The EPA has ordered additional dredging at three locations shown in red (Source: USEPA)

On July 25 2010, the 30-inch diameter Enbridge 6B pipeline ruptured near Marshall, Michigan. Roughly 1 million gallons of diluted bitumen (DilBit) from Canadian oil sands spilled into Talmadge Creek, a tributary to the Kalamazoo River. Nearly three years later, the cleanup continues.

Last week, the U.S. EPA issued a final Administrative Order requiring Enbridge, the owner of the pipeline, to conduct additional dredging to remove submerged oil from three portions of the Kalamazoo river and from sediment traps installed after the spill. EPA’s Cover letter and supporting documents are available here.

Rationale for Additional Dredging
Briefly, the EPA’s rationale for more dredging comes from data collected after extensive and ongoing monitoring of oil sheen and sludge. Analytical chemistry has identified the ruptured Enbridge 6B pipeline as the source. The dredging will remove sludge, a mixture of oil, sediment, and organic matter, from the river bottom. Oil contained in the sludge is located using a simple yet effective tool. “Poling” consists of using a long stick with a disk fastened to the end to manually agitate bottom sediments. Agitation mobilizes oil that floats to the surface as sheen and globules. Enbridge and its team created a standardized procedure of poling to describe “heavy,” “moderate” and “light” concentrations of submerged oil.

Results of the poling identified three areas of submerged oil accumulation, as shown on the above map (click for larger view). The three areas are located in segments of the river with relatively low flow velocities, consistent with a hydrodynamic model of the river’s variable current along its length. Sampling over the past year has shown that the oil is not stationary, however, but is slowly moving downstream and into portions of Morrow Lake.

Weak Natural Attenuation and Net Environmental Benefit Analysis
An EPA bench study found that potential biodegradation of the oil in the Kalamazoo River, under optimum conditions, is only about 25 percent. Cold, dark benthic conditions make for slow decomposition. This undoubtedly played a major part in the decision to order additional dredging. Most cleanup operations inflict some kind of negative impact on the environment. Accessing the contamination site, staging and operating equipment, disturbing sediments and aquatic habitat are inevitable, although they can be minimized through careful planning and procedures. These impacts were weighed against potential cleanup benefits in a Net Environmental Benefit Analysis.

Concerns over DilBit
Tar Sands Pipelines Safety Risks, a 2011 joint report by the Natural Resources Defense Council, National Wildlife Federation, Sierra Club and the Pipeline Safety Trust, raises concerns over the chemical and physical properties of DilBit. The diluents added to reduce the viscosity of tar sand crude in pipelines contain benzene and polycyclic aromatic hydrocarbons. Benzene is a carcinogen and levels found in the air following the Enbridge 6B discharge prompted a voluntary evacuation of nearby homes.

In what is called a “column separation,” the diluents can pass from the liquid to gas phase under certain temperature and pressure conditions, forming bubbles in the pipeline. It has been reported by the Chicago Tribune that Enbridge pipeline operators mistakenly interpreted the pressure changes from the pipeline rupture for one of these bubbles. Rather than shutting it down, twice they tried to restart the line, which , according to the report, allowed DilBit to leak into Talmadge Creek unabated for 17 hours.