Graduate Thesis Or Dissertation
 

Radon-222 as a tracer for performance assessment of NAPL remediation technologies in the saturated zone : an experimental investigation

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  • 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.
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file details GottipatoSarayu1996.pdf 2017-10-19 Öffentlich Herunterladen