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Polycyclic Aromatic Hydrocarbons in Nigerian Oil-based Drill-cuttings： Evidence of Petrogenic and Pyrogenic Effects

Compositional distribution and sources of polycyclic aromatic hydrocarbons (PAHs) in Nigerian oil-based drill-cuttings have been studied for evidence of petrogenic and/or pyrogenic effects with recommendations for the strict enforcement of the ban on the use of oil-based mud in drilling by government regulatory agencies and prior bio-treatment of the drill cuttings before final fate. Laboratory analysis of the drill cuttings with a Gas Chromatogram Model 6890N equipped with a flame ionization detector showed that the total initial PAHs concentration of the drill-cuttings was 223.52mg/kg while the initial individual PAHs concentrations ranged from 1.67 to 70.7 mg/kg, dry weight, with a 90% predominance of the combustion-specific 3-ring PAHs. The ratio of anthracene to (phenanthrene + anthracene) = 0.31 was found to be greater than the 0.10 pyrogenic source-criterion for PAHs fraction of molar-mass 178g/mol while the ratio of fluoranthene to (pyrene + fluoranthene) = 0.40 was found to be less than the 0.50 petrogenic source-criterion for fraction of molar-mass 202g/mol suggesting that the PAHs were from pyrogenic and petrogenic sources. Pyrogenic PAHs might be from a possible local heat effect while petrogenic PAHs might be from anthropogenic petroleum fractions in proprietary oil-based mud used in drilling. Consequently, the drill cuttings were composted by homogenizing 4000g of the drill cuttings, 920g of top-soil and 154g of saw dust and poultry droppings in a 10-litre plastic container of very low thermal conductivity in the ratio of 26:6:1. Mixing and watering of the reactors were done at 3 days interval under ambient temperature of 30°C for 42 days. After the bioremediation exercise, the total PAHs remaining in the drill cuttings were 4.25mg/kg (representing a 98.1% reduction). The half-lives of the PAHs in the medium ranged from 8 to 43 days.