Project:
Former Service Station
Bottom Line: Developed and refined a conceptual site model to tailor site management and remediation strategy to changing site.
Summary of Project
The site previously contained an automobile service station with one waste oil and three gasoline USTs. Site investigations defined gasoline-impacted soil beneath the former USTs down to the water table at approximately 120 feet below grade. LNAPL observed in source area wells has been submerged by significant increases in regional groundwater levels.
- Developed a conceptual site model (CSM) to describe the source area, distribution of contaminants, geologic/hydrogeologic conditions, and potential health and/or environmental risks. Identified data gaps that could potentially affect the understanding of site conditions. Used the CSM to develop, refine, and implement a site closure strategy and remedy selection and design.
- Observed significant changes (>20 feet) in groundwater elevations that affected the direction of groundwater flow and the distribution of LNAPL and dissolved-phase hydrocarbons in the vadose and saturated zones.
- Performed high-resolution vertical assessment to pinpoint locations of hydrocarbon mass in the subsurface.
- Designed and constructed an SVE system including design drawings, bid specifications, contractor selection, construction management, and system start-up. Operated the system continuously for approximately 2 years and observed generally asymptotic influent vapor concentrations. Transitioned to cycling system operation to maximize mass removal rates, while reducing the overall carbon footprint of the site. Removed an estimated 56,000 pounds of hydrocarbon mass over approximately 7 years.
- Performed long-term LNAPL monitoring and recovery at three source area wells using passive hydrophobic skimmers.
- Installed four wells using rotosonic drilling techniques to delineate a source area LNAPL plume. Used LNAPL-reactive tape for field screening of soil cores. A phased drilling schedule allowed for revisions to well construction based on field screening results.
- Designed a full-scale in situ chemical oxidation (ISCO) injection system consisting of a permeable reactive barrier. Installed 12 dual-nested wells at the southern property boundary for injecting hydrogen peroxide and ozone.
- Observed direct relationship between water levels and dissolved-phase hydrocarbon concentrations at barrier wall injection wells. Identified residual sorbed-phase mass in a low-permeability soil layer within the capillary fringe. Used the existing SVE system to remove approximately 500 pounds of hydrocarbon mass from the capillary fringe during a period of low water levels. Suspended implementation of the ISCO groundwater remedy as water levels and groundwater concentrations remain low.
- Conducted a 20-day multi-phase extraction (MPE) pilot test to evaluate the feasibility of the technology for remediating LNAPL and hydrocarbon-impacted groundwater. Evaluated the pilot test data including groundwater and vapor extraction and mass removal rates, radius of influence, and capture zone. Concluded that MPE was not feasible because of low air flow, high groundwater recovery, and low vapor-groundwater mass removal ratio.
- Performed a stepped-rate pumping test to evaluate the effect of water levels on recoverability of the submerged LNAPL. Observed a significant increase in apparent LNAPL thickness recovery during pumping. Established a threshold range of groundwater elevations that expedite LNAPL recovery in the source area.
- Procured an industrial wastewater discharge permit in preparation for an expanded LNAPL remediation event based on the results of the pumping test. Designed and constructed a sanitary sewer discharge lateral including design drawings, bid specifications, contractor selection, and construction management. Implemented an extended LNAPL remediation event to remove the recoverable portion of LNAPL at the site.