Background/Objectives: Naval Air Station North Island (NASNI) is a 2,800 acre facility located in Coronado, California where, between the 1940s and 1970s, Installation Restoration (IR) Site 9 was operated as a waste disposal area.
Today, a dissolved phase chlorinated volatile organic compound (CVOC) plume extends from the upland area at IR Site 9 toward San Diego Bay. An investigation in San Diego Bay in 1999 detected CVOCs in sediment porewater adjacent to IR Site 9, indicating the discharge of CVOC-impacted groundwater into the Bay sediments. In 2015, a second offshore investigation at IR Site 9 determined that the concentration of CVOCs in the sediment porewater decreased between 2 to 3 orders of magnitude, suggesting significant natural attenuation of the CVOCs via aerobic and / or anaerobic biodegradation. Methods to directly quantify intrinsic aerobic mineralization of chlorinated ethenes in marine sediments are not well established and the importance of aerobic biodegradation in natural attenuation of chlorinated ethenes is not well understood. The primary objective of this study was to develop and apply a novel isotopic method to directly measure aerobic biodegradation of cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC) by utilizing 13C-labelled substrates and measurement of 13CO2.
Approach/Activities: As part of method development, bioreactors were prepared using 13C-labelled cDCE and Polaromonas sp. strain JS666 (a culture that aerobically degrades cDCE) to generate 13CO2. 13CO2 in bioreactor headspace samples was quantified using gas chromatography isotope ratio mass spectrometry (GC-IR-MS) to confirm aerobic mineralization of 13C-labelled substrates. Following the method development, microcosms were constructed in triplicate using sediments and porewater from three shallow offshore locations adjacent to IR Site 9. These microcosms were spiked to 1 mg/L with individual 13C-labelled cDCE and VC and amended with oxygen. The spiking concentrations were greater than baseline which was necessary to meet the method detection limit for 13CO2. The microcosms were incubated for 16 weeks, sampled for CVOCs every two weeks, and re-amended with oxygen to maintain aerobic conditions as required. The aqueous phase in the microcosms was analyzed for cDCE and VC while the headspace was analyzed for 13CO2 via GC-IR-MS.
Results/Lessons Learned: The results of the method development were successful in demonstrating a novel approach that directly measures aerobic degradation of CVOCs in marine sediment porewater and is likely applicable to other sites where CVOCs are being discharged to marine sediments. In IR Site 9 microcosms, 13C-VC was mineralized at all three sediment/porewater locations resulting in the production of 13CO2; however, cDCE degradation was not detected at any location. The results demonstrate that aerobic VC-degrading bacteria are present and actively degrading VC in the shallow sediments and marine porewater in San Diego Bay adjacent to IR Site 9.