Formatted Title

Targeted Remedy Implementation to Reduce Contaminant Flux to Surface Water and Sediments

Background/Objectives

First developed in the late 1800s within a highly industrialized area, the site has been continuously operating under various owners as a refinery and bulk petroleum storage/transfer facility for approximately 150 years. During the remedial investigation of historical site operations, a chlorinated hydrocarbons source area was discovered in a portion of the site where groundwater ultimately discharges to a brackish tidal estuary.

Sediment and surface water sampled proximate to the site did not indicate impacts associated with the contaminated groundwater discharge, likely due to natural reductive dechlorination processes. To limit potential future impacts to surface water and sediments, a permeable reactive barrier was proposed. Given the apparent extent of groundwater contamination along the length of the shoreline, a barrier in excess of 900 feet long would be required to address the entire width of the contaminant plume.

During later phases of the remedial investigation, a high permeability zone was discovered in the glacial till aquifer linking the source area and waterfront. Implementation of the remedy was targeted to this specific zone to reduce contaminant mass flux to surface water and sediment, while at the same time reducing the barrier length to 225 feet.

Approach/Activities

Investigations of the chlorinated groundwater contamination and associated discharge to the estuary have been conducted using various methods including high-resolution site characterization via membrane interface probe (MIP) with hydraulic profiling (HP) tooling and long-term tidal study conducted with groundwater pressure transducers installed within 31 site monitoring wells. Based on results of high-resolution site characterization, additional monitoring wells were installed along the shoreline to confirm the extent of the contaminated groundwater plume.

Pre-design activities included in situ microcosm studies to evaluate potential electron donor materials and effects of bioaugmentation, as well as clean water injection testing to determine achievable radius of injection. During the clean water injection testing, groundwater displacement was measured with pressure transducers and analyzed by various aquifer test methods to estimate directional hydraulic conductivity and groundwater seepage velocity.

A full-scale in situ injection event was conducted in Fall 2021, consisting of direct injection utilizing a sonic drill rig of EHC liquid electron doner material and SDC-9 bioaugmentation culture. Injections targeted the freshwater groundwater flow discharging from the Site. Routine groundwater performance monitoring of the remedy is ongoing.

Results/Lessons Learned

A high permeability zone, linking the groundwater chlorinated hydrocarbon source area to the estuary, was observed through the evaluation of MIP and HP data overlaid on known soil stratigraphy and groundwater quality data. This concept was further supported by a comparison of directional hydraulic conductivity, long-term average water levels and evaluation of hour-by-hour groundwater flow directions over the tidal cycle. Implementation of a targeted PRB to this high mass flux zone will both reduce future potential chlorinated hydrocarbon surface water and sediment impacts while being more economical than a PRB installed across the entire width of the contaminated groundwater plume.

Pre-design and remedy implementation results will be presented and examined, including the in-situ microcosm studies evaluating different electron donor materials in the brackish groundwater environment, results of injection radius of influence tracer and aquifer testing, and the ability of the remedy to reduce contaminant flux to sediment and surface water. In addition, an evaluation of Bio-Trap® samplers compared to groundwater grab samples for long-term performance monitoring of Dehalococcoides and associated functional genes will be presented.