Editor's note: Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Michael Loya, graduate student, and Ken Sims, Professor of Geology and Geophysics, both at the University of Wyoming.

Building roads through Yellowstone is a considerable undertaking given all the thermal ground found throughout the park. Bridges, like the new span across the Yellowstone River near Tower Junction, are especially challenging.

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 currently 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 (https://highways.dot.gov/federal-lands/projects/wy/nps-yell-12-2). 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.

A particular concern related to the drilling was hydrogen sulfide gas (H2S)—a toxic gas often associated with Yellowstone’s hydrothermal systems. H2S is first noticeable to humans at 0.01–1.5 parts per million (ppm), and it has a faint rotten egg smell. At higher concentrations, H2S is odorless and extremely dangerous. Prolonged exposure, up to an hour or more, to concentrations between 10 and 50 ppm can cause nausea, headaches, fatigue, dizziness, and eye and respiratory tract irritation. Concentrations between 400 and 700 ppm can cause unconsciousness within five minutes and death if exposure is not reduced within 30–60 minutes, and concentrations above 1,000 ppm 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 H2S 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 H2S gas sensor to continuously monitor H2S 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 (TARP) 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 H2S gas up 10 ppm, monitoring intervals would be shortened. At Trigger Level 2, which indicates that even higher temperatures, higher acidity, and higher levels of H2S (~15 ppm) were detected in the drill shafts, the TARP officer would be notified immediately, and monitoring would be conducted even more frequently.

The TARP 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 H2S concentrations (greater than 20 ppm), 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 twenty 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.