Superfund Research Program
Mechanisms Influencing Long-term Mass Flux of DNAPL Contaminants
Background: Risk assessment and the implementation of efficient, cost-effective remediation systems at a hazardous waste site depend on accurate characterization of the type and magnitude of contamination, the possible routes of exposure, and the potential temporal and spatial variability of contaminant levels. This includes a thorough understanding of the distribution, transport, and fate of contaminants in the subsurface environment, which is physically and chemically heterogeneous at several scales.
The presence of dense organic nonaqueous-phase (immiscible) liquids (DNAPLs), including chlorinated solvents, greatly complicates the analyses required for risk assessment and determination of remediation strategies. NAPL source zones can be long-term sources of contamination and greatly impact the costs and time required for site remediation. Dr. Mark Brusseau at the University of Arizona Superfund Research Program has conducted extensive pore, bench, and field scale studies into NAPL source zone phenomena. His recent work is focused on processes known to occur at field sites that contribute to the delayed removal of contaminant mass during remediation via pump-and-treat and other methods.
Advances: In one study, Dr. Brusseau's group investigated low-concentration tailing behavior associated with sorption/desorption phenomena, with an eye towards the potential impact of contaminant residence time, or "aging". They used aquifer material collected during well installation at a Superfund site in Tucson, AZ, and conducted a geochemical characterization of the sediment's organic carbon and clay constituents. The scientists compared mass removal and long-term, low-concentration elution tailing in field-contaminated, synthetically-aged, and freshly-amended aquifer material, representing contact times of >40 years, ~4 years, and <8 h, respectively. With trichloroethene (TCE) as the model DNAPL contaminant, they found that elution patterns for the field-contaminated and synthetically-aged treatments were essentially identical to that observed for the fresh treatments. This finding suggests that long-term contaminant aging did not significantly influence the retention and transport of TCE in the low organic-carbon aquifer material, but that the interaction of TCE with physically condensed carbonaceous components of the aquifer material mediates the transport and fate behavior of TCE for this system.
In a second study, Dr. Brusseau's group used a newly available imaging tool to visualize, quantify, and compare DNAPL-water interfaces in various media. While most DNAPL-dissolution research has been conducted at the column scale, Synchrotron X-ray microtomography allows for the characterization of pore-scale morphology and distribution of TCE during water flushing so the scientists can examine dissolution dynamics. The scientists compared a "poorly sorted: natural medium with a large particle size distribution (Hayhook soil from Pima County, AZ) to an "ideal" medium (well-sorted natural sand). They packed columns with each medium, saturated the media with TCE, and then conducted dissolution studies by flushing the column under induced-gradient conditions to mimic pump-and-treat applications. Synchrotron X-ray images were collected at three test intervals: prior to flushing and after 95 and 180 pore volumes of flushing. The two-dimensional data sets obtained from the imaging were preprocessed and reconstructed to develop three-dimensional image arrays, which were analyzed to determine morphological characteristics of the TCE liquid blobs.
The scientists monitored the concentration of TCE in the effluent to assess mass-flux behavior. Ideal dissolution was observed with the well-sorted material. That is, there was an extended steady-state stage with effluent concentrations equivalent to aqueous solubility of TCE, followed by a transient stage with a rapid decrease in effluent concentration. The poorly sorted medium exhibited non-ideal dissolution, with multiple periods of relatively constant contaminant flux as opposed to the single steady-state stage observed for the well-sorted media (Figure 1). In addition, the dissolution front propagated less uniformly for the poorly-sorted medium than it did for the well-sorted. This indicates that direct contact between primary channels of water flow and organic liquid is constrained by the complex flow field in the poorly sorted material.
Significance: Using standard pump-and-treat systems, remediation of sites contaminated with DNAPLs may take hundreds of years to achieve health-based groundwater cleanup objectives. Dr. Brusseau's goal is to enhance the accuracy of risk assessments and improve the effectiveness of remediation strategies for sites contaminated by chlorinated solvents. Dr. Brusseau's research will contribute to an improved, mechanistic-based understanding of the mass-transfer behavior of DNAPLs, which will enhance the development and application of conceptual and mathematical models.
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To learn more about this research, please refer to the following sources:
- Johnson GR, Norris DK, Brusseau ML. 2009. Mass removal and low-concentration tailing of trichloroethene in freshly-amended, synthetically-aged, and field-contaminated aquifer material. Chemosphere 75(4):542-548. doi:10.1016/j.chemosphere.2008.12.004 PMID:19157496 PMCID:PMC2898734
- Russo AE, Narter M, Brusseau ML. 2009. Characterizing pore-scale dissolution of organic immiscible liquid in a poorly-sorted natural porous medium. Environ Sci Technol 43(15):5671-5678. doi:10.1021/es803158x PMID:19731661 PMCID:PMC3165041
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