Investigating the Potential for Colloid- and Organic Matter-Facilitated Transport of Polycyclic Aromatic Hydrocarbons in Crude Oil-Contaminated Ground Water

By Joseph N. Ryan, George R. Aiken, Debera A. Backhus, Karen G. Villholth, and Christine M. Hawley

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Abstract

The potential for colloid- and organic matter-facilitated transport of polycyclic aromatic hydrocarbons (PAHs) was investigated at a crude oil-contaminated field site near Bemidji, Minnesota. Field tests focused on sampling and characterization of the ground water colloids and assessment of the partitioning of PAHs to the colloids. The colloids were mostly iron-rich spheres of 100-300 nm diameter. Their concentration reached a maximum of about 2x10¹⁰ particles per liter (0.75 NTU) at a distance 50 m down-gradient from the oil spill and decreased with distance to a background concentration of about 7x10⁸ particles per liter at a distance of 140 m. During well purging, stable colloid concentrations decreased slightly with increasing pumping rate. Fluorescence quenching experiments revealed that partitioning of perylene to organic matter in the unaltered ground water was significant. Laboratory tests examined the effect of ferrous iron, present in high concentrations near the oil spill, on the partitioning of phenanthrene to aliphatic and aromatic carboxylic acids typical of crude oil degradation and hydrophobic acid fractions of natural organic matter. Ferrous iron increased the partitioning of phenanthrene to the organic acids by 11 to 250%, with the largest increase occurring for aromatic carboxylic acids. The data suggest that ferrous iron increased the partition coefficient by causing aggregation of the organic acids.