Formatted Title
Expedited Implementation of Combined In Situ Treatment Technologies to Address DNAPL at a Multi-Parcel Former MGP Site
Background/Objectives
Surfactant enhanced product recovery (SEPR) and in situ chemical oxidation (ISCO) using ferric iron-activated sodium persulfate were implemented in sequence at a former MGP Site where historical operations resulted in dense non-aqueous phase liquid (DNAPL) impacts remaining in outwash sands, collecting in wells up to several feet thick. General lithology consists of fill material underlain by loess, diamicton ablation till, clayey outwash sands underlain by basal till material consisting of organic clay, peat, gravel, and woody debris material. Depth to groundwater encountered varied from 6 to 10 feet (ft) below ground surface (bgs) under semi-confined conditions. The lateral extent of impacts remaining with the saturated zone was generally confined to coarser outwash sediments deposited between diamicton tills and basal tills. Injection intervals targeted the most permeable layers of sandy, clayey silts approximately ~15-35 ft bgs. The in situ treatment area consisted of 28,000 square foot area including a portion of the former MGP and off-site third party-owned properties including a city parking lot and street right-of-way.
Approach/Activities
Remediation of the main Site and adjacent parcels was conducted utilizing excavation and off-site disposal to depths of 39 feet bgs in 2021-2022. However, critical utilities including multiple utility poles and overhead electric lines emanating from a substation on an adjacent parcel, and a historical 36-inch diameter brick sewer in the center of the treatment area could not be relocated in time to meet the project timeline. Following bench-scale and pilot study activities that indicated the two-phased (SEPR+ISCO) approach was viable to address source material, the full-scale remedy began with the well installation phase in December 2022 (installation of 34 injection/extraction wells). The SEPR phase operated from February 2023 to June 2023. The ISCO injection phase delivering iron activated sodium persulfate via 200+ direct-push injection locations operated from May 2023 through July 2023. To meet the project deadline, overlap between the SEPR and ISCO phases was required, and was coordinated using a variety of factors including property-specific schedule considerations, hydraulic gradients, and daily collected performance monitoring data. Daily injection volumes have ranged from 1,600 to 2,400 gallons per day with concurrent extraction volumes averaging 2,500 to 3,000 gallons per day.
Results/Lessons Learned
A project deadline in August 2023 was required by the local government as part of an access agreement finalized during the well installation phase to ensure the treatment area was open for school buses for the start of the Fall 2023 school year. During a constructability-review following contractor selection, various design optimizations were implemented to enhance aquifer communication and improve recovery rates including increasing borehole diameters, selection of stainless-steel screens with significantly more open area than Schedule 80 PVC screens utilized during the pilot study, and supplementing with vacuum extraction. Treatment cells were developed around each injection/extraction well with final vertical treatment intervals determined during well installation activities using customizable screen lengths in the field. Interfacial tension (IFT) and total petroleum hydrocarbon (TPH) concentrations were collected daily from extraction wells to optimize total fluid recovery and used in a mass flux approach to prioritize treatment cells for ISCO injections so that injections could be completed prior to the project deadline. Total fluids recovered during the SEPR phase did not readily separate as they had during the pilot test, so pre-treatment options were re-evaluated in the field prior to selecting the best option to treat/dispose/recycle recovered fluids. When low levels of oxidant infiltrated into the historic brick sewer about 3 weeks prior to the completion of the ISCO phase, additional monitoring protocols and close communication with the local utility owner/township was key for completing the project on time in a satisfactory manner for the affected stakeholders. No measurable DNAPL was detected in any recovery or monitoring well following remediation. Confirmation soil, groundwater, and soil gas results are scheduled for collection from Fall 2023 through Spring 2024, and are anticipated to be presented during the conference.