Hydrocarbon Spills, Their Retention in the Subsurface and Propagation into Shallow Aquifers

Abstract

The two most important phases in the propagation of a hydrocarbon spill into the subsurface occur in the vadose or unsaturated zone, between the land surface and the water table, and in the groundwater flow zone at its contact with the free oil phase. The last determines the dissolution and spreading of contaminants through the most mobile flow zone. The first determines the spill geometry, the amount of free phase oil that reaches the groundwater body and the total retention of spilled oil. Oil retention studies carried out for this project show whether the volume of soil within a spill site is sufficient to retain the oil without it escaping into the groundwater and also gives a rough approximation of how' long it takes for the oil phase to reach pendular saturation. Grain size and sorting distribution and are useful factors governing retention, and are useful parameters in evaluating the potential damage of an oil spill and the immediate danger of groundwater contamination. Centrifuge air-oil-water displacement provides a quick and first order approximate means to measure site-specific oil and water retention within the unsaturated zone of a porous media. Results show a semi log relationship between total primary retention and grain size (in phi units) of laboratory samples with oil-water-air systems run under 38 g in the centrifuge. A correction factor adjusts retentions to gravity drainage column pack saturations after gravity drainage used as a standard of comparison. Field samples from an oil spill site assume the same relationship when plotted against effective particle size versus total primary retention. Effective particle size has a finer resolution than mean grain size because it emphasizes the role of the fine fraction and sorting characteristics that tend to increase retention capacity.