The Navy performed an interim remedial action to reduce gasoline concentrations in soil and groundwater at the Department of Defense Housing Facility (DoDHF) Former Underground Storage Tank (UST) Site 957/970 in Novato, California. The project was funded by Southwest Division, Naval Facilities Engineering Command in San Diego, California through the Naval Facilities Engineering Service Center (NFESC) in Port Hueneme, California. The interim remedial action consisted of in situ air sparging with soil vapor extraction (IAS/SVE). The system focused on four zones where maximum concentrations of benzene, ethylbenzene, toluene, and total xylenes (BTEX) had been detected in groundwater. The remedial action was initially targeted primarily at the removal of BTEX components; however, the concentrations of methyl-tert-butyl ether (MTBE) in both groundwater and the extracted vapor stream were monitored closely during system operation. As remedial activities progressed, MTBE became a greater concern among the scientific and regulatory community; therefore, its removal became more central to the specific remedial objectives at the Site. Results indicated that a conventional treatment technology designed for the removal of BTEX was also effective for significant removal of dissolved MTBE. The results of the remedial action that pertain to removal of dissolved MTBE are the focus of this paper.

SITE HISTORY

DoDHF Novato is located on former Hamilton Air Force Base (AFB) property in Novato, California, approximately 20 miles north of San Francisco. The Navy took over many of the housing units located on a portion of the Base and established DoDHF Novato. The Navy-occupied portion also included various operations to support the housing facility, including a Navy Exchange (NEX) gas station and Public Works Center (PWC) gas station (Figure 1) that were in use from the mid-1970s to the early 1990s. At that time, the gas stations were closed and the USTs that had previously stored gasoline were removed. The Navy-owned portion of the Site, now designated Former UST Site 957/970, comprises an area of approximately 13 acres of land (an approximate rectangle with dimensions 1,100 ft by 500 ft).

GROUNDWATER AND SUBSURFACE CHARACTERISTICS

Prior to removal of USTs from Former UST Site 957/970, gasoline releases impacted soils near some of the tanks and reached the groundwater in the shallow aquifer. In addition to typical organic compounds in gasoline such as BTEX, MTBE has been detected in groundwater at the Site. MTBE is a gasoline oxygenate that has been added to gasoline since the late 1970s and is a common constituent of Reformulated Gasoline (RfG) and California’s Cleaner Burning Gasoline. MTBE in groundwater originates from the former NEX gas station and extends to the north of the Navy property. BTEX compounds in groundwater are limited to the Navy property, and four years of quarterly groundwater monitoring have shown that these gasoline constituents are stable and that concentrations are decreasing over time. The extent of dissolved MTBE and benzene in groundwater is shown in Figure 1. The discussion of MTBE removal at the Site will be limited to the extent of the benzene plume, because this is the only portion of the MTBE plume that was actively treated by the IAS/SVE system.

The geology at the Site is heterogeneous, with sands and clay encountered in different proportions and at varying depths across the Site. Bedrock typically is encountered at 15 to 20 ft below ground surface (bgs) and increases in depth to the north of the Site. The bedrock underlies a shallow, thin aquifer encountered at about 9 ft bgs. A permeable sandy layer of varying thickness and depth was encountered in a majority of the borings during system installation. The aquifer zone is located in this permeable, sandy layer, throughout which the water table elevation fluctuates. Hydrocarbon-contaminated soils were encountered mostly in this permeable zone, with the majority of sorbed gasoline found in the smear zone formed by water level fluctuations and the initial release. The relative permeability of this zone, which contained the greatest levels of hydrocarbon impact, also made it the zone that is most conducive to effective air sparging.

IN SITU AIR SPARGING AND SOIL VAPOR EXTRACTION

A coupled IAS/SVE system was installed to reduce the mass of hydrocarbons in selected areas having elevated concentrations. The goal of the interim action was aggressive treatment and removal of “hot spot” areas and mass reduction of BTEX constituents, thereby reducing the potential of the groundwater plume to migrate. The interim action was not initially focused on MTBE, although it was known from past investigations to be present at the Site. Even though the interim action was not designed specifically for MTBE removal, MTBE was monitored in both extracted vapor and sampled groundwater throughout the duration of remedial activities. The IAS/SVE system operated at the Site from June 1998 to October 1999.

IAS is the injection of pressurized air directly into an aquifer. The injected air delivers oxygen to the groundwater and strips volatile contaminants from the groundwater. The air flows through permeable pathways in the saturated soil column driven toward the water table surface by buoyant forces. The shape and nature of the airflow pathways are determined by soil particle and sediment layering characteristics.

IAS removes hydrocarbons from saturated soil sediments by two primary mechanisms: (1) biodegradation: increased dissolved oxygen (DO) supports and enhances in situ biodegradation of hydrocarbons by indigenous microbes, and (2) stripping: volatile hydrocarbons partition into the vapor phase and are carried to the vadose zone with the air stream. The relative contributions of each of these removal mechanisms are dependent on site characteristics, contaminant type and concentration, subsurface microbiology, system design, and operational parameters. During the operational period of the IAS/SVE at Former UST Site 957/970, it was thought that MTBE could not be effectively biodegraded or stripped because MTBE did not appear to be readily biodegradable at this Site, and because of its low Henry’s Law Constant compared to other common gasoline constituents (i.e., BTEX). The discussion of results from IAS/SVE system operation at the Former UST Site 957/970 that follows indicates MTBE was likely removed by both biodegradation and stripping.

SVE systems are commonly coupled to IAS systems where the biodegradation capacity of the vadose zone is inadequate or unknown, or where the solutes to be removed are known to be recalcitrant to aerobic biodegradation, and where regulations do not permit the atmospheric release of the volatilized compounds of interest. The SVE system consisted of wells screened in the vadose zone and an extraction blower that imposed a vacuum on the soil voids near the well. Vapor was withdrawn to the surface and treated by a Vapor Check Vac 50 with a 500-scfm flow capacity prior to discharge.

The IAS/SVE system initially consisted of 10 air sparging wells and 6 SVE wells installed in May 1998. Sparge wells were screened as low as possible in the saturated permeable layer to allow air to traverse the maximum possible vertical distance through the aquifer sediments. SVE wells were screened in the vadose zone and across the water table to accommodate fluctuations in groundwater levels. Subsequent groundwater monitoring events revealed areas of elevated hydrocarbon concentrations outside of the areas for which the original air sparging and SVE systems were designed. For this reason, eight additional sparge wells and seven additional SVE wells were installed in October 1998 to remove gasoline from these areas.

RESULTS

Significant mass removal of MTBE was achieved through the operation of the air sparging and soil vapor extraction systems. As mentioned above, removal likely occurred through both biodegradation in the subsurface and stripping of volatile hydrocarbons which were captured by the SVE system and treated. Figure 2 shows the mass of gasoline and MTBE removed by the SVE system based on measured flowrates and hydrocarbon concentrations in the extracted vapor stream. The figure shows that approximately 10,000 kg and 450 kg of gasoline and MTBE, respectively, were calculated to have been removed through the SVE system. Note that these values are only indicative of the removal through stripping and do not account for removal that occurred in the subsurface as a result of biodegradation.

Groundwater monitoring was performed to determine the direct effect of IAS/SVE on groundwater concentrations. Groundwater samples were collected from monitoring wells and air sparging wells prior to startup of remedial activities to establish baseline concentrations. The initial (preremediation) sampling took place in May 1998. Groundwater samples were collected from monitoring and sparge wells again one year later (May 1999) and analyzed to determine the effects of remedial activities on groundwater concentrations. Air sparging wells were allowed to equilibrate for approximately one week prior to groundwater sampling. System expansion wells were not installed until October 1998; therefore, initial sampling data for these wells was conducted in November 1998, and the corresponding one-year sampling event took place in November 1999. Quarterly groundwater monitoring events were conducted between the baseline and annual monitoring; however, these sampling events consisted only of nearby monitoring wells and not the sparge wells because the system wells could not be monitored while operational.

Figure 3 shows groundwater MTBE concentrations in the immediate vicinity of the former NEX source area in a well located approximately 10 ft downgradient of the former NEX UST complex. Figure 4 shows groundwater MTBE concentrations in a well located approximately 250 ft downgradient from the former NEX UST complex. Groundwater monitoring results show that MTBE concentrations decreased significantly in the area of the former NEX gas station following one year of system operation (Figures 3 and 4, respectively). Groundwater concentrations in the well located in the immediate vicinity of the former NEX source area started at 190,000 µg/L of MTBE preceding system operation (Figure 3). After one year of operation, the concentration of MTBE in the same well decreased to 2,200 µg/L. This decrease represents a reduction of approximately 99% MTBE in this particular well as a result of only one year of operation. Note that Figure 3 shows MTBE concentrations increased slightly immediately after the IAS/SVE system was shut down due to diminishing returns. This slight increase of MTBE concentrations can be attributed to seasonal variations (i.e., the system was shut down during the dry season), and Figure 3 shows concentrations have decreased and remained at low levels over two years of quarterly monitoring. This long-term concentration trend indicates that rebound of gasoline constituents near the former NEX source area is not occurring. Groundwater concentrations in a monitoring well located approximately 250 ft downgradient of the former NEX source area showed a similar reduction to that observed in the immediate vicinity of the source area (Figure 4).

A microcosm study was performed using native soil and groundwater from the Former UST Site 957/970 to determine whether biodegradation of MTBE could occur at the Site (Magar et al., 2001). Results of the study showed that the indigenous microbes that exist in the native media were capable of aerobic MTBE biodegradation. These results can help to understand the decreasing MTBE concentration trends illustrated in Figure 5, which shows MTBE isoconcentration maps in the area of the Site influenced by the IAS/SVE system. Figures 5a and 5b show the MTBE concentrations in groundwater before and after IAS/SVE system operation, respectively. By comparing the two figures it is evident that the active treatment system was effective at decreasing MTBE concentrations. Figure 5c shows MTBE concentrations in groundwater two years after the active treatment system was shut down and indicates that MTBE concentrations did not rebound and continued to decrease without active remediation. This post-shutdown MTBE loss is attributed to aerobic MTBE biodegradation.

CONCLUSIONS

Although the interim remedial action system was designed to remove elevated levels of BTEX compounds, results showed that the system effectively removed significant MTBE mass. The MTBE removal was confirmed by the presence of MTBE in the extracted vapor stream and by the reduction of MTBE groundwater concentrations. In addition to significant groundwater concentration reductions during system operation, MTBE concentrations did not rebound and continued to decrease for two years after system operation stopped. The aggressive treatment of “hot spot” areas resulted in treatment to levels below those required to be protective of human health for the intended future site use (Battelle, 2001). Although specialized technologies are being developed to treat MTBE due to its unique characteristics, it should be noted that in some cases significant removal can be achieved with conventional treatment technologies such as IAS/SVE.

ACKNOWLEDGEMENTS

We would like to thank Thomas Macchiarella, the Navy BRAC Environmental Coordinator directing the work at Novato, for his contributions to technical decision-making and his ability to transform project and risk management needs into specific and unambiguous directions. Thanks also to James Hicks of Battelle for the many hours spent constructing volume and mass estimates.

REFERENCES

Battelle. 2001. “Final Revised Risk Assessment for Former UST Site 957/970 at Department of Defense Housing Facility, Novato, California.” Prepared for the Southwest Division, Naval Facilities Engineering Command under NFESC Contract No. N47408-95-D-0730. June 8.

Magar, V., K. Hartzell, C. Burton, J. Gibbs. 2001. “Aerobic and Cometabolic MTBE Biodegradation at the Novato and Port Hueneme Test Site.” Journal of Environmental Engineering. (submitted for publication December 2001).
 

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FIGURE 1. Sitemap of DoDHF Novato

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FIGURE 2. Cumulative mass of gasoline and
MTBE removed by the SVE system.

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FIGURE 3. Former NEX source area MTBE concentrations over time.

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FIGURE 4. MTBE concentrations over time downgradient of former NEX source area.

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FIGURE 5. MTBE plume contours a) prior to IAS/SVE treatment, b) immediately after IAS/SVE shut down, and c) two years after IAS/SVE shut down.

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