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

A gasoline release occurred at a site located within a fault system known as the Highlandville Fault System. Initial activities consisted of the installation of trenches and borings in an attempt to intersect and remove Light Non-Aqueous Phase Liquid (LNAPL). No LNAPL was encountered or recovered using these methods. Bedrock is encountered at depths of 18 to 60 feet below grade. A geophysical survey indicated the presence of a significant fracture trending to the Northwest. Subsequent investigation, resulted in the installation of monitoring wells that indicated the presence of wide spread voids rather then a single linear fracture. Dissolved chemicals of concern (COC) concentration data varied over time at many sampling locations and complicated delineation efforts. Based on the wide spread presence of the voids within the bedrock well locations at much greater distances were selected. A series of pressure transducers was installed to collect groundwater elevation data and was instrumental in developing fate and transport concepts related to both dissolved COC and LNAPL distribution. Groundwater elevations were shown to be very dynamic, seldom static and recharge during precipitation events via nearby losing drainages was rapid and dramatic. Groundwater Traditional methods of Light Non-Aqueous Phase Liquid (LNAPL) occurrences were sporadic and traditional methods of recovery were ineffective. Lithologic data indicates the mapped features of the Highlandville Fault System in the area, is more complex than previously documented. 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.