Abstract
Heavy hydrocarbons are a heterogeneous mixture of compounds consisting mainly of alkylated cyclics, resins and asphaltenes and, depending on the source, can form a significant proportion of crude oil. Their prevalence is expected to increase in the future as heavy oil reserves are increasingly exploited for growing worldwide energy demands. Despite their growing use, heavy hydrocarbons are generally overlooked when assessing the risk of hydrocarbons to human health, ecology and water reserves. Although their human and environmental health risks are considered low, heavy hydrocarbons are known to persist in the environment. This review considers the fate, transport and toxicity of heavy hydrocarbons. It provides a description of the possible mechanisms involved in heavy hydrocarbon attenuation and offers some interpretation of data that provides insight into their persistence in the environment. Supplementary material: The effect of hydrocarbon concentration and soil organic content on the partitioning of heavy hydrocarbon (EC >44) into soils is available at https://doi.org/10.6084/m9.figshare.c.3791473 .
Highlights
The conceptual diagram is split into three stages that describe transport and transformation processes: (1) initial surface release of crude oil and infiltration into the unsaturated zone; (2) partitioning of a non-aqueous-phase liquid (NAPL) body near the water table; (3) attenuation of oil resulting in progressive increase in proportion of heavy hydrocarbons and mass subsequent removal of heavy hydrocarbons by biodegradation
The unintentional release of heavy hydrocarbons will result in soil contamination and, depending on the volume released, the formation of free phase NAPL
Field evidence shows that biodegradation is dependent on the prevailing environmental conditions and that heavy hydrocarbon contamination can persist over long time periods in the subsurface
Summary
Crude oil is a complex mixture of hydrocarbon- and nonhydrocarbon-containing constituents. For the purposes of this review heavy hydrocarbons are defined as the fraction that cannot be distilled under atmospheric and vacuum distillation (McMillen et al 2001). This is equivalent to starting boiling temperature of c. As boiling points are dependent on molecular structure, chemical composition and molecular weight, hydrocarbons with a range of carbon numbers will constitute the starting point that defines the heavy hydrocarbon fraction. A survey of 41 crudes by McMillen et al (2001) found that heavy hydrocarbons ranged between 0.6 and 57% and had a mean value of 23%. This broadly agrees with work by Coleman et al (1978), who showed heavy hydrocarbon to be 0 – 70% in an assessment of over 800 US crudes
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