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EGLE's Air Quality Division provides update on Marathon incident during polar vortex

August 12, 2019

Dead Leg example illustration

In late January and early February of 2019, Michigan went through a weather event called a polar vortex. During the polar vortex, temperatures dropped below zero for several days. At that time, the Marathon Petroleum Company, LP, Michigan Refining Division (Marathon) located at 1300 South Fort Street, in Detroit, experienced a series of events resulting in unanticipated releases of air pollutants. The releases caused a significant number of citizen complaints to be relayed to the Michigan Department of Environment, Great Lakes, and Energy (EGLE), Air Quality Division (AQD). Complainants in communities located mainly downwind of Marathon expressed concern about strong odors and possible health effects.

Incident Details 

The AQD initially focused its investigation on a damaged coker flare system at Marathon. Upon further investigation, a broken pipe discovered at Marathon's neighboring gas concentration unit was determined to be responsible for the bulk of emissions released during the event.

Because of the quantity and nature of the complaints, air monitoring data collected by Marathon, the AQD, and the United States Environmental Protection Agency (USEPA) in the vicinity of the facility were examined. Monitoring equipment showed air quality standards were not exceeded. However, the air monitoring did not rule out the potential for odor impacts caused by sulfur compounds at ambient (outdoor) air concentrations near or below monitoring limits of detection. It is also possible for people to experience significant discomfort and nausea at levels below established standards.

Gas Concentration Unit Pipe Failure

On January 30, process units providing steam to Marathon were shut down due to cold weather freeze-ups. During normal operations, this steam is used to heat gas lines and provide adequate flow through the system. The shutdown caused a "dead leg" or low point in a pipe to accumulate water and eventually freeze (see Figure 1).

Sometime on February 2, water in a 2-inch pipe froze and caused the pipe to burst. From 12:40 p.m. to 5 p.m., a leak occurred from this pipe. It was determined that during the initial phase of the leak, approximately 96,000 pounds (lbs.) of hydrocarbon gas were released to the atmosphere, of which the majority was found to be propylene (a by-product of petroleum refining, which may also be used as a fuel), with about 20 lbs. of odorous hydrogen sulfide (H2S) co-emitted.

The H2S emitted from the broken pipe was the likely cause of odors in residential areas in the vicinity of Marathon. The odor threshold for H2S is very low, meaning at even low concentrations the odor from this gas can be noticeable, offensive, or cause odor-related health effects, such as those reported by community members. 

Coker Flare Failure

At around 1 p.m. on February 2, Marathon's flare gas recovery system was shut down as a precaution. When this happens, the gas is sent to the coker flare to be burned off. A plugged drain line in the coker flare system increased the pressure in the flare's molecular seal, which is a safety device. It was later determined that the manufacturer had installed a defective plug in the molecular seal. The pressure increased on the defective seal, and volume of gas routed to the flare, resulted in plug failure and a breach of the molecular seal. Marathon estimated approximately 138 lbs. of volatile organic compounds (VOC), 0.7 lbs. of H2S, and 3.5 lbs. of odorous mercaptans were released from the molecular seal breach. At the same time, Marathon estimates the flare itself emitted 55 lbs. of VOCs and 11 lbs. of sulfur dioxide (from the burning of H2S and mercaptans).

Modeling of Release Impacts 

Barr Engineering was hired by Marathon to perform dispersion modeling of the event's releases. This makes use of a computer model incorporating factors such as weather, emission types and quantities, and terrain, to determine plausible locations and amounts of impact. As previously mentioned, the monitoring immediately following the incident did not identify any exceedances of a health-based screening level. This modeling, being more conservative, identified the potential for a small area just outside the refinery fence line to be above the propylene short-term acute screening level. This screening level is based on an 8-hour average and is designed to be protective of even the most sensitive groups. Although propylene does have an odor, it is unlikely to have generated the community complaints or the health effects noted. The EGLE AQD screening levels are specifically designed for the AQD air permitting process. However, they may be used to evaluate air monitoring/modeling results as well, but this is not their primary purpose.

The incident also resulted in H2S concentrations exceeding the odor threshold (0.7 µg/m3)in a significant area downwind of Marathon. The exceedance of the H2S odor threshold is the most likely explanation for the amount and nature of the complaints received during this incident. Figure 2 shows a map of the area in which the odor threshold was likely to be exceeded according to the model, and the actual complaints received. Although the areas do not line up precisely, there is an evident link between the projected and actual areas of impact.Figure 2: Marathon Incident Odor Complaint Map

Next Steps

After the incident, it was clear more needed to be done to ensure a set of circumstances like this does not happen again, and to keep the community apprised of changing situations. The AQD met with the USEPA, Detroit’s Local Emergency Planning Committee, Emergency Management, the local health department, and Environmental Affairs at the Detroit Emergency Operations Center to devise ways to better communicate with community members in the future.

The primary outcome of this meeting was to further define the public notification process when incidents like this occur. The notification system is implemented by the Detroit Police and Fire Communications Department. Depending on the communications need, the Emergency Alert System (pushed to TV, radio, cable, and satellite services) and Wireless Emergency Alerts (pushed to a mobile device) will be activated. Residents are encouraged to sign up for Nixle, which is an opt-in text and email system for alerts.

The AQD continues to closely inspect and monitor operations at Marathon, as well as to continuously examine air monitoring data. Available air monitoring data can be found at Additionally, the AQD has initiated an enforcement action against Marathon that will include provisions to minimize the probability that a similar incident will occur in the future. More information on AQD's interactions with Marathon can be found on the Michigan EGLE Web Page.

Contact Jay Olaguer, AQD Assistant Division Director, with questions at

Complaints may be directed to the Detroit District Office at 313-456-4700.

Pollution Emergencies may be reported to 1-800-292-4706.

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