CHALLENGES OF INVESTIGATING AND REMEDIATING A GASOLINE RELEASE IN FAULTED KARST TERRAIN

A gasoline release occurred south of Springfield, Missouri within a fault system known as the Highlandville Fault System (HFS). Initial emergency response activities including the installation of trenches and borings did not encounter Light Non-Aqueous Phase Liquid (LNAPL). Subsequent investigative efforts including the installation of soil borings and monitoring wells encountered LNAPL that could not be recovered using skimming methods. Determining the magnitude and extent of dissolved chemicals of concern (COCs) was also problematic as dissolved COC concentrations at various locations located down-gradient ranged from several milligrams per liter (mg/l) to below method detection limits (MDL) at the same well over a one month period. Based on the wide spread presence of the voids within the bedrock well locations at much greater distances were selected. Pressure transducers were used to collect groundwater elevation data to better understand the dynamics of the groundwater. These data were important in developing fate and transport concepts related to dissolved COC and LNAPL distribution. Groundwater elevations were shown to be very dynamic gaining significantly during precipitation events via nearby losing drainages. Traditional methods of Light Non-Aqueous Phase Liquid (LNAPL) occurrences were sporadic and traditional methods of recovery were ineffective. Lithological data collected during the installation of monitoring wells and borings, indicates an additional previously unmapped Horst is present within the HFS. Pressure transducer data indicated that groundwater elevations are not static and that LNAPL is influenced by groundwater elevations. A vapor extraction system was effective at removing significant petroleum hydrocarbon mass when very specific groundwater elevations were present. Dissolved COC concentrations and distribution varied considerably dependent upon groundwater elevations and groundwater velocity. Poly diffusion bags (PDBs) were selected as a sampling method due to their residence time in aid in detecting rapidly changing concentrations. Interpretation of rock cuttings collected while drilling, drill logs, existing geological maps and groundwater elevation trends were all used to develop a conceptual site model (CSM) of the site and surrounding area.