APPLICATION OF PASSIVE SOIL GAS SAMPLING TO DETERMINE THE PRESENCE OF PETROLEUM HYDROCARBONS IN GAS STATION SUBSURFACE: TWO CASES IN THE CENTRAL VALLEY OF COSTA RICA

Passive soil gas sampling is an excellent tool that can identify quickly and directly the presence of volatile organic compounds in the subsurface, both in the unsaturated zone and VOCs volatilizing from groundwater plumes. In the case of fuel dispensing stations, pipelines or refineries, it provides for rapid determination of source areas and contamination plumes originating from hydrocarbon releases, such as diesel and / or gasoline. Volatile organic compounds (VOC) including petroleum hydrocarbons migrate through the vadose zone as a free phase liquid that forms an equilibrium between liquid phase, adsorbed phase, vapor phase and dissolved phase. The vapor phase equilibrium radiates from the source area to the surface where the passive soil gas samplers are installed in order to capture the equilibrium concentration at the time of sampling. Similar phase condition equilibrium is achieved once the non aqueous phase liquid (NAPL) reaches groundwater, however, the vapor phase is only at the surface of the water table. The dissolved phase hydrocarbons generate a plume spreading away from the point of entry into the groundwater and or location of NAPL or residual NAPL but does not always move linearly in the direction of groundwater flow.