Combining HVME and Enhanced Bioremediation for Gasoline Contamination in Groundwater

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Courtesy of REGENESIS

A leaking underground storage tank at a commercial petroleum retail site located in Haskell, Texas resulted in soil and groundwater petroleum hydrocarbon contamination. This contamination, including benzene, toluene, ethylbenzene, and total xylenes (BTEX) and total petroleum hydrocarbons (TPH) migrated down-gradient below a road and neighboring property. Access below the adjacent roadway was limited, and consultants at The Environment Company sought a solution that would allow for simultaneous treatment of on-site and off-site sections of the plume. As a result of the presence of separate phase hydrocarbons (SPH) a high vacuum multi-phase extraction (HVME) was used to extract product vapor and contaminated groundwater in a limited area of the site. Oxygen Release Compound (ORC®) was applied in the dissolved section of the petroleum plume for treatment on-site and off-site. The HVME treatment performed in October, 1998 successfully removed approximately 360 liters of gasoline equivalent as vapor, and over 19,000 liters of contaminated groundwater. ORC was applied via injection in March, 1998. The most recent sampling event results indicate a substantial reduction in the concentration of benzene (92%), total BTEX (76%) and TPH (81%).

INTRODUCTION

Groundwater at a commercial petroleum retail center in Haskell, Texas had separate phase hydrocarbons (SPH) and dissolved hydrocarbons as a result of a leaking underground storage tank system (LUST). The dissolved hydrocarbon plume had apparently migrated below a roadway and adjacent properties. Roadway access was limited, and a remedial solution was sought that would allow for simultaneous treatment of on-site and off-site sections of the plume. Due to off-site access restrictions, a remedial system that could be applied on-site and yet have a positive remedial effect off-site was deemed advantageous.

This project involves cleanup of gasoline released into a major aquifer, the Seymour Formation, which is unique in west central Texas due to relatively high groundwater velocity and shallow depth. This groundwater resource has been used as the domestic water supply for the city of Haskell and is currently used for domestic and agricultural irrigation.

In August of 1997, The Environment Company evaluated numerous remediation options, and recommended that Oxygen Release Compound (ORC®) be used to enhance the natural biological activity at the site. ORC is manufactured by Regenesis Bioremediation Products in San Juan Capistrano, California and is a patented formulation of magnesium peroxide that slowly releases molecular oxygen when hydrated, thereby facilitating aerobic bioremediation of organic contaminants (Koenigsberg et al., 1997). The ORC releases oxygen slowly over a period of approximately 6 to 9 months. Oxygen delivery is the most important aspect of in situ bioremediation (Norris, 1995). The indigenous aerobic microbes utilize this oxygen during respiration and can metabolize petroleum contaminants, resulting in reduction of petroleum hydrocarbon mass in groundwater. ORC enhances bioremediation of contaminants in-situ, without exchanging one environmental problem for another. One of the most important attributes of bioremediation is that harmful contaminants are metabolized by microbes, producing harmless byproducts (CO2 and water) and no further treatment is necessary. Other remediation methods may transfer contaminants to another medium which requires removal, transportation, and possibly additional cleanup efforts. While other treatment options may release volatile petroleum components to the atmosphere, ORC enhances bioremediation in-situ, thereby eliminating the potential for respiratory or dermal exposure to these contaminants.

The appearance of phase separate hydrocarbon (PSH) in some of the on-site wells was noted in July, 1998. Over a period of 3-6 months the PSH column continued to increase as groundwater levels declined due to local drought conditions. Additional wells were installed to facilitate additional high vacuum multi-phase extraction (HVME) treatment operations in October, 1998.

MATERIALS AND METHODS

A detailed ORC application design was performed using data from monitoring wells installed within the boundary of the plume. Additionally, biological and chemical oxygen demand analyses were performed to more accurately estimate total oxygen demand. The project approach called for the ORC installation in two separate injection phases, with the majority of the ORC installed during the first phase. This type of application approach optimizes the operational flexibility that can be realized with the ORC technology.

ORC was injected into the on-site aquifer in March, 1998 per the design plan. At that time 3,232 pounds of ORC were injected via 70 direct push points in the area downgradient of the UST’s and existing dispenser islands. The amount of ORC delivered to the subsurface within the dissolved phase plume was specific to the BTEX mass measured. To enhance degradation of the off-site plume, ORC was applied along the down-gradient property boundary. Natural groundwater velocity and flow direction was used to deliver oxygenated water to the off-site hydrocarbon mass.

An aggressive monitoring schedule was performed immediately prior and following ORC injection in order to measure the efficacy of the bioremediation process. The monitoring program included analysis of BTEX, TPH as well as field parameters such as dissolved oxygen (DO).

A second application of ORC has been delayed due to the unexpected presence of SPH. Upon completion of product recovery operations, ORC will be reapplied to polish off the residual hydrocarbon levels prior to closure request.

Cost Analysis. Implementation of a traditional remediation technology air sparging with soil vapor extraction (AS/SVE) at this site would have cost substantially more, as well as produced much greater site and operational disruption. In addition, an AS/SVE system would likely not have been as effective within the same time frame. For example, the estimated cost savings in the application ORC vs. installation of an AS/SVE system is approximately $75,000. As a result of no operation and maintenance costs using the ORC technology, an additional $25,000 savings was realized. Finally, the reduced frequency and extent of groundwater monitoring at the site provided an estimated savings of $20,000. The total estimated cost savings for this project are approximately $120,000.

The unexpected appearance of free product in several monitoring wells at the site resulted in plans for aggressive product removal via high vacuum multi-phase extraction events. Additional wells were installed in the product area to facilitate the process. The extraction events were scheduled to occur prior to additional ORC applications. The HVME did increase the remedial costs, but will ultimately reduce total project costs by decreasing the contaminant mass treated via bioremediation. To date the HVME has removed 360 liters of gasoline and 19,000 liters of contaminated groundwater.

If required, additional ORC applications will be designed to polish residual dissolved petroleum concentrations to closure limits following product recovery. To date, DO concentrations are sufficient in most of the treated area to support continued bioremediation.

CONCLUSIONS

This project involved application of two technologies. Treatment with HVME removed SPH primarily in soil and groundwater, as well as some dissolved mass in groundwater. This process was enhanced by a non-invasive and cost-effective means to enhance natural bioremediation of contaminants in groundwater. The two technologies have worked synergistically, thereby increasing the efficacy of the remedial approach.

The ORC offers a flexible and environmentally friendly method of enhancing nature’s own cleanup process: bioremediation (Morin, 1997). Since remediation with ORC is an in-situ approach, operations and maintenance requirements were minimal. The application of HVME and ORC injection in tandem was more cost-effective than conventional technologies such as air sparging and vapor extraction. ORC is an in-situ (non-invasive) and passive remedial option; this minimizes disruption of business activities at the site. The project approach did not require trenching, piping, and on-site equipment. Faster site cleanup was achieved compared to conventional technologies, saving both time and money.

 


 

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