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Quarterly Journal of Engineering Geology & Hydrogeology

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Physical and chemical properties of a DNAPL contaminated zone in a sand aquifer

¹ UNSW Groundwater Centre, School of Civil and Environmental Engineering, University of New South Wales, King Street, Manly Vale 2093, Australia

Minimally disturbed cores were recovered from two boreholes 25 m apart in the Botany Sands aquifer in Sydney, Australia. Physical and chemical analyses of the core subsections demonstrate that the formation is comprised predominantly of fine to medium sand from the surface to a depth of approximately 21 m. A 200 mm silty sand layer at 21.6 m depth partially supports a DNAPL (dense non-aqueous phase liquid) pool of chlorinated hydrocarbons. DNAPL has penetrated through this zone into the underlying sands. A more extensive sandy silt layer at 23.4 m has prevented further local penetration of the DNAPL. Free phase DNAPL mixture was recovered from a core subsection immediately above the silty sand layer and from several cores in the sand zone above the sandy silt.

A prominent thin high TDS band is associated with the top of the zone containing DNAPL. This zone forms a sharp bulk electrical conductivity anomaly on geoelectric sections compiled from conductivity cone penetration logs and induction logs carried out in the bores after coring. The anomaly is directly related to fluid conductivity variations, which have been measured on pore fluids derived from the cores. The bulk electrical conductivity (complex) measured at a range (10^–2–10⁵ Hz) of frequencies provides further confirmation of the source of the conductivity anomaly. Lithological detail is provided by grain-size analysis and by radiographs of the cores.

The chemical analysis data carried out on pore fluids indicate that the zones containing free phase DNAPL have low pH (3–4) and sodium to chloride ratios (m mol/l) below 0.20. The inorganic chemical data for 4 cores in the DNAPL contaminated zone is shown graphically and exhibits systematic changes within the section that suggest either strong reverse ion-exchange reactions in an environment where chloride is conservative or, more probably, mineralization of DNAPL.

Key Words: chlorinated hydrocarbons • geophysics • groundwater contamination • hydrochemistry