Retention of liquid contaminants in layered soils

Observations of organic contaminant liquids retained in layered soil profiles indicate contaminant saturations much greater than residual, where residual saturation would be expected, if one assumes a static distribution of fluid pressure. It is theorized that the increased retention of fluid is caused by coarse soil layers that reach a very small unsaturated hydraulic conductivity early in the drainage process, and thus prevent further drainage. The results from a retention experiment in a one-dimensional column were used to test this hypothesis. A vertical column was dry packed with a layer of fine sand (high entry pressure head) overlaying a layer of coarse sand (low entry pressure head). The column was saturated with an organic liquid and then drained. The saturation and pressure profiles were monitored over an extended period of time. A dual-gamma system was used to determine the liquid saturation, and specially fitted ring tensiometers were used to measure the capillary pressure at selected points along the column. The pressure readings and saturation profiles showed a high retention of oil in the fine layer above the interface of the two soils. The profiles were compared first with static pressure and corresponding saturation profiles, and secondly with profiles calculated assuming a very small steady flow through the column. The experimental profiles closely resemble those profiles that assume small flow rates, and display a much higher retention above coarse layers than the static profiles.