Formatted Title
Pre-Drill Emplacement of an Activated Carbon Reagent to Mitigate Glacial Till Refusal at a Heavily Impacted Retail Station
Background/Objectives
BOS 200®, an activated carbon reagent with nutrients and facultative bacteria, was previously injected into tight glacial till overburden with a Geoprobe 8040 rig to remediate a on- and off-site total petroleum hydrocarbon (TPH) sources at a retail station. The 8040 rig was selected for the first event as it easily achieved required depths during a pilot characterization event consisting of three borings, however, a significant number of points saw early refusal during the first full-scale injection program using this method. Although post-treatment groundwater concentrations in the treated volume indicate significant decreasing trends, intervals that received no reagent, due to refusal, were still heavily impacted with TPH. It was imperative that follow-on injections be designed to overcome the bony site conditions allowing reagent delivery to the remaining contaminated media.
Approach/Activities
A comprehensive evaluation of two years of post-treatment analytical datasets combined with the complex injection data enabled an understanding of how multiple previous direct push drilling refusals resulted in untreated underlying vertical intervals still heavily impacted with TPH. The design for a follow-up target treatment event targeting deeper soils utilized an air rotary drill rig to pre-drill 75 borings to between 20 and 57 feet below ground surface (bgs) that were then backfilled with hydrated bentonite. A Geoprobe 7822 was used to push high-pressure hose threaded tooling through the bentonite, allowing access to the deeper impacted intervals for reagent delivery. The pre-drilling activities occurred over 12 days immediately followed by 15 days of slurry injection. Although the delivery method impacted the field schedule and overall cost, reagent delivery beyond the local refusal zones was required to achieve contact and concentration reductions.
Results/Lessons Learned
Results from the first reagent application were favorable and continue to reduce groundwater concentrations over time within treated vertical intervals. The 2023 design and application addressed geological site limitations identified during the previous injection event and facilitated amendment delivery to deep vertical intervals inaccessible via direct push technology alone. While air rotary drilling met design requirements within glacial till, formation cave-in and short-circuiting of drilling fluid was observed within an unconsolidated unit of aggregate fill adjacent to an on-site structure. A different pre-drilling technology may have been more amenable than air rotary in fill material, such as more expensive sonic drilling. Additionally, although significant attention was focused on bentonite packing and hydration, the drilling method did not allow for optimized 2-foot lift emplacement, packing, hydration, and so on. Even given the limitations, the packing and hydration were sufficient to mostly mitigate surfacing, while allowing discrete interval reagent emplacement at target depth intervals. Post-treatment groundwater samples have been collected and quarterly collections are scheduled through Spring of 2024 and will be presented during the conference.