| Short-term (weeks to months) bench-scale accelerated weathering tests were designed to examine the effects of long-term (decades to centuries) natural attenuation processes on non-aqueous-phase liquid (NAPL) residuals in soils and their impact to groundwater. The weathering processes studied included dissolution, aerobic biodegradation, and volatilization. First-order approximation-based scaling rules were developed to relate bench-scale time frames to field-scale time frames for the dissolution and volatilization processes. Diluent obtained from the former Guadalupe Oil Field in California and a sample of diesel fuel (from a local gas station) thought to be representative of the NAPL release at the New Zealand Aluminium Smelters Ltd. Site in New Zealand were used for testing the bench-scale weathering protocols. Total petroleum hydrocarbons (TPH) in soil, dissolved TPH in leachates, vapor-phase TPH, and specific chemicals of interest (e.g., benzene, toluene, ethylbenezene, xylenes, and naphthalene) were analyzed via gas chromatography to identify the contaminant concentration and composition changes in the residual NAPL and leachates with weathering. When assessing composition changes, the complex petroleum hydrocarbon mixtures were characterized by mass distribution across carbon number ranges, based on retention times of a normal alkane standard. Other compositional change indicators that were evaluated included: (a) the relative changes in concentrations of normal heptadecane (n-C17) and octadecane (n-C 18), and pristane (2,6,10,14-tetramethylpentadecane) and phytane (2,6,10,14-tetramethylhexadecane) as these are suggested biomarkers for biodegradation process progress, and (b) the ratio of distinct and resolvable peak areas to the area of the underlying unresolvable complex mixture, as biodegradation processes often effect the reduction in distinct resolvable peaks in complex petroleum mixtures.; The experimental conditions were chosen to simulate tens to thousands of years of natural weathering under typical field conditions. These studies demonstrate the practicability of conducting these types of tests, and the value of the results to anticipating long-term changes to groundwater quality resulting from source zone natural attenuation processes. |
