P. S. Birak

Mobilization of Manufactured Gas Plant Tar with Alkaline Flushing Solutions

This experimental study investigates the use of alkaline and alkaline-polymer solutions for the mobilization of former manufactured gas plant (FMGP) tars. Tar-aqueous interfacial tensions (IFTs) and contact angles were measured, and column flushing experiments were conducted. NaOH solutions (0.01-1 wt.%) were found to significantly reduce tar-aqueous IFT. Contact angles indicated a shift to strongly water-wet, then to tar-wet conditions as NaOH concentration increased. Column experiments were conducted with flushing solutions containing 0.2, 0.35, and 0.5% NaOH, both with and without xanthan gum (XG). Between 10 and 44% of the residual tar was removed by solutions containing only NaOH, while solutions containing both NaOH and XG removed 81-93% of the tar with final tar saturations as low as 0.018. The mechanism responsible for the tar removal is likely a combination of reduced IFT, a favorable viscosity ratio, and tar bank formation. Such an approach may have practical applications and would be significantly less expensive than surfactant-based methods.

Effectiveness of Source-Zone Remediation of DNAPL-Contaminated Subsurface Systems

Concerted efforts to remediate subsurface systems contaminated with dense nonaqueous-phase liquids (DNAPLs) have met with limited success when measured by comparing solute concentrations to drinking water quality standards. One-dimensional and three-dimensional laboratory experiments and a field-scale experiment are used to investigate the effectiveness of source-zone remediation and to assess factors that contribute to the observed results. The three-dimensional laboratory experiment and the field-scale experiment used a surfactant flush followed by vapor extraction to reduce the DNAPL saturation, while vertical DNAPL mobilization was controlled using a brine barrier. DNAPL mobilization and recovery in the field-scale experiment was relatively ineffective due in part to the low saturation levels of the DNAPL. The results show essentially that complete removal of a DNAPL is required to reach typical cleanup standards and that details of the morphology and topology of a DNAPL distribution, in addition to the saturation, play an important role in determining the rate of mass transfer. The results are interpreted in terms of guidance for remediation approaches, realistic expectations for source-zone remediation, and elements needed for improved models of such systems.