Comparison of Eight Innovative Site Characterization Tools Used to Investigate an MTBE Plume at Site 60 , Vandenberg Air Force Base , California

Environmental assessment of contaminated sites often requires delineation of the pathways of contaminant migration. Often, the pathways are strongly affected by the subsurface geology. At Vandenberg AFB in California, an accidental release of gasoline containing MTBE occurred at a base gasoline station in 1994. Dissolution of the gasoline into groundwater has created a dissolved MTBE plume at least 1,700 feet long. The plume is flowing in a complex shallow alluvial aquifer system comprised of thin sand layers within a silt and clay matrix. Portions of the aquifer containing the dissolved plume have been characterized in considerable detail in order to assess the occurrence, distribution, and flux of MTBE and other solutes within the dissolved plume. In one area(Transect B), the geology and plume geochemistry was assessed by collecting continuous stratigraphic and geochemical profiles at 40-foot intervals across the entire width of the MTBE plume. Eight innovative characterization tools were used in the subsurface investigation along Transect B, providing a unique opportunity to compare and contrast the various characterization tools. The subsurface geology was investigated by collecting and logging continuous soil cores, performing cone penetration test (CPT) soundings, direct-push resistivity probes, Geoprobe electrical conductivity probes, and surface dipole-dipole resistivity surveys. The groundwater geochemistry was assessed by collecting one time “snapshot” samples using a Geoprobe sealed screen sampler and the Waterloo Groundwater ProfilerTM. Permanent, seven-zone multi-level monitoring wells have been recently installed along Transect B to assess the hydraulic head distribution in the shallow aquifer and to provide a way to monitor the plume geochemistry over time. Characterization data along Transect B shows that the subsurface stratigraphy exerts a strong control on the distribution and flow of MTBE and dissolved electron acceptors in the shallow alluvial aquifer. Continuous soil cores were useful to provide samples needed to measure the physical properties of the various strata and to calibrate the CPT data. The CPT provided the most detailed, cost-effective information about the site stratigraphy. The multi-level monitoring system yielded important information regarding the vertical distribution of hydraulic head in the aquifer and on-going samples of groundwater for geochemical measurements. Introduction In early 1998, researchers from the University of Waterloo selected a former gas station (Site 60) at Vandenberg Air Force Base (VAFB) in Central California as the location for an API-funded field study of natural and enhanced attenuation of dissolved MTBE (Figure 1). Since then, detailed laboratory and field experiments have been undertaken to assess the feasibility of stimulating in situ biodegradation of dissolved MTBE (Mackay et al., 1999 [these proceedings] and Wilson et al., 1999 [these proceedings]). In addition, numerical simulations are being performed to better understand the characteristics and longevity of MTBE source zones (Durrant et al., 1999 [these proceedings]). Prior to beginning the field experiments and computer simulations, detailed characterization of the hydrogeology of the site was performed. The site characterization utilized a number of innovative technologies due to the necessity of defining the stratigraphy, hydraulics, and groundwater geochemistry of the study area in considerable detail. Many of the technologies were used in parallel, providing an unprecedented opportunity to compare the various characterization tools and assess their particular strengths and weaknesses. While the technologies were employed at many locations at the site, particular effort was spent delineating the subsurface geologic and geochemical conditions along a transect (Transect B) oriented perpendicular to the prevailing groundwater flow direction. Transect B is approximately 200 feet downgradient from the source zone, near the location of the small-scale pilot tests described by Mackay et al., 1999 (Figures 2 and 3). Detailed stratigraphic, hydraulic head, and groundwater chemical profiles were collected at closely-spaced points (maximum spacing of 40 feet) along Transect B (Figure 3). Eight innovative subsurface characterization or monitoring technologies have been used to date at Site 60, VAFB. Comparison of these innovative characterization technologies along Transect B is the focus of this paper. These eight characterization tools are: 1. Dipole-dipole surface resistivity survey 2. Enviro-CoreTM Soil Sampling System 3. Waterloo Piston SamplerTM 4. Geoprobe Electrical Conductivity Probe 5. CPT – Resistivity Probe 6. Waterloo Groundwater ProfilerTM 7. CMT Multi-Level Monitoring SystemTM 8. Geoprobe Membrane Interface ProbeTM Release History and Plume Formation A plume of petroleum hydrocarbons and MtBE resulted from leaking fuel storage facilities at a General Services Administration (GSA) gas station at VAFB. The GSA service station and associated subsurface contamination is referred to as Site 60. The service station, which historically dispensed diesel, leaded, and unleaded fuel, was taken out of service in 1994 after a significant fuel leak was noted. Reconciliation of inventory records suggested that a total of 572 gallons of unleaded fuel had been lost (Lee and Ro, 1998; see Figure 2 for the approximate location of the former fuel tanks, labeled as “source zone”). The underground storage tanks and piping were removed in 1995 via two excavations. Immediately after tank removal, the northern pit was partially filled with pea gravel. In late 1995, the southern pit and remaining 2 to 3 feet of the northern pit were filled with sand (Lee and Ro, 1998). Based on investigations performed by consultants to the Air Force, the MtBE plume has been mapped as historically being from 250 to 300 feet wide and extending approximately 1,700 feet beyond the source area (Figure 2). A BTEX plume is also present at the site but apparently attenuates to below detection limits within 50-100 feet of the source. 101 SANTA MARIA GUADALUPE VANDENBERG AIR FORCE BASE ORCUTT LOMPOC P U RI S SI MA HI L LS S ANT A RI T A HI L L S CASMALIA Pt. Arguello PACIFIC OCEAN Purisimo Pt. Pt. Sal S NT A Y N E Z MT N S. S A N T A M A RI A V A L L E Y