THE NATURAL ATTENUATION OF A PETROLEUM CONTAMINANT PLUME IN GROUNDWATER OF THE SOUTHERN CUMBERLAND PLATEAU OF TENNESSEE

The study area is located on the southern Cumberland Plateau in Sewanee, Tennessee. Here, shallow, sandy loam soils (average of 1m thick) overlie lower-Pennsylvanian quartz arenites and conglomerates. In 1996, a leaking underground gasoline storage tank was excavated from the grounds of the University of the South in Sewanee. No remediation efforts were made besides removal of soil immediately surrounding the tank. A hydrocarbon contaminant plume was discovered in the groundwater that extended from the tank to a small stream 150m away. Subsequent installation of shallow monitoring wells by students revealed concentrations of total BTEX (benzene, toluene, ethylbenzene, and xylene) up to 30,000 parts per billion (ppb) in some wells. Movement of BTEX down-gradient was controlled by linear depressions in the bedrock surface. Two subsequent student studies were conducted in 2004 and 2012. In 2004 the BTEX concentrations had decreased by natural attenuation to a maximum of 13,000 ppb per well and the plume had separated into two lobes. In 2012 the research revealed that the petroleum plume had attenuated further, with a decrease in hydrocarbon concentrations. In 2018, water samples were again collected and analyzed for BTEX. All samples were free of any detectable contaminants, signaling the complete disappearance of the contaminant plume within 22 years since removal of the contaminant source. This is the first study to chronicle the complete life cycle of a petroleum groundwater contaminant plume by natural attenuation on the Cumberland Plateau of Tennessee.