This August 2020 photo shows the bridge across the Yellowstone River near Tower Junction. The bridge was constructed in the early 1960s and has structural problems that require it to be replaced. The bridge will be removed once the new Yellowstone River Bridge is completed, which is scheduled for 2026. (Jacob Frank/Yellowstone National Park)
As you drive through Yellowstone National Park, have you ever wondered how roads are safely built through and near hydrothermal areas?
While roads are usually not constructed through major thermal features, the hydrothermal activity is so pervasive throughout the park that building roads through and near hydrothermal areas is sometimes unavoidable — like the road that passes near Beryl Spring and proved to be a major engineering challenge, and the road that crosses thermal ground in Lower Geyser Basin and is the source of the “melting roads of Yellowstone” story.
Furthermore, the absence of hot springs and mud pots doesn’t mean that there is not thermal activity just below the surface. This is why bridges require special planning, as they usually involve drilling. Such is the case with the bridge under construction across the Yellowstone River near Tower Junction.
The Yellowstone River Bridge project involves replacing the existing 60-year-old bridge and rerouting part of the Northeast Entrance Road to the intersection of Grand Loop Road. This new, 1,285-foot-long and 175-foot-high steel girder bridge is located within a hydrothermally active zone with multiple gas vents along the river’s edge.
Because of its proximity to thermal activity, the large-drilled shafts (5–10 feet in diameter and 40–60 feet in depth) required sulfate-resistant cement and thermal monitoring of below-grade concrete curing to assure a stable bridge structure. The actual drilling of these large shafts also posed a significant safety risk for the drillers.
Workers drill for the new Yellowstone River Bridge near Tower Junction. A small, yellow, personal 4-gas safety monitor can be seen on the upper left breast pocket on the person in the far right of the picture. (Ken Sims/University of Wyoming).
A particular concern related to the drilling was hydrogen sulfide gas — a toxic gas often associated with Yellowstone’s hydrothermal systems. The gas is first noticeable to humans at 0.01–1.5 parts per million, and it has a faint rotten egg smell. At higher concentrations, the gas is odorless and extremely dangerous.
Prolonged exposure, up to an hour or more, to concentrations between 10 and 50 parts per million can cause nausea, headaches, fatigue, dizziness and eye and respiratory tract irritation. Concentrations between 400 and 700 parts per million can cause unconsciousness within five minutes and death if exposure is not reduced within 30–60 minutes, and concentrations above 1,000 parts per million can cause death in minutes.
The hazard is not inconsequential and is exemplified by an accident that occurred in this same location on June 26, 1939. While building an earlier bridge across the Yellowstone River, three Bureau of Public Roads employees were conducting a routine test pit excavation when hydrogen sulfide gas overwhelmed two of the workers in the pit. The two victims were eventually rescued, but unfortunately, one worker died the following day.
To determine if the modern-day drilling was impinging on the adjacent local hydrothermal system, geologists from the University of Wyoming measured and recorded temperature and pH variations in groundwater and “drill-spoils” (the dirt and rock removed from the drill holes), as well as changes in groundwater electrical conductivity at specified time and depth intervals.
They also monitored gas concentrations to help ensure a safe work environment. This monitoring involved equipping each worker with a personal hydrogen sulfide gas sensor to continuously monitor gas levels around the drill site. In the event of an accidental and hazardous gas exposure, an oxygen supply and full protective gear were on site to ensure a fast and effective response.
To establish a decision tree in response to a hazardous event, a Trigger Action Response Plan was implemented to address any geologic hazards or worker risks that may occur during drilling operations. If hazardous conditions were detected, the plan outlined three levels of response.
At Trigger Level 1, which indicates elevated temperatures, more acidic conditions in the soil and water, and levels of hydrogen sulfide gas up 10 parts per million, monitoring intervals would be shortened.
At Trigger Level 2, which indicates that even higher temperatures, higher acidity, and higher levels of the gas (~15 parts per million) were detected in the drill shafts, the Trigger Action Response Plan officer would be notified immediately, and monitoring would be conducted even more frequently. The officer would also notify project managers and Yellowstone National Park officials of a Trigger Level 2 event.
Even higher temperatures, soil and water acidity, and hydrogen sulfide gas concentrations (greater than 20 parts per million), would initiate a Trigger Level 3 causing a work stoppage and immediate evacuation of the area until conditions were deemed safe.
Lastly, each day’s activities were documented with detailed daily records of the drilling activities and a final drilling log for each of the 20 drilled shafts as part of the project.
The drilling phase of the Yellowstone River Bridge project was completed in September 2023 without incident. Potential hazards were mitigated through careful planning, monitoring of geologic conditions, and implementation of a Trigger Action Response Plan. Construction is above ground is ongoing, and the bridge is scheduled to be completed in the fall of 2026.
In this Aug. 4, 2023 photos, workers pour sulfate-resistant cement for one of the bridge footers after a drill hole has been completed. The existing bridge can be seen through the trees on the left side of the picture. The black tarps surrounding the site are designed to prevent drilling sludge from escaping the site and ending up in the Yellowstone River. (Ken Sims/University of Wyoming)
Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory.
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