Abstract:
Persistence of Non aqueous phase liquids (NAPLs) in the subsurface at residual
saturations eventually contributes to undesirable groundwater contamination. Proper
characterization of subsurface NAPL, its location, composition and distribution, is
essential for the chosen remediation technology to be effective. It is also desirable to
assess the performance of remedial actions at NAPL-contaminated sites in order to verify
the technoeconomic viability of the selected method. The unique properties of radon-222
gas make it a good indicator for organic phase liquids. It is ubiquitous in the subsurface,
chemically inert, radioactive, and most importantly, partitions into NAPLs. This research
explores the practicality of using radon to indirectly monitor the progress of NAPL
remediation efforts.
The effectiveness of surfactant flushing in remediating NAPL contamination was
also studied in the process. Preliminary studies were conducted using micro-columns to
evaluate the efficiency of the surfactant selected for the study, triton. These studies show
that triton is more effective at higher concentrations in solubilizing residual soltrol and its
solubilizing capacity is greatly enhanced after batch equilibration. These observations
suggest that surfactant solubilization of NAPLs is rate-limited rather than instantaneous.
These studies also indicate the adverse effect of aged NAPL on surfactant solubilizing
capacity. Two independent methods, total organic carbon analysis and HDPE strip test,
were also designed for analyzing the aqueous and sand samples and estimating the level of
cleanup achieved.
Since triton proved to be effective in micro-column studies, the remediation of the
soil columns was performed by flushing triton through the columns in a sequential batch
mode. The soil columns employed in the study had been previously packed and used by
Hopkins (1994). The influence of the decrease in residual soltrol saturations on
breakthrough of radon was observed. The gradual cleanup of columns at various initial
residual soltrol saturations (1.0%, 5.0%, and 8.0%) through surfactant flushing was well
reflected by radon. The aqueous radon concentrations increased and the retardation of
radon lessened as residual soltrol was removed from the columns. The linear equilibrium
partitioning model of radon was used to estimate the initial residual NAPL saturation in
each column and the subsequent saturations as the remediation proceeded. The saturation
estimates were based on retardation factors obtained from maximum aqueous radon
concentrations and breakthrough of radon. These estimates correlated fairly well with
those based on TOC analyses and HDPE strip tests, supporting radon's capability of
detecting and quantifying NAPLs, and monitoring the progress of NAPL remediation.
The results of this study demonstrate the potential of radon as a tracer for
evaluating the performance of NAPL remediation techniques. This study also
substantiates the ability of surfactants to enhance NAPL recovery from subsurface.
However, clogging problems have been encountered, which are believed to be caused by
surfactant micelles, while sampling columns. Hence, careful selection of appropriate
surfactant, among other criteria, is essential to get maximum benefits of surfactant-enhanced
NAPL remediation technology.
