Formatted Title
Lessons Learned: A 20-Year Review of the Effectiveness of Containment Barrier Walls in the Pacific Northwest
Background/Objectives
Containment barrier walls are implemented on sites where remediation is either impractical and/or cost-prohibitively expensive. Barrier walls are implemented through connection to an aquitard with low-permeability properties, similar to the barrier wall, effectively creating a containment cell to prevent the contaminants of concern from further migration off of the site. These containment cells are commonly paired with a low permeability cap, e.g., pavement, and a pumping system to maintain an inward hydraulic gradient with set performance requirement, e.g., 1-foot inward differential, to increase the protectiveness of the remedial measure.
Through long-term management of two barrier wall containment cells installed as interim actions, the goal has been to prove the implemented remedy is functioning properly by providing containment and preventing downgradient impacts from the source area and reduce overall cost of maintaining the remedy, i.e., reduce pumping rates or show the containment cell can provide the required protectiveness without extraction of groundwater.
Approach/Activities
The Pacific Northwest’s geologic conditions and historical industrial development occurring in areas of deltaic deposits and marshlands in close proximity to maritime industry, soft soil conditions allow for use of vibratory-beam constructed barrier wall systems with aquitards as deep as 90 feet below ground surface. This barrier wall construction method consistently achieves an installed maximum permeability of 1 × 10-7 cm/s and commonly permeabilities on the order of 1 × 10-9 cm/s. These barrier walls are usually a mixture of granular ground blast furnace slag (GGBFS) and attapulgite or sepiolite clay, for example.
Since installation of the containment cells (2003 and 2004), data have been collected including groundwater extraction rates over time, groundwater contaminant concentrations, and water levels across the barrier wall. These data have allowed for assessment of inward hydraulic gradient compliance, observations of monitored natural attenuation downgradient/outside of the barrier wall, and modeling to project the impacts if pumping were ceased. For one of the two sites, pumping was ceased in 2021 and remains off as part of a pilot study. Data will be presented on the ongoing pumping cessation pilot study at one of the sites and how the containment wall has functioned throughout this plot study.
Results/Lessons Learned
During the long-term assessment of these two barrier wall systems, the importance of design, including the cap, can have large cost implications related to groundwater extraction if pumping cannot be ceased. In addition, long-term data collection have confirmed that:
- Field construction complications (obstructions, chemistry, etc.) can be successfully mitigated;
- Hydraulic control optimization can significantly reduce pumping volumes (and cost);
- Design predictions of permeability of the barrier walls were correct that wall permeability was less than existing aquitards;
- Operational modelling were consistent with annual average pumping rates; and
- Existence of the barrier walls eased implementation and regulatory approval of other remedial actions.