Abt Global’s staff supported the state of Montana and co-trustees on the July 2011 Yellowstone River oil spill Natural Resource Damage Assessment (NRDA). As a part of this work, we characterized the movement of the oil and resulting extent of the contamination by sampling sediment, soil, and debris – such as decomposed plant material – downstream of the spill site. The analysis contributed to a settlement of the case by state of Montana and co-Trustees and a proposed restoration plan.

Abt staff analyzed the collected samples for oil compounds that are toxic, in particular polycyclic aromatic hydrocarbons (PAHs), and used the pattern of PAHs in the samples to confirm the oil’s pathway and exposed resources. We further analyzed the ratio of particular individual PAHs to distinguish the oil PAHs from naturally occurring background PAHs.

The relative amount of individual PAHs present in a sample varies depending upon the source – for example, the type of oil, or shale – and it changes over time with weathering. However, some PAHs weather at the same rate. Therefore, the ratio of particular PAHs can be used as a fingerprint to trace back spilled oil to the original source, and to differentiate between different sources.

We used known relationships between two specific types of PAHs, dibenzothiophenes and phenanthrenes, to fingerprint the spilled oil and distinguish it from background sources of PAHs. Specifically, we compared the ratio of C2-Dibenzothiophenes/C2-Phenanthrenes to C3-Dibenzothiophenes/C3-Phenanthrenes – a ratio that remains constant and does not change with weathering – to confirm the source of PAHs measured in the collected samples was from the oil, and not naturally occurring background shale.

Read the Yellowstone Damage Assessment and Restoration Plan