Officials can focus on cleanup now, rather than effect
11-21-07

GUNNISON—Up to now, much of the news coverage of the Gunnison Top Stop gas leak has dealt with the effect of the spill on the city and its residents, the response to the leak and remediation efforts to clean it up, and the fact that the city, the company, the consultant and the state had some difficulty all getting on the same page.
But now that efforts and communication seem to be more coordinated following developments last week, space can now be devoted to examining what is at work underground, the factors influencing the direction, rate and distance of the leak’s migration, and how the leak goes from liquid in the soil to vapor in homes and businesses.
The Messenger spoke to two people to explain the technical side of the situation. Lance Hess of Remedy LLC, the consultant hired by the city, spoke of factors influencing leaks generally; Les Penington of Wasatch Environmental, Top Stop’s consultant who is handling the remediation effort, spoke of those factors in relation to the Gunnison leak.
Hess said that petroleum, like about anything else, has a tendency to flow downhill. However, he added, “if there’s any pressure or gravity-driven force then sometimes these things can flow uphill. That’s called density-driven flow,” he said.
Penington said the Gunnison leak is indeed moving downhill.
“The conditions we’re discovering indicate that the gasoline is moving along low spots in the hardpan, which underlies much of Gunnison [at about 9-15 feet deep],” Penington said. “The gasoline seems to have traveled along at least two and possibly three channels in this hardpan, making it very difficult for us to track.”
Penington said the leak, or “plume,” has traveled very quickly, about 1,800 feet since July to August. “This thing has moved very fast,” he said.
Hess said the major factor influencing the rate a leak moves underground is the size of soil particles. For example, a leak generally moves slower in silt (relatively small-sized particles, than in sand (relatively larger), and slower still in clay.
Other factors that can affect the rate of migration is the amount of organic carbon in the soil, which can filter the petroleum and slow it down, and also the presence of certain bacteria that actually metabolize petroleum, again slowing it down, Hess said.
The distance a plume will travel is a “function of how much has been released,” Hess said. For instance, a tablespoon of liquid poured in soil will not spread as far as a gallon will.
Another factor influencing the distance of the Gunnison leak is the hardpan layer Pennington referred to.
Hess described the effect a relatively impermeable surface would have. “It would be like pouring molasses on your cutting board—it will spread out. Once it hits hardpan ... it will want to migrate laterally.”
As for how the liquid gasoline ends up as vapor, both Hess and Penington said that the gasoline simply, in effect, evaporates, and those vapors then rise up through the space between soil particles.
“Not unlike well water,” Hess said. “It’s just coming up out of the pore space.”