Formatted Title
Unsaturated Zone Angled Boring Desorption Utilizing Iron Activated Persulfate at an Active Commercial Restaurant
Background/Objectives
Historical manufacturing operations at the subject Site have resulted in chlorinated volatile organic compound (cVOC) and semi–volatile organic compound (sVOC) impacts within unsaturated soils. Recent Site re-development and engineering controls restricted ex situ treatment via excavation within portions of the property boundary (Area of Concern). Remaining residual soil impacts are located on a sloped embankment, along the property boundary of a busy commercial fast-food chain and adjacent railway. Additionally, the Client was imperative that remedial activities do not inhibit ongoing business nor compromise adjacent railway structural integrity.
The target treatment area consists of native silty sand overburden bordering quarry process fill from the previous excavation extent. The target treatment area was covered by landscaping shrubs which were not to be disturbed or damaged from the remedial activities.
The remedial goals were to treat sorb unsaturated mass, while reducing a flux in dissolved groundwater concentrations, as well as minimizing disruption to Site infrastructure or active business activities.
Approach/Activities
Iron activated sodium persulfate was the chosen remedial application to target the contaminants of concern, utilizing its desorption ability for sorb mass, along with continual reactivity to reduce dissolved concentration spikes. Residual impacts were located in an embankment approximately 80 feet in length by 5 feet wide based on historical boring delineations. The target treatment area was based off a 12-foot treatment thickness; however, the top of the treatment area was approximately 3 to 4 feet above the asphalt lot grade, and inaccessible due to vegetation shrubbery and the slope of the embankment.
Drilling implementation required calculated angled borings to specifically target the narrow residual mass and specific treatment thickness. Injections were sequenced methodically to specifically target the shallowest interval. Per the remedial design, the shallowest interval within each temporary injection point was flooded with the highest reagent volumes to enhance mass desorption.
The remedial approach utilized 5,510 pounds of the sodium persulfate remedial material along with 18 gallons of ferrous activator. In total, 30 temporary injection points spaced approximately 7.5 feet apart were installed during the injection event. Based on Site conditions, daily coordination was required with the commercial fast-food manager to limit disruptions to business, and minimize parking and drive-thru disruptions.
Results/Lessons Learned
Based on groundwater analytical performance data, significant sVOC and cVOC reductions were observed without increases within dissolved phase concentrations. Multiple injection events were initially proposed; however, after the first event the Site has moved into monitored natural attenuation (MNA).
The remedial approach provided the client with significant cost savings and reduced onsite disruption compared to alternative remedial approach considers, which was excavation and shoring.