Influence of physical factors and geochemical conditions on groundwater acidification during enhanced reductive dechlorination

Enhanced reductive dehalogenation is an attractive in-situ treatment technology for chlorinated contaminants. The process includes two acid-forming microbial reactions, that is, fermentation of an organic substrate to hydrogen and short chain fatty acids followed by dechlorination, with hydrochloric acid as side-product. The accumulation of acids and the resulting drop of groundwater pH are controlled by the mass and distribution of chlorinated solvents in the source zone, type of electron donor, alternative terminal electron acceptors available and soil mineral phases able to buffer the pH (such as carbonates). Groundwater acidification may reduce or halt microbial activity, thus increasing the time and costs required to clean-up the aquifer. For this reason, research in this area is gaining increasing attention. In previous works (Robinson et al., 2009, Sci. Tot. Environ, Robinson and Barry, 2009, Environ. Model. & Software, Brovelli et al., 2010, submitted) a detailed geochemical and groundwater flow model able to predict the pH change occurring during reductive dehalogenation has been developed. The model accounts for the main processes influencing groundwater pH, including its composition, the electron donor used and dissolving soil minerals. The model has now been applied to study in detail how spatial variability occurring at the field scale affects groundwater pH and dechlorination rates. Numerical simulations were conducted to study the influence of heterogeneous hydraulic conductivity on the distribution of the injected fermentable substrate and on the accumulation/dilution of the acidic products of reductive dechlorination. The influence of the geometry of the DNAPL source zone was also studied, as well as the spatial distribution of soil minerals. The conclusions drawn and insights gained from this modeling study will be useful to design improved in-situ enhanced dechlorination remediation schemes.

Effect of heterogeneity on enhanced reductive dechlorination: Analysis of remediation efficiency and groundwater acidification

David Andrew Barry, Alessandro Brovelli, Christof Holliger, Elsa Marina Lacroix, Clare Robinson

Enhanced reductive dehalogenation is an attractive in situ treatment technology for chlorinated contaminants. The process includes two acid-forming microbial reactions: fermentation of an organic substrate resulting in short-chain fatty acids, and dehalogenation resulting in hydrochloric acid. The accumulation of acids and the resulting drop of groundwater pH are controlled by the mass and distribution of chlorinated solvents in the source zone, type of electron donor, alternative terminal electron acceptors available and presence of soil mineral phases able to buffer the pH (such as carbonates). Groundwater acidification may reduce or halt microbial activity, and thus dehalogenation, significantly increasing the time and costs required to remediate the aquifer. In previous work a detailed geochemical and groundwater flow simulator able to model the fermentation-dechlorination reactions and associated pH change was developed. The model accounts for the main processes influencing microbial activity and groundwater pH, including the groundwater composition, the electron donor used and soil mineral phase interactions. In this study, the model was applied to investigate how spatial variability occurring at the field scale affects dechlorination rates, groundwater pH and ultimately the remediation efficiency. Numerical simulations were conducted to examine the influence of heterogeneous hydraulic conductivity on the distribution of the injected, fermentable substrate and on the accumulation/dilution of the acidic products of reductive dehalogenation. The influence of the geometry of the DNAPL source zone was studied, as well as the spatial distribution of soil minerals. The results of this study showed that the heterogeneous distribution of the soil properties have a potentially large effect on the remediation efficiency. For examples, zones of high hydraulic conductivity can prevent the accumulation of acids and alleviate the problem of groundwater acidification. The conclusions drawn and insights gained from this modeling study will be useful to design improved in-situ enhanced dehalogenation remediation schemes.

2011

pH control for enhanced reductive bioremediation of chlorinated solvent source zones

David Andrew Barry, Clare Robinson

Enhanced reductive dehalogenation is an attractive treatment technology for in situ remediation of chlorinated solvent DNAPL source areas. Reductive dehalogenation is an acid-forming process with hydrochloric acid and also organic acids from fermentation of the electron donors typically building up in the source zone during remediation. This can lead to groundwater acidification thereby inhibiting the activity of dehalogenating microorganisms. Where the soils’ natural buffering capacity is likely to be exceeded, the addition of an external source of alkalinity is needed to ensure sustained dehalogenation. To assist in the design of bioremediation systems, an abiotic geochemical model was developed to provide insight into the processes influencing the groundwater acidity as dehalogenation proceeds, and to predict the amount of bicarbonate required to maintain the pH at a suitable level for dehalogenating bacteria (i.e., > 6.5). The model accounts for the amount of chlorinated solvent degraded, site water chemistry, electron donor, alternative terminal electron- accepting processes, gas release and soil mineralogy. While calcite and iron oxides were shown to be the key minerals influencing the soil’s buffering capacity, for the extensive dehalogenation likely to occur in a DNAPL source zone, significant bicarbonate addition may be necessary even in soils that are naturally well buffered. Results indicated that the bicarbonate requirement strongly depends on the electron donor used and availability of competing electron acceptors (e.g., sulfate, iron(III)). Based on understanding gained from this model, a simplified model was developed for calculating a preliminary design estimate of the bicarbonate addition required to control the pH for user- specified operating conditions.

Elsevier2009

Insights and Modelling Tools for Designing and Improving Chlorinated Solvent Bioremediation Applications

David Andrew Barry, Alessandro Brovelli, Clare Robinson

The chlorinated solvents tetrachloroethene (PCE) and trichloroethene (TCE) have been used extensively in industry and are now amongst the most common and hazardous groundwater contaminants. These solvents are typically present as dense, non-aqueous phase liquids (DNAPLs) and represent long-term source zones that produce persistent contamination plumes in aquifers. Under anaerobic conditions, chlorinated ethenes may be biodegraded via reductive dechlorination (the biologically mediated, step-wise removal of chlorine) to form ethene, a relatively innocuous end-product. The rate of reductive dechlorination can be enhanced by stimulating the activity of dechlorinating bacteria by injection of an electron donor (typically an organic substrate that generates hydrogen upon fermentation), nutrients and, in some cases, microbial communities known to dechlorinate effectively to ethene (i.e., bioaugmentation). Reductive dechlorination has been shown to be a viable technology for in situ treatment of dissolved chlorinated solvent plumes, and recent laboratory studies have suggested that this strategy may also be effective for chlorinated solvent DNAPL. Here, the source zone is targeted directly, with the aim of reducing its lifespan by enhancing dissolution from the DNAPL and sorbed phases and coupling this with effective and sustained dechlorination near the DNAPL-water interface and within the plume. This bulletin focuses on modelling of enhanced dechlorination processes in groundwater, including the modelling tools developed in the SABRE project (under which this report was written, http://www.claire.co.uk/index.php?option=com_content&task=view&id=53&Itemid=47&showall=1), insights gained from the models concerning factors controlling the rates and extent of enhanced source zone DNAPL bioremediation, and how the modelling tools can be used to assist future applications of this technology.