The presence of dissolved-phase plumes of chlorinated solvents in many US aquifers was documented by the Council on Environmental Quality in 1981. Mackay and Cherry summarized the evidence that these plumes were derived from the dissolution and volatilization of chlorinated solvents and other dense, non-aqueous phase liquids (DNAPLs) present, but not necessarily observed, in sediment and rock beneath spill sites.
To understand the problem of chlorinated-solvent remediation first requires some familiarity with the basic principles of the physics of multiphase flow in porous media. However, such familiarity is insufficient to explain the difficulty that hydrogeologists have had in locating the subsurface DNAPL sources generating the TCE plumes. The difficulty is due largely to the heterogeneity of typical alluvial systems. This heterogeneity causes DNAPL migration and trapping patterns that are complex in geometry and expensive to characterize. On the other hand, the aqueous plumes of TCE, which are generated by these zones of trapped DNAPL, are much easier to detect and map.
Up to 1989, the standard remedy for groundwater contamination was pump and treat. Then the landmark paper by MacKay and Cherry announced that there were persisting sources of contamination that were being ineffectively remediated by pump and treat. These sources were undissolved organic contaminants that had become trapped in the subsurface due to capillary forces, and this type of contamination was given the name Non-Aqueous Phase Liquids (NAPLs). Essentially, it was recognized that the removal of NAPLs by pump and treat was limited by the dissolution of this separate phase of contamination into the groundwater. Many of these NAPLs have low aqueous solubilities; therefore the dissolution process is a slow one. Thus began the search for a method to overcome this mass transfer limitation posed by NAPLs. One of the remedies sought was chemical enhancements to pump and treat.
Surfactant Enhanced Aquifer Remediation (SEAR), in its most basic form, could thus be considered a chemical enhancement to pump and treat. A chemical solution is pumped across a contaminated zone by introduction at an injection point and removal from an extraction point. To cover the entire contaminated zone, a number of injection and extraction wells are used; the well configuration is determined by the subsurface distribution of NAPL and the hydrogeologic properties of the aquifer.
SEAR is a source zone remediation technology. SEAR removes the residual phase contamination from which the dissolved phase plume is derived. Free phase contamination is typically removed by conventional pumping before SEAR is employed. SEAR does not have an immediate effect on the dissolved phase plume concentrations and is not a dissolved phase plume remediation technology. Removal of the source does however cause an intermediate and long-term reduction in dissolved phase contaminant concentrations.
Surfactants are unique chemical agents that greatly enhance the solubility of organic contaminants in aqueous media. They are also able to reduce the interfacial tension (IFT - that force existing where two fluids meet that keeps them as separate fluids) between the aqueous and organic phases to mobilize the organic phase. To illustrate the two mechanisms, we can use the familiar examples of the cleaning action of household cleaning detergents, which contain surfactants as a common constituent. We have witnessed surfactant-induced solubilization in the oily solution resulting from soaking oily pots and pans in dish detergent; we have observed a reduction in IFT from oil droplets or a sheen of oil coming off the pan due to the presence of a detergent. A surfactant flood can be designed to remove contaminants either primarily by solubilization or primarily by mobilization. Surfactant mobilization can remove more DNAPL in less time; however, there is greater risk of uncontrolled downward movement of DNAPL, as DNAPL is being physically displaced by the surfactant solution. Thus, to conduct a mobilization flood, it is necessary to have an aquitard as a barrier to prevent vertical DNAPL migration. It is important to identify from the outset whether solubilization or mobilization of DNAPL is desired, because not all surfactants can be used to conduct a mobilization flood.
The primary objective in SEAR design is to remove the maximum amount of contaminant with a minimum amount of chemicals and in minimal time while maintaining hydraulic control over the injected chemicals and contaminant. Each step in the design process must keep this in mind. Design challenges include precisely locating the DNAPL, finding the optimum surfactant solution for a given DNAPL composition and soil type, and fully characterizing the hydraulic properties of the aquifer, particularly the heterogeneities typically present in the subsurface environment. Because it is impossible to know with certainty the variations in aquifer properties over the treatment zone, numerical modeling tools are used to simulate how the system may respond in the presence of these unknown factors. Numerical modeling is also necessary to understand the dynamics of the flooding process under the hydrogeologic conditions at the site. SEAR has been acknowledged to be a promising, innovative technology for the removal of DNAPLs primarily because of the history of the use of surfactant-enhanced oil recovery by the petroleum industry.
Gary A. Pope
Center for Petroleum and Geosystems Engineering
1 University Station C0304
The University of Texas at Austin
Austin, Texas 78712-0228
Phone: (512) 471-3235 FAX: (512) 471-9605