Delineation and weathering of gasoline contamination in three dimensions using multi-level soil and groundwater samples

Date of Completion

January 1997


Geology|Agriculture, Soil Science|Environmental Sciences|Geochemistry




The use of multi-level soil samples to delineate and quantify the weathering of gasoline contamination is evaluated. Gasoline constituents with high effective solubilities (e.g., MTBE and BTEX) were found to be present over a wide range of total soil concentrations, with or without actual gasoline being present, in the soil samples. Conversely, the heavier molecular weight gasoline constituents (e.g., napthalene, propylbenzene, trimethylbenzene) were found to be present in significant concentrations only when gasoline was present in the soil samples. For the soil samples that contained gasoline, the measured contribution from the aqueous and adsorbed phases was less than 10% of the total soil concentration for all of the measured constituents, except MTBE and benzene. The use of equilibrium based aqueous and adsorbed concentrations significantly overestimated the measured non-gasoline contributions to total soil sample concentrations. Evaluation of the gas chromatograms of the soil samples, and the presence and concentrations of the heavier molecular weight compounds, improved the use of soil samples as a method to delineate gasoline.^ The evolution of gasoline contamination is quantified by comparing the proportions of constituents in recovered gasoline to those in the gasoline in spatially distributed soil samples. The relative proportions of constituents in the least weathered gasoline found in soil samples are shown to match the relative proportions in the residual gasoline in a soil sample collected 5 years earlier. However, calculation of the mole fractions of residual gasoline indicate that the losses of unknown constituents increased the absolute concentrations of all of the constituents by approximately a factor of 2 to 3 over the 5 year period. Significant soil concentration losses from the gasoline in the soil are documented for MTBE, benzene, and toluene at the site. An average of 99, 75, and 50% of the total initial mass of MTBE, benzene, and toluene have been lost to weathering. Concentration loss per time estimates range from 0.0164 to 0.1446 mg/kg-day for MTBE and the BTEX constituents. ^