Effect of chromium and lead on degradation of South Louisiana crude oil in sediment

Abstract

The effect of chromium (Cr) and lead (Pb) on degradation of South Louisiana Crude oil in sediment collected from a produced water discharge site was measured in laboratory microcosms under oxidized and reduced conditions. Chromium was added to the sediment at rates of 0 to 5,000 μ,g, g‐1 sediment. Lead was added at rates from 0 to 2,500 μ,g, g‐1 sediment. The effect on degradation of alkane petroleum hydrocarbon fractions was determined over time. Metal content had no impact on total hydrocarbon degradation content based on changes in concentration. Even on a percentage basis, which normalizes for differences in hydrocarbon concentrations no statistical influence of Cr (5,000 μ,g, g‐1) and Pb (2,500 μ,g, g‐1) on degradation could be measured under reducing conditions. A more sensitive analysis of metal impact on degradation depicted by an analysis of reduction of selected alkane fractions also showed little effect on degradation. Only degradation of octadecane was shown to be statistically inhibited at 5,000 μ,g g‐1 of added Cr and only under reducing conditions. The sediment, due to the clay content, sequested much of the added metals. Under reducing conditions, 25 percent of water soluble and exchangeable Cr and Pb remained in solution at the highest Pb (2,500 μg, g‐1) and Cr (5,000 μg, g‐1) levels compared to oxidized sediments. The amount in solution was not great enough to impact hydrocarbon degradation. The study demonstrates that Cr and Pb entering sediment at this produced water site would have minimal impact on degradation rates unless extremely high levels were added.