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Description
Two surfactants, Witconol SN-70 and Igepal CO-530, were evaluated to determine whether they may be used to enhance remediation for non-aqueous phase contaminants such as diesel fuel. Because previous studies have shown that surfactants may actually decrease the hydraulic conductivity of soils (particularly finer-grained soils), various laboratory tests were performed to determine whether the two surfactants actually do decrease the hydraulic conductivity of soils and when it would be worthwhile to use them. Two different diesel phases (dissolved-phase and pure product phase) were evaluated and compared to each other using the multi-phase flow version of Darcy's Law. This evaluation was accomplished by observing when the mobility of the dissolved-phase (with surfactants) of diesel fuel is greater than or less than the mobility of the pure-product phase of diesel fuel. Beaker tests were performed to determine the density and viscosity of the dissolved phase of diesel in various surfactant-water solutions. Falling-head permeameters were used to determine the saturated hydraulic conductivities of four different soil types (SW, SM with 15% fines content, SM with 30% fines content, and SC) at various surfactant concentrations (0.5, 1.0, 1.5, and 2.0% by volume). Results indicated that the two surfactants, particularly Igepal CO-530, decrease the saturated hydraulic conductivity (with an increase in surfactant concentration) of the four soil types, especially for the finer-grained soils. Studies were also performed to determine whether the surfactants also affect the intrinsic permeability of the four soil types. Results indicate that the intrinsic permeability of the four soil types, particularly the finer-grained soils, also decreases with an increase in surfactant concentration, especially for Igepal CO-530. For the pure-product phase of diesel, pressure plate extractors were used to determine the soil characteristic curves for the four soil types. The results were inputted into to the computer programs SOHYP and OILSAT to determine the relative permeability as a function of oil saturation for each soil type. When evaluating the dissolved-phase of diesel, it is clear that the mobility decreases with an increase in surfactant concentration. This is true for Igepal CO-530 when dealing with both coarse-grained and fine-grained soils, which is why it is not recommended to use this surfactant, or similar ones, in remediation enhancement. For Witconol SN-70 though, the mobility only decreases with the finer-grained soils (SM30 and SC soil types). For the coarser-grained soils (SW and SM15 soil types), the dissolved-phase mobility for Witconol SN-70 actually increases with an increase in surfactant concentration. As a result, Witconol SN-70 may be useful in remediation only when dealing with soils containing up to about 15% fines content. Witconol SN-70 was found to be useful (such that it's mobility is greater than the pure-product phase mobility) when monitoring wells contain hydrocarbon thicknesses ranging from less than 96 to 115 cm (for the SM15 and SW soils, respectively). This indicates that it would be best to first conduct free-product skimming in the well(s) to reduce the hydrocarbons down to residual levels. Thereafter, Witconol SN-70, or similar surfactants, may be introduced into the remediation approach to expedite mitigation efforts, since it has been shown that certain surfactants increase hydrocarbon solubilities.
