Letter pubs.acs.org/journal/estlcu
Effect of CO2(aq) Exposure on a Freshwater Aquifer Microbial Community from Simulated Geologic Carbon Storage Leakage Djuna M. Gulliver,†,‡ Gregory V. Lowry,†,‡ and Kelvin B. Gregory*,†,‡ †
Office of Research & Development, National Energy Technology Laboratory, Pittsburgh, Pennsylvania 15236-0940, United States Department of Civil & Environmental Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213-3890, United States
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ABSTRACT: Geological carbon storage is likely to be part of a comprehensive strategy to minimize the atmospheric release of carbon dioxide (CO2), raising concerns that injected CO2 will leak into overlying freshwater aquifers. While a high concentration of CO2 is toxic to bacteria and causes large community changes, little is known about how exposure to low concentrations of CO2 may impact bacterial communities in aquifers. Microbial communities in aquifers provide important functionality through carbon and nutrient cycling in aquifer and connected aqueous ecosystems. The impact of in situ CO2(aq) exposure on the microbial community in a freshwater aquifer was examined. CO2(aq) was introduced into the formation to simulate leakage from a geologic carbon storage unit. On the basis of the 16S rRNA genes recovered from subsurface samples, the population numbers were unaffected by CO2(aq) up to 1.53 g of CO2(aq)/L, but diversity decreased with an increased CO2(aq) concentration, suggesting at least short-term disruption of ecosystem functions may occur.
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INTRODUCTION Geologic carbon storage (GCS), one of the proposed comprehensive solutions to control the rising atmospheric CO2 concentration, has raised concerns over the potential for leakage of CO2 into overlying freshwater aquifers.1,2 During GCS, CO2 is injected into a deep subsurface storage formation, which may be transported into overlying freshwater aquifers through aggravated caprock fractures or through preexisting wells.3−5 Trautz et al. revealed that the introduction of CO2(aq) alters the chemical environment in a freshwater aquifer.6 In this in situ study, a freshwater aquifer underwent a controlled release of CO2-saturated groundwater. Downgradient monitoring wells displayed a temporary increase in CO2(aq) concentrations, accompanied by a decrease in groundwater pH and an increase in the level of dissolved cations such as Ca, Fe, and Mg. It is likely that these CO2-driven changes in pH and dissolved ions will also accompany changes in the freshwater microbial community. The microbial community in freshwater aquifers contributes to the stability of the freshwater aquifer and connected aqueous ecosystems by allowing critical ecosystem functions such as secondary productivity, pollutant degradation, and nutrient cycling.7−10 A loss of microbial diversity may be deleterious to these ecosystem functions.11,12 In some cases, a change in microbial diversity has negatively affected water quality.13−15 The change in microbial community that arises after CO2(aq) exposure will influence the postleakage ecosystem functions and may ultimately affect the stability and water quality of the © 2014 American Chemical Society
impacted aquifer. Therefore, a need to better understand how a microbial community may change and which are the tolerant and intolerant microbial populations in CO2-exposed environments exists. Previous laboratory experiments revealed that increasing the CO2 concentration decreased the population numbers and diversity in the microbial community in a deep subsurface storage site.16 However, a freshwater microbial community and a deep saline community may respond to CO2 exposure differently, as lower total pressure and dilution following leakage will result in lower CO2(aq) concentrations. The objective of this study was to determine the effect of CO2(aq) exposure expected during geologic carbon storage leakage on a freshwater microbial community. Trautz et al. introduced CO2(aq) into a freshwater aquifer and collected samples upgradient and downgradient from the injection well.6 The microbial communities in these samples were examined using pyrosequencing and 16S rRNA gene clone libraries. The aim of this study was to understand the bacterial population and diversity changes that may occur in a CO2(aq)-exposed freshwater aquifer and identify the populations tolerant and intolerant to CO2(aq) exposure. Received: Revised: Accepted: Published: 479
October 21, 2014 November 20, 2014 November 21, 2014 November 21, 2014 dx.doi.org/10.1021/ez500337v | Environ. Sci. Technol. Lett. 2014, 1, 479−483
Environmental Science & Technology Letters
Letter
Figure 1. Relative proportions of phylotypes recovered from well samples downgradient of a CO2-saturated-water injection well. The amount of dissolved CO2 of each well at the time of sampling was measured by CarboQC, and the microbial community was revealed by 16S rRNA gene clone libraries and qPCR. Clones were assigned a genera based on >97% similarity to cultured organisms. Clones that were
