In recent years, commercially available chemical compounds designed to slowly release oxygen have demonstrated some success in aiding the in situ biodegradation of certain fuel hydrocarbon constituents such as benzene, toluene, ethylbenzene, and xylenes (BTEX). These commercial compounds are designed to increase the dissolved oxygen content in groundwater, increasing aerobic biodegradation of hydrocarbons by naturally occurring microorganisms that would otherwise be oxygen limited. While methyl tertiary butyl ether (MTBE) had been thought to be recalcitrant to bioremediation, several recent studies have noted MTBE attenuation rates higher than those anticipated by dispersion alone (Barker et al, 1998). Other studies suggest that in mixed contaminant systems MTBE is consumed, but only after the BTEX constituents have been depleted.
Reports by the manufacturer of a patented magnesium peroxide compound called Oxygen Release Compound (ORC®) that their product has been successful in biodegrading fuel release sites with significant MTBE concentrations dissolved in the groundwater (Regenesis, 1998a) led to its use at the subject site. The following is an examination of the effectiveness of ORC®in accelerating the biodegradation of fuel hydrocarbons in the groundwater in general, and specifically its effectiveness in affecting the biodegradation of MTBE. Although not implemented as a research project, sufficient data was collected to warrant discussion.
Site Description. The site is an active service station dispensing gasoline stored in two underground tanks (Figure 1). The service station is located along the coast of southern California at the mouth of the San Juan Creek Valley, approximately one-third mile (0.5 km) to the north of the Pacific Ocean.
Site Background. Following replacement of the underground storage tanks in the early 1990’s, hydrocarbon-affected soil was encountered beneath the tank cavity and one of the dispenser islands. Subsequent site assessment activities performed following overexcavation of the tank cavity area indicated that total petroleum hydrocarbon (TPHg) concentrations of up to 9,000 mg/kg remained in the soil and extended to groundwater, present at a depth of approximately 18 feet (5.5 m) below grade.
Dissolved-phase hydrocarbon concentrations of up to 25,000 m g/L benzene and 13,000 m g/L MTBE have been present in the groundwater beneath the site. The direction of groundwater flow is toward the west and southwest at an average gradient of 0.0037 ft/ft. Both the dissolved-phase BTEX and MTBE plumes appear to be stable.
Soil vapor extraction was employed at the site for approximately 13 months during the mid 1990’s but did not have a lasting impact on dissolved-phase hydrocarbon concentrations in the saturated zone. Due to the fine grained nature of the materials within the vadose zone, a significant mass of fuel hydrocarbons are suspected to remain adsorbed to the soil.
Geology and Hydrogeology. Subsurface materials encountered in the vadose zone and capillary fringe beneath the service station are composed of silts and clays, from the surface to a depth of approximately 20 feet (6.1 m), and silty, fine-grained sands to medium-grained sands in the saturated zone.
Though used intermittently by a local water district, the groundwater quality for this area of the basin is listed as poor due to high concentrations of dissolved solids (Woodside, 1986). Groundwater beneath the service station contains a chloride concentration of 950 mg/L and a total dissolved solids concentration of 4,700 mg/L. The darcy velocity of the groundwater was calculated to range from 0.067 to 0.24 ft/day (0.020 to 0.073 m/day).
TECHNICAL APPROACH TO USING MAGNESIUM PEROXIDE
While the oxygen release rate of ORCâ is dependent upon the level of contaminant flux, this reaction is normally expected to last about six months. However, due to the brackish nature of the groundwater, it was anticipated that the reaction time could be decreased by as much as 50% (Regenesis, 1998b). The subsequent increase in available dissolved oxygen is intended to metabolize the dissolved fuel hydrocarbons by stimulating aerobic biodegradation.
Other factors that were considered prior to implementation of this approach were the groundwater velocity, biological oxygen demand (BOD), soil texture, porosity, pH, temperature, and the concentrations of alternate electron acceptors. Although the salinity of the groundwater caused some concern, most other factors appeared to be within an acceptable range for the use of this product.
Oxygen Demand Calculations. A conservative estimate of the oxygen to carbon stoichiometry of 3:1 has been employed by the manufacturer of ORCâ. Based on the dimensions and average concentration of the dissolved fuel hydrocarbon plume (BTEX + MTBE) in the groundwater and the estimated porosity of the saturated soils, a mass of 3.1 pounds (1.4 kg) of fuel hydrocarbons were estimated to be dissolved in the groundwater. An additional oxygen demand factor of 8.075 was also used to account for oxygen that would be used by competing, non-targeted, organic material such as organic carbon in the matrix other than the fuel hydrocarbons and by fuel hydrocarbons adsorbed on the soil particles within the saturated zone and capillary fringe. Based on these factors the amount of oxygen required to metabolize the dissolved hydrocarbon was calculated using equation 2.
(3 lbs Oxygen/1 lb Carbon)(3.1 lbs HC)8.075 DF = 75 lbs (34 kg) Oxygen
Where HC = fuel hydrocarbons, DF = additional oxygen demand factor
Injection Point Field Design. Injection into direct-push bore holes was chosen as the preferred application for this site so that the entire amount of ORCâ could be applied evenly across the source area of the dissolved-phase fuel hydrocarbon plume. As an added measure to prevent further migration of the dissolved phase hydrocarbon plume, an oxygen barrier was placed at the downgradient property boundary using an additional application of direct injected ORCâin that area.
A total of 750 pounds (340 kg) of ORCâ was injected through 18 direct-push bore holes located within the source area by pushing hollow rods with removable tips to a depth of 28 feet (8.5 m), then pumping the slurry through the rods as they were raised from a depth of 28 feet (8.5 m) to 18 feet (5.5 m).
FIGURE 1. Site Plan Showing ORCâ Injection, Points and Baseline MTBE Plume
An additional 150 pounds (46 kg) of ORCâ was divided between 12 direct-push bore holes located at the property line and injected as in the source area (Figure 1).
To monitor the effectiveness of the magnesium peroxide compound in biodegrading the dissolved fuel hydrocarbons, baseline groundwater samples were collected from representative groundwater monitoring wells just prior to injection, and then monthly for a period of five months after the injection. The groundwater samples were analyzed for TPHg, BTEX, and MTBE. Prior to collection of the groundwater samples, field measurements of depth to water, dissolved oxygen, temperature, pH, oxidation/reduction potential, and conductivity were collected.
Figure 2. Dissolved MTBE and BTEX Concentrations and Oxidation/Reduction Potential vs.
Time in MW-14.
Dissolved Hydrocarbons. Dissolved-phase MTBE and BTEX concentrations in Monitoring Well MW-14 (the well located within the source area) were plotted over time both prior to and following injection of the magnesium peroxide compound (Figure 2). Along with an 82% reduction of dissolved BTEX baseline concentration of 16,600 m g/L within the first month after the injection of ORCâ, dissolved MTBE concentrations were reduced from the baseline concentration of 2,200 m g/L by 77%. Dissolved phase BTEX and MTBE concentrations dropped an additional 1% and 20%, respectively, during the second month after injection. By the third month after injection, however, dissolved phase BTEX concentrations had rebounded to 151% of the baseline results and dissolved MTBE had rebounded to 82% of the baseline. Subsequent analysis of groundwater samples collected during the next two months indicated that dissolved BTEX and MTBE concentrations had returned to pretreatment levels.
Dissolved Oxygen and Oxidation/Reduction Potential. Dissolved oxygen readings taken at the beginning and following injection of ORCâ never varied more than 1 mg/L (between 0 and 1 mg/L) for the selected wells tested. Oxidation/reduction potential readings were not collected prior to injection but were collected in the months following. As expected the lowest potential was observed in Monitoring Well MW-14, located in the center of the dissolved fuel hydrocarbon plume. The oxidation/reduction potential peaked for MW-14 during the second month following ORCâ injection and then steadily declined (Table 1 and Figure 2). The oxidation/reduction potential for the two wells located within the oxygen barrier peaked during the third month after injection and then declined.
Table 1. Chemical and Physical Groundwater Properties of MW-14