In situ bioaugmentation and biostimulation experiments were conducted at the USN Hydrocarbon National Environmental Test Site at Port Hueneme, CA (PH), where the dissolved MTBE groundwater plume is over 1500 m long. Laboratory microcosm experiments prepared with PH groundwater showed that MTBE was rapidly metabolized (t 1/2 ≤ 2 weeks) after inoculation with a high-activity MTBE-degrading bacterial consortium (MC-100). Microcosm studies also showed that natural ether degraders were present at PH; however, the rates were 3−5 times slower than with the bioaugmented treatment. The field pilot test was conducted to assess the efficacy of creating an MTBE biobarrier by inoculating with MC-100 and maintaining well-oxygenated conditions. Three test plots located in the MTBE-only portion of the plume included control (no treatment), O2-only (intermittent O2 gas injection), and O2 + bioaugmented (MC-100) zones. Initial MTBE and dissolved oxygen (DO) concentrations in the plots prior to treatment varied from 2 to 9 mg/L and ≤1 mg/L, respectively. DO levels increased in the O2-only and O2 + MC-100 plots from 5 to >20 mg/L within a few weeks of O2 gas injection. MTBE levels decreased in the O2-only plot to 0.01−0.1 mg/L after a lag period of 186−261 days, indicating the apparent stimulation of naturally occurring ether degraders. In contrast, in the O2 + MC-100 plot, MTBE concentrations decreased after 30 days and throughout the 261-day experiment eventually to ≤0.001−0.01 mg/L. tert-Butyl alcohol (TBA) concentrations also declined in the bioaugmented plot to <0.01 mg/L.

Knowledge of the effective vapor-phase porous medium diffusion coefficient is important for many applications, including risk-based volatilization and vapor migration calculations and remediation performance estimates. A procedure for measuring the effective vapor-phase porous medium diffusion coefficient in situ is presented here. The approach utilizes transient changes in volume-averaged concentrations of an inert tracer gas, rather than changes in point concentration measurements, to determine the effective vapor-phase porous medium diffusion coefficient D v eff . Measurements are made over intermediate time frames (minuteshours), and the procedure is easily altered to probe scales ranging from approximately 0.1-1 m. The data reduction leads to the determination of D v eff / (θ v ) 1/3 , where θ v denotes the air-filled porosity. Given this reduced dependence on θ v (relative to approaches based on transient point concentration changes), in many settings it would not be necessary to make independent measurements of moisture content and total porosity to determine D v eff . For example, if θ v falls in the range 0.13 e θ v e 0.43 cm 3 -vapor/cm 3 -soil, then using an assumed value θ v ) 0.28 would contribute to less than a 23% error in determining D v eff from experimental data. The theory, a general protocol, and an example field protocol are presented along with sample field data.

The use of compound-specific isotope analysis (CSIA) as a diagnostic tool for MTBE biodegradation in aquifers was tested at the Port Hueneme, CA site. There, a 1500-m long dissolved MTBE plume and associated engineered aerobic flow-through biobarrier have been well-studied, leading to delineation of regions of known significant and limited bioattenuation. This allowed comparison of field-scale CSIA results with a priori knowledge of aerobic MTBE biodegradation, leading to conclusions concerning the utility of CSIA as a diagnostic tool for other aerobic biodegradation sites. Groundwater samples were collected and analyzed for both 13C and 2H (D) in MTBE through the bioactive treatment zone and within the larger MTBE plume. For reference, the 13C enrichment factor for MTBE biodegradation in laboratory-scale microcosms using site groundwater and sediments was also quantified. Aerobic microcosms showed a 13C enrichment of 5.5 to 6.4 +/- 0.2 per thousand over a two-order of magnitude concentration decrease, with an average isotope enrichment factor (epsilon(c)) of -1.4 per thousand, in agreement with other aerobic microcosm studies. Less 13C enrichment (about 25%) was observed for similar MTBE concentration reductions in groundwater samples collected within the aerobic biotreatment zone, and this enrichment was comparable to the scatter in delta13C values within the source zone. Increasing enrichment with decreasing MTBE concentration seen in microcosm data was not evident in either the 13C or D field data. The discrepancy between field and laboratory data may reflect small-scale (<1 m) spatial heterogeneity in MTBE biodegradation activity and the mixing of water from adjacent strata during groundwater sampling; for example, relatively nonattenuated MTBE-impacted water from one stratum could be mixed with highly attenuated/low-MTBE concentration from another, and this could produce a sample with both reduced MTBE concentration and low enrichment. Overall, the results suggest that 13C data alone may produce inconclusive results at sites where MTBE undergoes aerobic biodegradation, and that even with two-dimensional CSIA (13C and D), an increase in the confidence of data interpretation may only be possible with data sets larger than those typically collected in practice.