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Separate-phase hydrocarbons (SPH – gasoline/diesel mixture) in six wells ranging in thickness from 0.02 to 0.91 feet (1.99 feet aggregate thickness) covering ~10,000 sq. ft.
Weathered shale and limestone bedrock overlain by silty clay. The bedrock surface at this site is variable and undulating.
Groundwater is present largely in a bedrock aquifer that consists of weathered limestone and shale. Monitor wells at the site have been drilled and completed into the bedrock aquifer. Static groundwater levels in each well are present above the well screens.
SPH was present in two wells at thicknesses of <0.10 feet twelve months following SURFAC® (90% SPH removal).
The subject site is a former trolley and bus terminal located in Cincinnati, Ohio. Business activities since the early 1900s have resulted in soil and groundwater contamination from diesel fuel and gasoline releases. SPH covered an area of approximately 10,000 square feet. Previous remedial activities included excavation of all underground storage tanks. A pump and treat system that has been inactive for 5+ years recovered over 1,000 gallons of SPH. Additionally, another provider has performed twelve mobile dual-phase/multi-phase extraction events. Historically, SPH thicknesses have increased during spring months.
The object of EcoVac Services’ patented SURFAC® process was to remove all SPH from the site.
Groundwater is present in a confined bedrock aquifer that consists of a weathered limestone and shale at depths ranging from approximately 20 to 37 feet below grade. The bedrock surface at this site is variable and undulating. This bedrock aquifer is overlain by silty clay. Monitor wells utilized for treatment have been drilled and completed into the bedrock aquifer. Static groundwater levels, ranging from approximately 15 to 28 feet below grade in each well are present above the well screens, posing an impediment to the SURFAC® treatment process.
EcoVac Services was contacted to implement SURFAC® at this site due to the challenging conditions (i.e. geology/hydrogeology conditions, extent of SPH plume, and ongoing presence of SPH despite previous remedial attempts) and lack of funds the owner of the site was able to spend. EcoVac Services’ patented SURFAC® process is the combination of dual-phase/multi-phase extraction and surfactant injection. EcoVac Services employs its EFR® (enhanced fluid recovery) process for the dual-phase/multi-phase extraction component of EcoVac Services’ patented SURFAC® process. The process described herein is patent-protected and represents the intellectual property of EcoVac Services, Inc.
SURFAC® Pilot Test #1
An initial one-day SURFAC® pilot test was conducted at the site to: (1) assess the potential need for additional injection wells, (2) determine if additional EFR® events are necessary prior to surfactant injection, (3) achieve contaminant removal, (4) reduce the aerial/vertical extent of the plumes, (5) determine the surfactant injection volumes/locations/sequence, and (6) determine the duration of the surfactant injection/capture events.
The SURFAC® pilot test was conducted at four monitor wells (MW-4, MW-5, MW-22, and MW-24). SPH was detected in four monitor wells (MW-4 – 0.03 feet, MW-5 – 0.02 feet, MW-22 – 0.13 feet, and MW-24 – 0.02 feet) prior to the pilot test. SPH was not detected in the extraction wells immediately following extraction from the wells; however, SPH was detected in MW-22 (0.05 feet) and MW-28 (0.55 feet) upon conclusion of the pilot test. Historically, SPH has never been detected in MW-28.
A calculated total of 255 pounds of petroleum hydrocarbons (approximately 36 equivalent gallons of gasoline/diesel fuel) was removed during this pilot test. Extraction was conducted individually at monitor wells MW-4, MW-5, MW-22, and MW-24 and collectively at monitor wells MW-4, MW-5, and MW-24. The hydrocarbon removal rate was considerably higher during extraction from MW-5, followed in decreasing order by MW-24, MW-4, and MW-22. Increasing removal rates were only observed during extraction from MW-5. Groundwater recovery rates ranged from 1.3 gallons per minute (MW-4) to 4.7 gallons per minute (MW-5).
Differential pressures were only detected in a two monitor wells (MW-13 and MW-26) during extraction at the various wells. The differential pressure detected in MW-25 was most likely due to the six feet drop in groundwater created by extraction and not vacuum created in the subsurface. This is due to MW-5’s well screen installed below the groundwater surface. The lack of observed vacuum during the test is most likely due to submerged well screens. Groundwater drawdown created by the pilot test ranged from –0.23 feet (MW-5) to –6.00 feet (MW-25). The wide range in drawdown may be attributed to the placement of wells in certain bedrock fracture zones and the interconnectivity of these fractures.
As stated, SPH was present MW-28 (0.55 feet) at the conclusion of the event despite that it was not used as an extraction well and never has contained SPH. This may be due to a couple of explanations. EFR® may have mobilized SPH into the well during extraction at other wells. Also, the drawdown observed in MW-28 may have exposed the well screen as well as pocket of SPH that may exist in the bedrock aquifer.
SURFAC® Pilot Test #2
An additional SURFAC® pilot test was performed in July 2008. This pilot test was conducted to further determine additional wells to utilize for surfactant injection and to determine the amount of water/surfactant mixture for injection. The pilot test was conducted at six extraction points, consisting of MW-4, MW-5, MW-21, MW-24, MW?27, and MW-28. SPH was not detected in the extraction wells following this event.
A calculated total of 95 pounds of petroleum hydrocarbons (approximately 16 equivalent gallons of gasoline) was removed during this pilot test. Increasing removal rates were only observed during extraction from MW-28. Extraction from the individual monitor wells revealed higher removal rates during extraction from MW-21 (13 to 31 pounds per hour) followed by MW-28 (0.7 to 2.0 pounds per hour) and MW-27 (1.1 to 1.3 pounds per hour). Groundwater recovery rates ranged from 07 gallon per minute (MW-28) to 2.1 gallons per minute (MW-21).
Differential pressures were only detected in four monitor wells (MW-27 and MW-28 during extraction from MW-21, MW-23 during extraction from MW-21, MW-22, MW-27, and MW-28, and MW-21 during extraction from MW-4, MW-5, and MW-24) during this event. It appears that differential pressures were detected in the surrounding wells when the monitor well screens were exposed due to previous extraction from that well or drawdown created by the extraction well. It appears that vacuum is being created in the subsurface, but it is not being detected due to well screens installed below the groundwater surface. Groundwater drawdown created by the pilot test ranged from –0.02 feet (MW-12) to –7.24 feet (MW-28). The wide range in drawdown may be attributed to the placement of wells in certain bedrock fracture zones and the interconnectivity of these fractures.
SURFAC® Treatment Program
The addition of three monitor wells as well as additional EFR® were recommended at this site; however, due to the lack of funds the site owner was able to spend at the site, SURFAC® was implemented. The additional monitor wells would have allowed for more complete coverage of the surfactant/water mixture to come into contact with the SPH. Additional EFR® events were recommended in order to mobilize more SPH toward the extraction wells and further reduce SPH thicknesses at the site.
Two SURFAC® injection events were conducted at the subject site in December 2007 followed by four SURFAC® capture events in December 2007 and January 2008. A calculated total of 572 pounds of petroleum hydrocarbons (approximately 95 equivalent gallons of gasoline/diesel fuel) was removed as vapor phase during the extraction events, as well as 14,731 gallons of groundwater. Approximately 2,650 gallons of a surfactant aqueous solution was gravity fed into MW-4, MW-5, MW-21, MW-22, MW-27, and MW-28 during the two SURFAC® injection events. Approximately 12,901 gallons of liquid were removed during the four SURFAC® capture events.