Spatial and Temporal Variability in Hydrocarbon and Oxygen Concentrations beneath a Building above a Shallow NAPL Source

摘要

A field research program is being conducted to study the spatial and temporal variability of oxygen (O2) and total petroleum hydrocarbon (TPH) concentrations beneath and around an unoccupied maintenance warehouse, located at the former Texaco refinery in Casper, Wyoming (Figure 1). Residual light-end petroleum distillate (e.g., gasoline- range components) is present in the soil as a light, non-aqueous phase liquid (LNAPL) beneath the foundation. The interpolated depth to the top of the residual hydrocarbons, as determined by visual evidence (i.e. stained soils) and odors at 31 locations is presented in Figure 2. As can be seen, the depth to the top of the residual hydrocarbons varies between 0.5 - 1.0 ft in the northeast quadrant of the foundation and 4.5 - 5.0 ft below ground surface (bgs) immediately outside the south and southwest edge of the foundation. The installation of a nearby hydraulic barrier and corresponding groundwater pumping has lowered the water table from historic highs near 2 ft bgs, to a current depth of approximately 14 ft bgs. This has created a thick vadose zone interval containing residual LNAPL, which acts as a vapor source. The soil is primarily a mixture of sands and gravels, and the warehouse is built on a slab-on-grade foundation below which there are water and electrical utility lines.