On April 20, 2016, a gasoline tanker truck towing a pup trailer was traveling westbound on Montana Highway 10 east of Alberton in Missoula County, Montana. At a bend in the road near mile marker two, the wheels of the pup trailer dropped off the paved roadway onto a soft shoulder, causing the pup trailer to break free and tumble down an embankment. The trailer came to a rest wheels-up, the tank cantilevered over the edge of a small cliff. The tank was breached in the accident and approximately 2,200 gallons of gasoline were released.
Initial response crews included the local volunteer fire department and towing companies from Missoula, Montana. Olympus Technical Services was contacted to provide air monitoring to ensure that explosive atmospheric conditions were avoided to protect the health and safety of the responders. We also helped transfer gasoline that remained in the wrecked trailer to another tanker.
Following initial response operations and extraction of the wrecked pup trailer, Olympus was contracted to provide remediation services at the site. We sought and received an access agreement with the landowner, filed for an encroachment permit with the Montana Department of Transportation, ordered the location of buried utilities, and contracted for traffic control as needed.
Olympus began excavating impacted soil on April 25, 2016. An abandoned railroad right-of-way was at the base of the cliff where the trailer came to a stop. Excavation began at that lower elevation, guided by visual observations and a photoionization detector (PID). Pooled water and non-aqueous phase liquid (NAPL) were encountered at a depth of approximately three feet, indicating the presence of a perched water table. A test pit was dug 20 feet from the excavation site. Water and NAPL were encountered there as well.
The following day, Olympus moved to the top of the cliff and excavated between the highway and the cliff, using a PID to identify impacted soil. The excavation site was approximately 19’ x 35’ and no more than six feet deep. Upon completion of excavation and sampling activities at the upper elevation, the excavation was backfilled, shaped, and compacted.
Focus returned to the lower elevation site on April 27. Olympus continued excavating impacted soil and NAPL encountered in the excavation was removed using sorbents. Due to the presence of water and NAPL, only the portion of the excavation adjacent to the cliff was backfilled to stabilize the slope. The rest of the excavation was left open to allow for access to the NAPL and water in the bottom of the excavation. Following excavation activities, the area was sampled in accordance with Montana Department of Environmental Quality (DEQ) guidance.
Following standard operating procedures, Olympus identified 12 groundwater supply wells within a half-mile of the site and received permission from landowners to sample 11 of the wells. Samples were collected from the wells several times over the course of two years to monitor for impacts to drinking water.
Montana DEQ requested that a sentinel soil boring be advanced to 100 feet below ground surface in a location down-gradient of the site to ensure that NAPL had not traveled through fractured bedrock, and to investigate the presence of a more regional shallow groundwater aquifer. Olympus contracted with a drilling company and the soil boring was completed on May 31. Measurable concentrations of volatile organic compounds were not detected at any depth.
To expedite the remediation process in the lower excavation area, a sprinkler system was used to circulate the water. On June 6, 7, and 8 a two-inch pump with large sprinkler heads was used to draw water from the excavation and spray it back into the excavation to volatilize petroleum hydrocarbons. Samples collected before and after indicated that circulating the water decreased petroleum hydrocarbon constituents. Based on the positive results of the initial sprinkler system, a solar-powered sprinkler system was installed on July 28 to provide a more long-term, low maintenance remediation system
In parallel with work on the sprinkler system, a front-end loader and mini excavator were mobilized to the site to dispose of the impacted soil that has been stockpiled there. Approximately 1,610 tons of impacted soil was hauled offsite for disposal.
In January 2017, Montana DEQ notified the responsible party that additional remediation work was required, including noxious weed management, the installation of a monitoring well and ongoing sampling and analysis of that well and the three closest residential water wells. Olympus performed this work for the responsible party.
In July 2020, Montana DEQ notified the responsible party that the site did not appear to pose an unacceptable risk to human health, or the environment and no further remedial action was required.
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