Theme: 10. Characterization, Fate and Transport

Topic Area: 10f. High-Resolution Site Characterization (HRSC)

Formatted Title:

Biogeochemical Characterization of CVOC-Impacted Bay Sediments Using High-Resolution Passive Profiler

Background/Objectives:

Onshore groundwater and nearshore sediment porewater at Installation Restoration Site 9, Naval Air Station North Island in Coronado, California (Site) are impacted by chlorinated volatile organic compounds (CVOCs) including trichloroethene (TCE) and its dechlorination daughter products. Over the last 35 years, the nature, extent, and transformation of CVOCs in onshore and nearshore Site areas have been investigated. A primary focus of Site characterization efforts has been the collection of evidence regarding the occurrence of CVOC natural degradation. This presentation focuses on biogeochemical characterization of nearshore sediments at the Site in the context of assessing natural degradation of CVOCs. High-resolution passive profilers (HRPPs) were added to the nearshore assessment scope in Spring 2023 to supplement a larger Site characterization effort that included multiple sample collection methods (piezometers, Trident Probes and seepage meters). The objectives of this presentation are 1) to discuss how the HRPP data affect the existing Site conceptual model, and 2) to evaluate the performance of HRPPs as a Site characterization technology compared to other established Site sampling methods.

Approach/Activities:

The HRPP is a direct-push stainless steel passive equilibrium sampler capable of measuring concentrations of CVOCs and geochemical indicators, porewater velocity, microbial community composition, and compound specific isotope analysis at high-resolution (~8-inch) intervals. A total of 14 HRPPs were deployed at nine Site locations up to 8 feet below bay floor (bbf) in February 2023. HRPPs were deployed from a boat using one of two methods: 1) winch, push-pole, and slide hammer, or 2) vibracore with assistance from commercial divers. HRPPs were retrieved and sampled in March 2023 after approximately 3 weeks of in situ equilibration. From the 14 HRPPs, a total of 98 samples were collected and analyzed for CVOCs and geochemical indicators, 74 samples were collected and analyzed for biomarkers and CSIA, and 33 samples were collected and analyzed for bromide to estimate porewater velocity. The HRPP data are evaluated against contemporaneous velocity data from seepage meters and CVOC data from Trident Probes, seepage meters and piezometers.

Results/Lessons Learned:

The HRPP sampling results indicate the potential for natural degradation of CVOCs at the Site. This conclusion is based on low CVOC detections relative to historical Site concentrations, detection of biomarkers indicating the presence of dechlorinating microorganisms and functional genes, and concentrations of methane, sulfate, sulfide and dissolved iron that indicate a strongly reducing sediment environment, favorable for reductive dechlorination. These results supplement prior microcosm studies, which observed rapid natural dechlorination of CVOCs in bay sediment slurries. CVOC concentrations detected in HRPPs were similar to or slightly less than concentrations detected in Trident Probes, seepage meters and piezometers. HRPP porewater velocity estimates were similar to or slightly higher than seepage velocity measurements. The collective results of the HRPP data provide further validation of the HRPP as a tool for collection of representative environmental samples in sediment porewater, and the density and resolution of HRPP data surpass datasets from previous Site nearshore investigations.

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Michael Pound