Natural Gradient Tracer Test to Evaluate Natural Attenuation of MTBE Under Anaerobic Conditions

Abstract

A natural gradient tracer test using perdeuterated MTBE was conducted in an anaerobic aquifer to determine the relative importance of dispersion and degradation in reducing MTBE concentrations in ground water. Preliminary ground water chemistry and hydraulic conductivity data were used to place the tracer within an existing dissolved MTBE plume at Port Hueneme, California. Following one year of transport, the tracer plume was characterized in detail. Longitudinal dispersion was identified as the dominant mechanism for lowering the perdeuterated MTBE concentrations. The method of moments was used to determine the longitudinal and lateral dispersion coefficients (0.85 m2/day and 0.08 m2/day, respectively). A mass‐balance analysis, carried out after one year of transport, accounted for 110% of the injected mass and indicated that no significant mass loss occurred. The plume structure created by zones of higher and lower hydraulic conductivity at the site was complex, consisting of several localized areas of high tracer concentration in a lower concentration plume. This is important because the aquifer has generally been characterized as exhibiting fairly minor heterogeneity. In addition, the tracer plume followed a curved flowpath that deviated from the more macroscopic direction of ground water flow inferred from local ground water elevation measurements and the behavior of the existing plume. Understanding the mass balance, plume structure, curvature of the tracer plume, and consequently natural attenuation behavior required the detailed sampling approach employed in this study. These data imply that a detailed understanding of site hydrogeology and an extensive sampling network may be critical for the correct interpretation of monitored natural attenuation of MTBE.

Design of an MTBE Remediation Technology Evaluation

“…The impact of the aquifer heterogeneity on tracer distribution at the source area of the Port Hueneme plume was also suggested by others (Amerson and Johnson 2003). About 6 months before the initiation of the technology evaluation, the average MTBE concentrations in the deep screens at the four locations were relatively high including downgradient test plot at 5329 lg/L (s ¼ 2112), upgradient test plot at 4900 lg/L, downgradient control plot at 5989 lg/L (s ¼ 1852), and upgradient control plot at 4800 lg/L.…”

“…Over 300 g of 2 H 12 -MTBE were uniformly (23 mg/well/d) injected into the aquifer at a rate of 10mL/well/d. Since a total of 7 g of 2 H 12 -MTBE had previously (1998) been injected in the source zone, several hundred feet upgradient from our site (Amerson and Johnson 2003), it became necessary to perform background sampling on two occasions to assure that it was not present (detection limit, 0.2 lg/L). Stable isotope-labeled tracers are valuable tools for field-scale retardation and biodegradation studies since they avoid the limitations of using a more brute-force monitoring approach or regulatory approval problems associated with the use of radiolabeled isotopes.…”

Anaerobic Biodegradation of MTBE at a Gasoline Spill Site

“…To date, most field studies of MTBE have been conducted on large plumes that show little indication from their behavior that natural biodegradation is an important mechanism. Amerson and Johnson (2002) injected deuterated MTBE into a large MTBE plume at Port Hueneme, California. At the time of their experiment, the plume was more than 1000 m long and the leading edge of the plume continued to advance into the aquifer.…”

Modern geochemical and molecular tools for monitoring in-situ biodegradation of MTBE and TBA

“…Tracer techniques are feasible when biodegradation occurs without a lag phase over the duration of the experiment, so that the degradation effect are present and can be measured. Deuterated MTBE flow line tracer was used in an MTBE plume mass-balance study of Amerson and Johnson (2003), however, in this case only advective contaminant transport and no biodegradation were observed. There is a significant body of work on insitu application of isotope tracers to study biodegradation, but no examples of MTBE or TBA applications.…”

“…Alternatively, stable isotope-labeled tracers can be used to distinguish the tracer plume from the existing plume of the same chemical species in mass balance studies (cf. Amerson and Johnson 2003). In such studies, it is assumed that the properties of labeled and unlabeled chemicals are identical.…”