Formatted Title
Nested Horizontal Wells Selected to Assess, Inject, and Monitor a Multi-Aquifer Chlorinated Solvent Plume
Background/Objectives
An initial assessment to characterize the contaminant impacts at a former dry cleaner site in central Florida was completed with a limited amount of vertical monitoring wells and temporary groundwater and soil sampling points utilizing a direct push drill rig.
Approach/Activities
Due to the infrastructure constraints at the site, a robust nested horizontal well network was installed using EN Rx VertebraeTM Well Systems (VWS) beneath the onsite buildings and parking areas, a four-lane highway, and an off-site property scheduled for future development. A total of 84 independent well segments were installed in seven VWS. Three of the VWSs were installed in the shallow zone with lengths of 300 to 400 feet and four VWSs were installed in the deeper contaminant zone with lengths of 600 to 800 feet. Forty-three of the independent well segments from the VWS network were sampled to produce detailed contaminant contours and further refine the conceptual site model (CSM). Two contamination zones were defined: a shallow surficial zone moving southeast and a deeper zone moving 700 feet northeast of the site. The sampling data collected from the independent well segments identified significant contamination directly under the building resulting in a 7-fold increase of contaminant mass when compared to the limited vertical assessment information. This additional assessment information significantly impacted the remedial design and prompted modifications to the preliminary ISCO dosing.
Results/Lessons Learned
Following completion of the site assessment the VWS well network was then utilized to complete two years of automated chemical oxidant injections while continuing to sample select well segments to evaluate the effectiveness and completeness of the treatment. While the wells indicated bias sampling, careful examination and trends allowed the project engineers to judge the progress of the injection and reapportionment of the chemical reagent where needed. The network continued to be sampled after the injection events were completed to monitor for rebound.
The site was assessed and remediated under the available escrow funding that was allocated based on the vertical data and prior to the VWS well network installation. Despite limited funding based on the revised CSM, the primary goal of reducing the plume mass to below Natural Attenuation Monitoring levels and minimizing plume transport off site was achieved.
This presentation will provide further details of the nested horizontal well system design, sampling protocol and assessment, remedial design modifications made in real time based on updated sampling data, and the overall process from start to finish.