A geochemical evaluation of enhanced in-situ bioremediation of chlorinated ethenes in groundwater

Includes bibliographical references (page 51)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