Residual NAPL saturations in unsaturated soils for two- and three-fluid phase systems

At contaminated sites throughout the U.S. recent risk-based analyses have shown that acceptable levels of crude oils in soil, as measured by TPH (total petroleum hydrocarbons), will often be limited by the mobility of the oil in the unsaturated zone, i.e., the level at which oil will not migrate as a free phase to ground water. As a result it has become important to better quantify residual oil contents and to determine the effect of oil and media type on residual levels.;For this research residual oil contents were first measured in gravity drainage experiments for two-fluid phase systems (oil and air) using vertical columns. Measurements were made for four oils: dodecane (a model oil), two crude oils, and a gas condensate in four different porous media, 250-300 microm glass beads (a model media), a fine-to-medium grain size Ottawa sand, a fine-to-coarse grain size Ottawa sand, and a silty loam soil. Residual oil content measurements (corrected for volatilization) for the various oils were in the range of 0.012 to 0.019 gm-oil/gm-soil for glass beads, 0.019 to 0.056 mum-oil/gm-soil for the fine-to-coarse Ottawa sand, 0.016 to 0.038 gm-oil/gm-soil for fine-to-medium Ottawa sand, and 0.09 to 0.10 gm-oil/gm-soil for the silty loam soil.;Residual oil contents were then measured for three-fluid phase systems (oil-air-water). Measurements were made using the same glass beads and Ottawa sands that were used in the two-fluid phase experiments. Experiments were conducted with the same oils, representing a range of spreading coefficients (based on individual measurements of interfacial tensions for oil-air, oil-water, and water-air). The residual oil contents for the three-phase experiments were lower for the spreading oils (crude oils) compared to the two-phase experimental data. Higher oil contents were observed for dodecane (non-spreading) for both the Ottawa sands. These results are consistent with limited literature data that indicate that residual oil contents are lower for spreading oils in fractionally water wet systems.;In the final set of experiments, residual oil contents were measured by centrifugation. The centrifugation experiments were conducted for a range of bond numbers for selected oils and media for both two- and three-fluid phase systems. In general the oil contents were lower for the centrifugation experiments compared to gravity drainage experiments. These data demonstrate that the impact of centrifugation on capillary desaturation needs to be considered when trying to estimate residual oil contents for the more environmentally relevant gravity drainage condition.