associated with the mobilization of arsenic from the aquifer sediment.
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METHODS Field Site. We collected aquifer sediment and surface recharge water from a well-characterized field site located in Bashailbhog village in the Munshiganj district of Bangladesh (Figure S1 of the Supporting Information). This site has hosted a range of investigations focused on understanding arsenic contamination of groundwater.9,13,18 Collection, Storage, and Processing of Sediment. We collected aquifer sediment cores in December 2011 from a depth of 9.1 m (30 feet) below the ground surface where dissolved arsenic concentrations are low (>10 μg/L).18 See section 1 of the Supporting Information for a description of the sediment coring method. Upon retrieval, we heat-sealed cores in gas-impermeable Escal bags (Mitsubishi Gas Chemical Co., Inc.) with oxygen scavenging packets and oxygen indicator tablets (BD Diagnostic Systems). Cores were kept on ice in coolers at the field site and during transport to the University of Washington, where they were then stored at 4 °C. Inside an anaerobic chamber with an anaerobic meter (Coy Lab Products), we opened, pooled, and hand-homogenized cores that remained anoxic during transit (according to the oxygen indicator tablet). We sealed half of the homogenized sediment in Escal bags with oxygen scavenging packets and again stored it at 4 °C until starting the experiment. We dried the other half inside the anaerobic chamber and packaged it in glass bottles that we sealed in Escal bags with oxygen scavenging packets. The dried sediment was gamma irradiated at Boeing Radiation Effects Laboratory using an MDS Nordion Gammacell 220 Excel radiation cell at 10465 R/min for 3.98 h, for a total dose of 25 kGy. Drying sediment before irradiating it minimizes alterations of the sediment’s chemical properties.19 Collection of Sediment Porewater Prior to Incubations. We vacuum-filtered an aliquot of the native sediment through a 0.2 μm polyethersulfane membrane filter to collect sediment porewater, which we analyzed for organic carbon (see Processing and Analysis of Samples for details of the analysis). Sediment properties, including porewater organic carbon concentrations, are listed in Table S1 of the Supporting Information.
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RESULTS AND DISCUSSION Release of Sedimentary Organic Carbon. After sediment had been homogenized, the dissolved organic carbon (DOC) concentration measured in sediment porewater was 1059 ± 186 mg/L (Table S1 of the Supporting Information), which equates to 0.33 ± 0.06 mg of organic carbon/g of sediment. This concentration is notably higher than those typically measured in Bangladeshi aquifers (e.g., ∼1−15 mg/ L18,20,21) and represents 5 ± 1% of the sediment’s organic carbon content [6.5 ± 1.5 mg/g (Table S1 of the Supporting Information)]. It appears that sampling, homogenizing, and/or handling the sediment disturbed the sedimentary organic carbon pool, perhaps by breaking open carbon-containing aggregates, by changing equilibrium conditions for sorption of organic carbon to sediment, or by stimulating microbial and enzymatic processing of sedimentary organic material with warmer temperatures and/or trace inputs of oxygen.15,16 When the recharge waters were mixed with the aquifer sediment, dissolved organic carbon concentrations reached ∼150−250 222
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mg/L (Figure 1A,B and Table S3 of the Supporting Information). The rice field and pond recharge waters initially contained 17 ± 7 and 30 ± 3 mg of organic carbon/L, respectively. (These values represent the average and standard deviation of measurements made from three different BOD bottles.) Thus, the input of sedimentary organic carbon overwhelmed the dissolved organic carbon contribution from the recharge waters. In fact, the organic carbon content of the sediment porewater can account for all of the dissolved organic carbon measured at the start of the experiment. Given our experimental sediment:water ratio (∼1:1), a dissolved concentration of 150−200 mg/L equates to the sediment contributing ∼0.2 mg of carbon/g of sediment, which is similar to what we measured in the sediment porewater. Microbial Consumption of Released Sedimentary Organic Carbon. After ∼20 days, DOC concentrations in the untreated (native) sediment incubations began to decrease. Dissolved organic carbon concentrations dropped by a factor of 5 within 80 days (∼150 to ∼30 mg/L), approaching the initial DOC concentrations in the recharge waters (Figure 1A,B). In contrast, DOC concentrations in the gamma-irradiated sediment did not decrease with time (Figure 1A,B). After recharge waters had been mixed with the sediment, pH values were similar in all treatments and relatively constant over time (see Figure S2 of the Supporting Information); thus, the divergent behavior of the biotic and abiotic treatments cannot be attributed to pH-induced re-equilibration of organic carbon between the sediment and water phase. Headspace samples collected at the end of the experiment show instead that the native microbial community fermented the sedimentary organic carbon into methane (Table S5 of the Supporting Information). Methane is also generated in the aquifer,9,13 and thus, the rapid consumption of the released organic carbon suggests that if it had been available to the microbial community in situ it would have been utilized, with the caveat that the added recharge waters contain more nutrients (e.g., nitrogen and phosphorus) than the shallow groundwater13,18 and can potentially sustain greater amounts of carbon utilization than the resident groundwater. These results align with those emerging from other studies, namely that easily degradable forms of organic carbon can persist for thousands of years in the subsurface if the microbial community lacks physical access to the carbon or experiences biochemical limitations (e.g., nutrient and/or energy availability) that inhibit respiration.15,16 Arsenic Mobilization. Consumption of the released sedimentary organic carbon did not mobilize arsenic off of the aquifer sediment. In both the sterilized and native treatments, dissolved arsenic concentrations decreased or did not change when the recharge waters mixed with the aquifer sediment, and then concentrations remained relatively constant over time (Figure 1C,D and Table S4 of the Supporting Information). This behavior implies arsenic is abiotically equilibrating between the dissolved and solid phases, as has been observed previously,22 with the initial decrease in dissolved concentrations explained by sorption to the solid phase. Dissolved arsenic concentrations were higher with the gamma-irradiated sediment, suggesting that anaerobic drying and/or irradiation decreased the sorption capacity of the sediment. The tested aquifer sediment does not contain abundant iron (hydr)oxides,18,22 the mineral phase associated with carbon-fueled microbial mobilization of arsenic [via reductive dissolution of iron (hydr)oxides and/or reduction of arsenic].23 Given a different aquifer sediment composition,
Figure 1. Concentrations of dissolved organic carbon (A and B) and arsenic (C and D) vs days since the addition of rice field (gray symbols) and pond (black symbols) recharge water to the aquifer sediment. Empty symbols show data for gamma-irradiated sediment and filled symbols show data for native sediment. Values plotted before day zero represent those in the recharge waters before they were mixed with the aquifer sediment. Sediment porewater (not plotted) had an initial dissolved concentration of 1059 ± 186 mg/L. (Note that this porewater concentration reflects a higher sediment:water ratio than the plotted incubation concentrations.) Error bars in all panels represent analytical uncertainty based on multiple injections of samples and standards. Plotted data are listed in Tables S3 and S4 of the Supporting Information. 223
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apparently conflicting lines of evidence that implicate either surface organic carbon or sedimentary organic carbon in fueling microbial activity associated with the mobilization of arsenic into groundwater.
namely the presence of arsenic-bearing iron (hydr)oxides that exist in other arsenic-contaminated aquifers in South and Southeast Asia,24,25 consumption of the released sedimentary organic carbon could promote arsenic mobilization. While microbial consumption of the mobilized sedimentary organic carbon did not release arsenic off the tested aquifer sediment, the carbon source identified here does represent a pool of bioavailable organic carbon neglected in previous arsenic mobilization studies. For example, incubation studies have tried to assess the impact that labile organic carbon has on arsenic mobilization by adding carbon substrates (e.g., lactate, acetate, and glucose) to aquifer sediments.22,26−34 However, none of these studies measured the total amount of organic carbon dissolved in the incubation water. If a similar mobilization of sedimentary organic carbon occurred in these studies as observed here, it could have inflated or masked the impacts of added carbon, depending on the experimental setup and controls. Broader Implications. Our data demonstrate that aquifer sand with a relatively low organic carbon content (not finegrained channel fill or peat material associated with arsenic hot spots at other field sites2−8) contains biodegradable organic carbon and that perturbations to the sediment matrix can make this carbon available to the native microbial community. In our experiment, sampling, homogenizing, and/or handling of sediment released the bioavailable sedimentary organic carbon. In the aquifer, pumping-induced rearrangement of groundwater flow paths could create rapid changes in groundwater chemistry that could theoretically disturb the sediment matrix. Similarly, fresh inputs of young dissolved organic carbon could stimulate microbial reactions that target the solid phase and destabilize mineral-associated sedimentary organic carbon.35,36 Once mobilized, this pool of sedimentary organic carbon could fuel microbial reactions that further perturb the sediment matrix. Radiocarbon data from the site where we collected the tested sediment9 show that at the depth where arsenic concentrations are elevated, dissolved inorganic carbon is ∼700 years old and dissolved organic carbon is ∼3000 years old. Previous calculations13 explained the dissolved inorganic carbon (DIC) age as an ∼90:10 mixture of DIC from microbial respiration of young allochthonous organic carbon and calcite dissolution, and explained the dissolved organic carbon age as an ∼40:60 mixture of young allochthonous organic carbon and old sedimentary organic carbon mobilized from the aquifer matrix. In our experiment, the high dissolved organic carbon concentrations measured in the sediment porewater (i.e., 100−1000-fold higher than those measured in situ) support the notion that sedimentary organic carbon can be mobilized into the aqueous phase and indicate that the older component of the dissolved inorganic carbon pool at this site could be from microbial respiration of mobilized sedimentary organic carbon. This reinterpretation implies that subsurface microbes use both young allochthonous and old sedimentary organic carbon to support respiration. Such a scenario is consistent with radiocarbon data from other arsenic-contaminated aquifers in Bangladesh, West Bengal, and Cambodia10,11,37 (see section 3 of the Supporting Information). If the biodegradable sedimentary organic carbon detected in our incubations can be mobilized in situ and if its existence is a widespread phenomenon, it suggests that perturbations to subsurface conditions could stimulate microbial reactions fueled by sedimentary organic carbon. Conceptually, this pool of stabilized sedimentary organic carbon could bridge the
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ASSOCIATED CONTENT
S Supporting Information *
Five figures, five tables, and three sections providing data and information supporting statements made herein. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: (206) 221-2298. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Mehedi Hasan Tarek and the people of Bashailbhog village for their assistance in the field, Jerry Wert at Boeing for gamma irradiating our sediment, Colby Moorberg for help with the methane analysis, and two anonymous reviewers for input that improved the manuscript.
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