Design of an MTBE remediation technology evaluation

This study examines the intrinsic variability of dissolved methyl tert-butyl ether (MTBE) concentrations in ground water during the course of a pilot-scale bioremedial technology trial in Port Hueneme, California. A pretrial natural gradient tracer experiment using bromide was conducted in an anaerobic test section of the aquifer to characterize hydrogeology. The results showed the presence of a complex velocity field in terms of vertical stratification and preferential flowpaths. The hydraulic conductivity at the test area varied by > 2 orders of magnitude, and the effects of vertical stratification were made apparent by the tracers' detection pattern, which was predominately higher in the lower part of the aquifer. Since historically the lower portion of the aquifer significantly influenced MTBE transport, it was emphasized by increasing the sampling frequency for MTBE and tracers during the pilot test that involved the intermittent addition of oxygen and propane into the aquifer. A second tracer experiment using bromide and deuterated MTBE (H-2(12)-MTBE) was conducted at the onset of the technology trial and after the aquifer was made aerobic. The continuous metering of the tracer solutions into the test area was maintained for 300 d. The results showed that H-2(12)-MTBE behaved as a conservative tracer since (1) its concentrations increased throughout the study approaching its designed injected level and (2) the pattern of its detection resembled that of bromide. On the other hand, H-2(12)-MTBE, which was purposefully introduced into the aquifer, behaved differently from that of the existing dissolved MTBE plume that emanated from a non-aqueous phase liquid (NAPL) source over a decade ago, thereby undergoing years of diffusion. The data imply that a detailed understanding of the complexity of the flow field was not possible by observing the intrinsic MTBE data alone.