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.