In situ bioventing of the vadose zone of multi-scale heterogeneous soils

Abstract

A bioventing pilot test was performed on one cell of the vadose zone of a low permeability and fractured site polluted by jet fuel. Air was injected and extracted through hydraulic fractures that were installed to enhance effectively the horizontal permeability of the soil. The biodegradability of pollutants was evaluated with lab-scale studies in batch reactors and soil columns, respectively. To assess the process efficiency, soil samples were collected from seven depths of twelve wells of the pilot Cell, the hydrocarbons were extracted with dichloromethane, and analyzed with gas chromatography/mass spectroscopy and gas chromatography/flame ionization detection. Data obtained from the complete characterization of the pore structure of the soil were employed to understand the main mechanisms of pollutant removal. Due to the heterogeneous nature of the porous matrix over a broad range of pore sizes (from 1 nm to 1 mm), the rate of intrinsic biodegradation of jet fuel in matrix was limited. The main pollutant removal mechanism was the ventilation in which most volatile hydrocarbons vaporized and diffused toward the gas phase that was flowing along the preferential flow pathways. Hydraulic fractures facilitated and accelerated the pollutant remediation rate by enhancing the interconnectivity of the porous matrix with natural fracture systems or any other type of natural preferential flow pathway.