Following the removal of several Underground Storage Tanks, a release was discovered at an active bulk plant. After years of inefficient and costly assessments, GRI was contracted to determine if a different approach could be utilized to bring the release to closure. After analyzing multiple different approaches to the project, GRI determined that a Limited Excavation with Dewatering and Off-Site Disposal of Impacted Soil and Water approach would be the most effective. The site went from having over 5 feet of free product to being effectively remediated in a total of 5 days.
In 1994, a release was discovered at this active bulk plant following the removal of Underground Storage Tanks (USTs) located in separate basins on the site. In accordance with the applicable North Carolina Department of Environmental Quality (NCDEQ) guidelines at that time, the consultant conducting the closure returned all removed soils to the excavation and did not conduct initial abatement of contaminated soil and the source area. The release was covered by an insurance policy and was not trust fund eligible.
Over the years, previous consultants installed fifteen Type II monitoring wells, three Type III monitoring wells and one 4-inch pumping test well during assessment and corrective action activities were conducted over an 8 year span. Three active water supply wells were in use within 250 feet of the site and the site was considered high risk. Free product had been historically reported in monitoring wells MW-1, MW-4, MW-6 and MW-12.
Geological Resources, Inc. (GRI) was contracted to determine if a different approach could be utilized to bring the release to closure.
GRI completed an updated receptor survey, comprehensive sampling event and delineated the extent of soil contamination. The ground water concentration exceeded the Gross Contamination Levels (GCLs) and the 3 water supply wells remained in use within 250 feet. However, public water was available in the vicinity of the site. GRI was successful getting the 3 water supply well owners to agree to connect to municipal water and abandoned the water supply wells, dropping the risk classification to intermediate. GRI conducted three Mobile Multi-Phase Extraction (MMPE) events at the site over 6 years resulting in the removal of 841.91 gallons of free product.
GRI collaborated with the insurance company to develop a plan to more thoroughly delineate the contaminant mass at the site so that free product recovery and reduction in dissolved phase contamination could be more cost effective. GRI installed 19 temporary monitoring wells at the site to depths ranging from 15 to 20 feet below ground surface to define the free product plume. Free product was observed in 11 of the 19 temporary wells and the 4 monitoring wells MW-1, MW-4, MW-6 and MW-12. Free product thicknesses ranged from 0.01 feet to 5.39 feet across the site.
The proper delineation of the contaminant mass indicated that the presence of free product was widespread across the site and allowed GRI to evaluate adequate clean-up alternatives for the site and not just limited to those wells historically containing free product. GRI evaluated the following remedial options:
Vacuum Enhanced Pump and Treat (Dual-Phase Extraction)
This remediation method can be used to extract ground water, recover free product, or vent contaminated soils. The process can accomplish all of the above with a single vacuum blower/extraction pump servicing a network of extraction wells with a vacuum line applying a vacuum at each well to affect simultaneous water and vapor withdrawal, or by a combination of pneumatic ground water recovery pumps in each of the recovery wells. Off-site discharge permits would be required for the discharge of treated ground water. The only discharge point was a pond located approximately 2,000 feet of the site. The remedial option was feasible, but was not the most cost effective approach.
Mobile Multi-Phase Extraction (MMPE)
This remediation method employs the same technology and processes as dual-phase extraction but at a fraction of the costs and with almost no maintenance. This method does not involve any significant installation costs since the extraction piping is connected directly to the well head. Given the site was an active bulk plant with limited space and heavy truck traffic, this method was not deemed cost-effective.
Limited Excavation with Dewatering and Off-Site Disposal of Impacted Soil and Water
Free product present in MW-1, MW-4, MW-6 and MW-12 was increasing in thickness and correlating with a decreasing water table, indicating that secondary contamination may be occurring and removal of the contaminated soil would likely reduce the concentrations and decrease or eliminate the free product. Excavation followed by limited dewatering of the contaminated water in the vicinity of the worst impacted monitoring wells was selected to eliminate a significant portion of the contaminant mass.
Based on the defined extent of the contaminant mass, the area to be excavated was divided into two areas: Area #1, which consisted of three areas measuring approximately 1,056 square feet, 1,220 square feet and 352 square feet were proposed to be excavated to a depth of 15 feet and Area #2, which was approximately 400 square feet, was proposed to be excavated to a depth of approximately 10 feet. The excavation was designed to include the saturated soil extending from the water table to the terminal depth of the excavations due to contact with contaminated ground water. An approximate volume of contaminated soil that was proposed to be excavated and disposed was calculated as 1,654 cubic yards (2,481 tons). Limited dewatering activities would be conducted in support of soil excavation.
A total of 2,650.22 tons of petroleum contaminated saturated and unsaturated soils were removed from the site during the soil excavation activities. The site remained operational during the excavation. The excavations were dug to depth, then immediately backfilled and compacted to avoid the accumulation of recharging ground water in the excavation basin and the need for dewatering.
Based upon the soil volume removed and the depth to water at the site, approximately 41,840 gallons of petroleum contaminated ground water and/or free product was removed from the surficial aquifer by excavation alone.
Subsequent to backfilling the excavations, the destroyed monitoring wells were replaced in the same locations. Semi-annual ground water sampling events conducted 6 months apart reported that no free product was present in any of the monitoring wells and none of the reported contaminant concentrations exceeded the GCLs and generally showed a trend of decreasing concentrations. The site was issued a conditional No Further Action (NFA) following the deed recordation of a Notice of Residual Petroleum (NRP).
The site went from having over 5 feet of free product to being effectively remediated in a total of 5 days.
This would not have been possible if the contaminant mass had not been adequately defined and the secondary source of contamination removed.