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., N6.5). 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.

© 2009 Elsevier B.V. All rights reserved. 

To assist in the design of enhanced reductive dechlorination systems for in situ remediation of chlorinated solvent source zones, the software BUCHLORAC (BUffering of deCHLORination ACidity) was developed to predict the amount of buffer required to maintain the groundwater pH in a DNAPL treatment zone within the optimal range for dechlorinating bacteria. Reductive dechlorination is an acid forming process with hydrochloric acid and organic acids typically building up in the treatment zone. Remediation of source zones is associated with such extensive localized dechlorination that it may be common for the soils’ natural buffering capacity to be exceeded. As groundwater acidification may inhibit the activity of dechlorinating microorganisms and thus slow or stall the remediation process, sufficient alkalinity must be present to maintain a near-neutral pH. BUCHLORAC is a Windows Graphical Interface based on an abiotic geochemical model that is implemented through the program PHREEQC. BUCHLORAC allows users to estimate the buffer requirements for their specific operating and design conditions including, for example, site water chemistry, mineralogy, amount of chlorinated solvent to be degraded, design inhibition pH, and type of organic substrate and buffering additive.

© 2009 Elsevier B.V. All rights reserved. 

Seventh International Conference on Remediation of Chlorinated and Recalcitrant Compounds, May 24 - 27, 2010, Monterey, California

“Acidity Production and Buffer Requirements for In Situ Bioremediation of Chlorinated Solvents.”  A. Brovelli, C. Robinson, D.A. Barry, and J.I. Gerhard. Clare E. Robinson (University of Western Ontario/CANADA)