Natural attenuation of diesel fuel in heavy clay soil

petroleum hydrocarbons in the extractable fraction decreased rapidly in a heavy clay soil contaminated with diesel fuel at 5000 mg kg(-1) demonstrating natural attenuation. Natural attenuation rates for untilled soil were estimated using two distinct extraction and analysis methods, the Oil & Grease Content (O&G) (US EPA 418.1; ASTM D 95) and extractable organics (EO) (EPA 3520B; EPA 8000A). The time to 50% decrease (t(1/2)) in the fraction varied with extraction method, Oil & Grease Content (11 d) and Extractable Organics (26 d). Complete attenuation appeared to have occurred after 74 d as Extractable Organic values for soil extracts were below the detection limit (5 mu g g(-1)). An enzyme-linked immunosorbant assay (ELISA) was also used to monitor the hydrocarbons present in the soil. This also effectively demonstrated the near-complete disappearance of the hydrocarbons but was not quantitative. Soil microbial biomass carbon (chloroform fumigation-extraction) and surface CO2 flux measurements did not show any significant impact of diesel contamination or tillage in the active microbial population. This observation serves to demonstrate the applicability of natural attenuation as a remedial strategy for heavy clay soils contaminated with diesel fuel of the type used in this study.