Hillsborough Street retail site, Raleigh, N.C.
Mobile multi-phase extraction (MMPE) technology, in conjunction with enhanced natural attenuation, has proven a highly effective means of free phase petroleum product, contaminated groundwater, and volatile soil vapor remediation when site restrictions prevent large scale excavations and loud, high-maintenance, in-ground systems. While standard remediation techniques typically require extensive site excavation, the MMPE/enhanced attenuation method offers the benefits of mobility and less invasiveness for tenants and patrons of commercial sites.
The MMPE system is an in-situ groundwater remediation technology that uses a high-pressure vacuum system to pull a mixture of free phase petroleum product, contaminated groundwater, and volatile soil vapors from the subsurface. Groundwater monitoring wells are used as the extraction points on which the MMPE rig draws contaminants (see Figure 1). The MMPE system itself is a trailer-mounted rig comprised of a high-power generator and vacuum system in which contaminants pulled from the subsurface are volatilized and released as a vapor from the system.
Figure 1: Schematic diagram of combined MMPE and enhanced natural attenuation system.
Natural attenuation consists of the natural degradation of petroleum compounds into carbon dioxide and water by microbes and bacteria. Enhanced natural attenuation involves injecting additional microbes into the shallow groundwater along with air and hydrogen peroxide, which acts as a catalyst for the microbes and increases the rate of petroleum degradation. Using enhanced natural attenuation with MMPE technology can increase the rate of remediation. This method proved to be a convenient and cost-effective means of remediation at a heavy-trafficked, mixed-use development in Raleigh, N.C.
Hillsborough Street in Raleigh is the location of a 2.02-acre parcel, originally developed in 1930 as an industrial baking distribution facility. The facility operated continuously as a baking distribution center for more than 60 years until the property was sold and renovated into its current state as a mixed-use retail development. The site’s location, adjacent to Meredith College and near North Carolina State University and interstate access, is a premium location for retail development. However, in 2002, a notice of violation was issued to the site by the North Carolina Department of Environment and Natural Resources (NCDENR) regarding the removal of four petroleum product underground storage tanks (USTs) in 1993.
Through the installation of numerous groundwater monitoring wells and soil borings, more than 6,500 square feet of free phase petroleum product was identified in the shallow groundwater along with extensive soil contamination. The area of contamination was located immediately behind the main facility building and partially underneath the building slab of the parking garage. The contaminated area is used as a parking lot for the recently redeveloped mixed-use retail center.
Terracon Consultants, Inc., was retained to perform a comprehensive site assessment, as well as to develop a corrective action plan to remediate the site to acceptable levels of subsurface contamination. Terracon examined three options for remediation to develop the corrective action plan.
Option 1—Remove more than 6,800 tons, or 136,680 cubic feet, of contaminated soils from beneath the asphalt paved parking lot. In addition, contaminated groundwater would need to be removed using a vacuum extraction method. Cost estimate was more than $350,000.
Option 2—Use MMPE technology in conjunction with enhanced natural attenuation. Cost estimate was $107,000 (based on eight MMPE events).
Option 3—Use a vacuum extraction method, similar to MMPE technology, but involving installation of an underground trenching into the shallow groundwater. A low-pressure vacuum would be used to remove free phase petroleum product and contaminated groundwater from the subsurface; removed product is forced into a settling tank, requiring weekly maintenance and pumping. Cost estimate was $166,000 (based on two years of operation and maintenance).
In addition to higher costs, options one and three would have disturbed the current site tenants by excavating large portions of the asphalt paved parking area. Option two was approved by the current site owner because the MMPE events could be scheduled to avoid conflicts with an on-site sports bar on game days, and local college events that draw high traffic and business to the retail center.
Using MMPE and enhanced natural attenuation technologies was also considered more advantageous than the other options because the state of North Carolina does not currently require air permitting for the MMPE volatile vapor discharge, which reduced the cost for disposal of free phase petroleum hydrocarbons as would be required for option three.
Terracon performed 96-hour MMPE events at the site on a monthly basis planned around tenant schedules. The monthly schedule worked well with the remediation efforts because the time between events allowed the free phase petroleum product to recharge in the wells sufficiently after the high-pressure vacuum extraction events took place. An MMPE rig and tanker truck, required to dispose of groundwater pulled during the MMPE process, were placed strategically so a minimal portion of the parking area was blocked off and traffic through the area was not impeded.
An initial six MMPE events were performed at the Royal Bakery facility. As a result, more than 2,000 gallons of free phase petroleum product were removed from the groundwater. Based on the amount of product removed, it became evident that more contaminants remained in the subsurface as free phase petroleum product, contaminated groundwater, and volatile soil vapors. The NCDENR approved additional MMPE events, and 27 MMPE events had taken place at the site as of Jan. 21, 2008. An estimated total of 18,419 gallons of petroleum product were removed as vapor from the subsurface and off-gassed through a stack as part of the MMPE rig.
From the beginning of the MMPE remediation, microbes and hydrogen peroxide were injected monthly into wells surrounding the petroleum product plume. To date, the dissolved petroleum contaminated groundwater extents have shrunk significantly. Through groundwater sampling, it became evident that the dissolved contaminant plume was reduced to within several feet of the free phase petroleum product plume, a remarkable gradient change. In 2006, the product was measured to be 83 octane gasoline, with a benzene concentration greater than 10,000 parts per billion (ppb), while groundwater sampled from a well approximately 15 feet away was found to be less than 1 ppb.
In examining the most current free phase petroleum product plume, it can be inferred that the contaminant extents have decreased significantly in lateral width and vertically. Initial sampling of the on-site wells indicated a product thickness ranging up to more than 3 feet. Following 25 MMPE events, estimates indicated that the product plume is generally less than 0.5 feet thick with two small areas ranging up to 1 foot thick.
In examining the effectiveness of the MMPE system in conjunction with enhanced natural attenuation technology, it is clear that both the dissolved product plume and the free phase product plume have substantially decreased in both horizontal and vertical extents.
The MMPE system has also been able to adapt to fluctuations in groundwater elevation identified during the remediation process. Stingers, the vacuum extraction points lowered into wells to the groundwater interface and during a rise or fall of groundwater, are easily adjusted to accommodate the changes in groundwater elevation.
Perhaps the most important indication of MMPE and enhanced natural attenuation as an effective remediation system is the approval from state regulators along with the current site owner and tenants. Terracon successfully planned the MMPE events around tenants’ schedules. Frequent sporting events draw a large crowd to the on-site sports bar and blocking a portion of the parking area would have caused major inconvenience for the tenants and patrons, as would noise generating from the high-powered vacuum system. Disruptions were easily avoidable with simple planning.
Patrick Crile is a staff geologist with Terracon in Raleigh, N.C. He can be contacted at firstname.lastname@example.org. Michael W. Minett, REM, is a registered environmental manager and environmental services department manager with Terracon’s Charlotte, N.C., office. He can be reached at email@example.com.