Impact of aquifer sediment grain size on petroleum hydrocarbon distribution and biodegradation

A site located in Hanahan, South Carolina, U.S.A., was contaminated in 1975 by JP-4 jet fuel. These coastal plain sediments are comprised of unconsolidated, fine-grained sands, but have interfingering lenses of clay. Heterogeneity in subsurface sediment grain size and contaminant distribution was examined in sediment collected in adjacent boreholes at discrete depth intervals (15-20 cm) in the saturated zone. Sediments were analyzed for JP-4 and for benzene, toluene, ethylbenzene and xylene (BTEX) by GC-MS, and for sediment grain size to determine percentages of sand vs. clay and silt. Spatial heterogeneities in sediment grain size were found on a small vertical scale in adjacent boreholes, and between boreholes separated by 21-m distance, although coastal plain sediments are considered to be homogeneous. Jet fuel contamination was positively correlated with clay content of the sediment. In addition, biodegradation of radiolabeled amino acids, toluene and benzene was measured in laboratory microcosms from sediment collected from contiguous depths from a single borehole in the same area. Biodegradation of the three substrates was less in the predominantly clay sample than that in the sandy sample. Increased sorption and limited biodegradation may contribute to higher concentrations of contaminants in clay sediments.