Shana McCarthy
Shana McCarthy

A Geochemical Evaluation of Enhanced In-Situ Bioremediation of Chlorinated Ethenes In Groundwater

Shana McCartrhy
M.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Kathy Thorbjarnarson

Friday, December 14th, 2012
CSL 422, 10:00am
watch Shana’s defense here

Sites impacted with chlorinated solvents  present unique technical challenges when compared to most other groundwater  contaminants.  Chlorinated volatile  organic compounds (CVOCs) such as such as tetrachloroethene (PCE),  trichloroethene (TCE), or 1,1,1-trichloroethane (1,1,1-TCA) generally do not  degrade naturally in the environment.   Therefore, more aggressive source depletion methods such as enhanced  in-situ bioremediation (EISB) may be implemented to treat the groundwater  plume.  EISB involves injection of an  electron donor to promote reducing conditions followed by inoculation of  groundwater with dechlorinating bacteria.   When conditions are favorable, the dechlorinating bacteria sequentially  remove chlorine ions from the chlorinated solvent compound until an innocuous  end product is produced.  This process  creates unique and dramatic changes in the natural geochemistry of the aquifer  system.
A CVOC-impacted site located at Naval Air  Station North Island (NASNI) was used as a test case for this study.  NASNI is an active military base located  adjacent to the City of Coronado in San Diego County, California. The site, or  Operable Unit 24 (OU 24), is a chlorinated solvent groundwater plume which may  have originated from an acid waste pump station associated with a historic  industrial waste pipeline.  The purpose  of this study was to evaluate the geochemical changes that occur during EISB  and how they relate to the effectiveness of remediation. An evaluation of the  redox conditions present in groundwater and the observed reduction in CVOC  concentrations was used to evaluate the effectiveness of EISB. In addition,  geochemical modeling was performed to develop an understanding of the effect of  redox conditions on observed dissolved inorganic constituent concentrations due  to precipitation or dissolution of minerals present in the aquifer.
Based on the results of the geochemical evaluation, Groundwater  generally became more reducing and VOC concentrations decreased following  implementation of EISB.  In addition, minerals  containing Fe, Mn, and SO4 were sensitive to redox transformations.  Conversely, Ca, Mg, Na, and some  Mn-containing minerals were not sensitive to redox conditions.  Reduced minerals FeS, FeS2, and H2S have the  potential to precipitate and oxidized Fe and Mn minerals have the potential to  dissolve as groundwater becomes more reducing.   This can create problems with groundwater treatment systems that expose  groundwater to oxygen. When reduced groundwater containing high concentrations  of dissolved Fe and Mn is exposed to oxygen, the Fe and Mn hydroxide minerals  will precipitate and may foul remediation equipment.  Additionally, redox transformations can  potentially mobilize toxic metals such as arsenic, chromium, lead, and mercury  through oxidation and reduction processes.