You are here:
AN EXPERIMENTAL ASSESSMENT OF THE IMPACTS OF PARTIAL DNAPL SOURCE ZONE DELETION USING SPARGING AS A REMEDIATION TECHNIQUE
Citation:
BOB, M., M. BROOKS, S. C. MRAVIK, AND L. WOOD. AN EXPERIMENTAL ASSESSMENT OF THE IMPACTS OF PARTIAL DNAPL SOURCE ZONE DELETION USING SPARGING AS A REMEDIATION TECHNIQUE. Presented at The Second International Conference on DNAPL: Characterization and Remediation, Niagra Falls, NY, September 24 - 28, 2007.
Impact/Purpose:
To present information at the Second International Conference on DNAPL
Description:
The contamination of the subsurface environment by dense non-aqueous phase liquids (DNAPL) is a wide-spread problem that poses a significant threat to soil and groundwater quality. Implementing different remediation techniques can lead to the removal of a high fraction of the DNAPL mass present in the source zones, as demonstrated by a number of field- scale studies. Removing all the DNAPL mass present in the source zones using current technologies is, however, difficult or impractical due to the complex entrapment of the DNAPL in the subsurface. Several studies have shown that as increased mass of DNAPL is removed from source zones, access to the remaining DNAPL mass becomes hydrodynamically limited and removal efficiency drops significantly. There is a need thus, especially with limited financial resources, to evaluate the impact of partial DNAPL source mass deletion. To this end, the contaminant flux (ML-2T-1), has been proposed as a metric for source zone remediation activity. It is argued that the contaminant mass discharge, rather than groundwater contaminant concentration, is the main factor that determines the risks associated with a contaminated site. A careful investigation of the relationship between contaminant mass present in the source zone and the mass flux generated thus becomes very essential. To date, laboratory and field studies have mainly focused on evaluating the efficiency of treatment technologies, with only a limited number of laboratory studies investigating the flux reduction associated with contaminant mass reduction. In this research, well-controlled laboratory experiments were carried out to assess the impact of partial DNAPL source zone remediation on dissolved contaminant flux generated following the implementation of an aggressive in situ treatment technology, namely sparging. Three experiments were conducted using large two-dimensional (2-D) chambers (48.26 cm by 48.26 cm by 1.4 cm) packed with silica sands. The effect of both homogeneous and heterogeneous packing on DNAPL architecture, mass reduction and flux reduction was studied. A known volume of tetrachloroethylene (PCE), a model DNAPL, was released in the chamber and at least three sparging events were conducted in each experiment to achieve incremental PCE mass removal. A modified light transmission visualization (LTV) method was developed and used to calculate PCE saturation distribution in the source zone before and after each sparging event allowing for DNAPL architecture characterization. In addition, a gas chromatography-mass spectroscopy (GC/MS) instrument and carbon columns were utilized to quantify the reduction in PCE mass by quantifying vapor-phase PCE leaving the chamber during each sparging event. Following each sparging experiment, the chamber was flushed with water for a sufficient time to re-establish a steady state condition, and effluent aqueous samples were collected and analyzed for dissolved PCE concentration which was used to calculate the PCE mass flux. Results showed a significant reduction in dissolved PCE mass flux as more PCE mass was removed from source zone, and that sparging can remove as high as 70% of PCE mass present. The relationship between mass reduction\flux reduction was close to 1:1, which is similar to results obtained by other researchers who studied this relationship using different remediation techniques. LTV analyses clearly showed the reduction in PCE saturation as a result of the sparging process and also showed that a significant fraction of the remaining PCE was at areas having initially high saturation. The results presented here provide important new information regarding the effect of partial contaminant source zone removal by aggressive in situ treatment technologies, and should also be helpful in designing effective remediation strategies and in the development of remediation guidelines by regulatory authorities.