FluOil: Tracking Sunken Oil in Rivers

When oil spills into a river, most response effort goes to the slick on the surface. But a river's turbulence can break a slick into droplets that mix with sediment or organic debris and sink into the water column, forming oil-particle aggregates (OPA). These travel below the surface, where they are easily missed. FluOil, a U.S. Geological Survey tool, estimates how fast OPA move downstream and when and where they are likely to settle, to help with preparing for, responding to and recovering from spills.

Why sunken oil matters

• How much: no one knows exactly, but anywhere from 1 percent to 10–20 percent or more of the oil in a river spill may submerge, mix with sediment, and be carried many kilometers downstream or deposited on the bed and banks.
• Which oils: most, including light crude. Flow speed and turbulence, and the oil's contact with suspended sediment, matter more than the oil's own properties, and droplets can form immediately after a spill.
• What it threatens: drinking water intakes, sensitive habitat buried under deposits, aquatic life including bottom-dwelling organisms, and long-lasting sheens when deposits break up again.
• It comes back: OPA tend to collect with fine silt and clay in backwaters, oxbows, side channels and pools during low flow, and can be stirred up and moved downstream by the next high flow — so problems may outlast a normal spill response. Where OPA settle in organic-rich sediment, warm weather can release oil and sheen with gas bubbles from anaerobic decay, a process called ebullition.
• Recovery: OPA change how organisms are exposed to oil and how toxic it is, and how fast leftover oil biodegrades — a major factor in how quickly habitat recovers.

So a response may need early monitoring and containment of submerged oil, not just recovery of floating oil.

How oil-particle aggregates form in a river (modified from Fitzpatrick and others, 2015). USGS figure.

Lessons from the Kalamazoo River

In 2010 a pipeline ruptured and spilled diluted bitumen — a thick oil-sands product thinned with gas condensate — into Michigan's Kalamazoo River. Conventional methods recovered the floating oil, but the long cleanup of submerged oil needed many lines of evidence: mapping of river forms and sheens, submerged-oil assessment, flow and sediment modeling, oil chemistry forensics and a net environmental benefit analysis. Ultraviolet epifluorescence microscopy confirmed that OPA had formed.

Laboratory work during and after the cleanup showed OPA take many forms depending on how oil and sediment mix:

• droplet-type, where particles coat the outside of a round oil droplet;
• solid-type, where particles penetrate the oil and form an irregular mass — including flat, disc-like "flaky" particles;
• aggregates of several droplets and particles.

These differences change how OPA travel and settle. OPA can float, stay suspended or sink, and range widely in density, shape and size.

How FluOil works

FluOil is a particle-tracking model that simulates the movement of OPA by advection, diffusion, deposition and resuspension. It is open-source, written in MATLAB, and adapted from FluEgg, a USGS program built to simulate how carp eggs drift and settle.

FluOil's inputs, computation and outputs. USGS figure.

Inputs

• Existing hydraulic data for the river, from sources such as the U.S. Army Corps of Engineers' HEC-RAS models (imported directly) or a spreadsheet. Reaches can include dams and rapids.
• OPA properties set by the user: size, settling velocity (reflecting shape and density) and critical shear stress. Settling velocity controls where OPA sit in the water column; critical shear stress controls where they settle and get picked up again. Because even one oil can form OPA with widely varying properties, users can run a range, drawn from laboratory studies or modeling, to reflect seasonal and flow changes.
• Choices for how turbulence and velocity vary with depth, to suit conditions.

Outputs

• graphs of how OPA are distributed through the water column;
• boxplots of travel times;
• a three-dimensional animation of OPA transport.

A Google Earth utility for mapping suspended and deposited OPA was being restored.

Assumptions and limits

• OPA are assumed to have already formed and not to change as they travel.
• FluOil doesn't adjust flow for frozen conditions.
• No MATLAB license is needed, but users must install the free MATLAB Runtime R2020a (version 9.8), which may need help from IT staff.

Sources

• U.S. Geological Survey: "FluOil—A tool for estimating the transport and deposition of oil-particle aggregates in rivers," Fact Sheet 2025–3055.
• Works cited include Fitzpatrick and others (2015); Li and others (2022); Zhu and others (2022); Waterman and Garcia (2015); Berens and others (2021); Ji and others (2021, 2023); Garcia and others (2013), on FluEgg; Dollhopf and others (2014); U.S. Environmental Protection Agency (2016); and Sortor and others (2023), on a FluOil training dataset for the St. Croix River, Minnesota.