Division of Waste Management

UST Branch Turns to ‘Soil Mixing’ to Clean Thousands of Gallons of Spilled Petroleum

Energy and Environment Cabinet staff and a team of contractors stand at the site of a petroleum spill in Sonora, Ky. The crew employed a technique called “soil mixing” to rehabilitate the site. Photo by the Energy and Environment Cabinet.

By Robin Hartman 

Not too many people describe their work as mesmerizing, but that’s exactly how the crews working with the Energy and Environment Cabinet’s Underground Storage Tank (UST) Branch viewed their task at the site of Sammy’s Market in Sonora. 

In late 2018, the branch faced a unique challenge: How do you clean up thousands of gallons of spilled petroleum — 25 feet below the surface, so deep that traditional clean-up methods would be cost-prohibitive? The Hardin County property, now owned by a local bank, provided an opportunity for the team to clean up the spill using “soil mixing,” a process that involves adding a combination of activated carbon, sulfate and a blend of microbes to the contaminated soil. Once the ingredients are mixed, the petroleum is bound to the activated carbon and “eaten” by the microbes.  

The process fascinated everyone who witnessed it, said West Johnson, the project manager and an environmental scientist with the UST Branch. 

“The track-hoe operator for the mixer loved doing it,” Johnson said. “He had never operated a track-hoe with a mixing head before, so it was something new and fun for him.” 

The Sammy’s Market site prior to cleanup.

Three underground storage tank systems supporting fuel pumps at the market — a combination of leaded, unleaded, diesel and kerosene tanks — contributed to the overall contamination. Leaks with the systems, installed as early as 1980, began in 1985 and continued for decades.  

Over the years, the owner of the property faced multiple violations from the then-Natural Resources and Environmental Protection Cabinet. In 1996, the owner reached an agreement with the cabinet to submit required documents, perform environmental testing and conduct corrective action. However, the owner eventually defaulted and declared bankruptcy. The last of the unleaded gasoline tanks were removed in 2015. 

By then, more than 7,000 gallons of petroleum had been released at the site, causing significant impact to both soil and groundwater.  

“At one time, more than four feet of gasoline was measured atop the water table at Sammy’s,” Johnson said. “This is one of the larger petroleum releases I’ve ever worked on.” 

Site Analysis Begins

There are more than 500 UST sites being investigated and remediated for petroleum releases in Kentucky. Some are closed sites while others are at active facilities. Nearly all pose some level of risk to the environment; many have the potential to impact public health.  

Kentucky’s Petroleum Storage Tank Environmental Assurance Fund (PSTEAF), supported by a $0.014 fee assessed on each gallon of fuel imported to Kentucky, provides reimbursement to UST owners for eligible costs associated with the investigation and corrective action required to clean up the releases.  

When a there is an immediate threat due to a significant leak, the Division of Waste Management’s Emergency Response Team (ERT) is the first line of defense. Once the immediate threat is alleviated, the UST Branch takes over for site investigation and corrective action. Working with the property owner and contractor, the nature and extent of contamination is determined and a plan is developed. 

For many UST release sites, contaminated soil is hauled away and disposed of at permitted landfills.  Because of the size and features of the site at Sammy’s — roughly a quarter acre and more than 5,000 tons of contaminated material  — removing it and replacing it with clean backfill would have been far too costly. Favorable factors like having unrestricted access to the site made soil mixing a more achievable option. 

For these reasons, Johnson was confident the process could work.  

“I had been involved in similar projects while working in the private sector and was just waiting for the right opportunity to try it at the state level,” he said.  

An aerial photo of the work site taken on Dec. 20, 2019.

However, the team needed to make some key modifications to account for the depth of contamination. 

Usually, the activated carbon and microbe product is injected into the ground, but due to the large mass of contamination extending so far below the surface, this likely wouldn’t be an effective strategy. A sufficient amount of amendment was needed, and contact with the contaminants was essential.  

“Those in the business call injecting and soil mixing a ‘contact sport,’ meaning if the amendment doesn’t come into contact with the contaminant, then it most likely won’t be successful,” Johnson said.  

An Effective Strategy

After reviewing several options, the team agreed the most effective and affordable choice was to excavate and stockpile the top layer of uncontaminated soil, down to 15 feet below the surface, and focus on treating the impacted soil underneath. 

“We’re always looking for new technologies and approaches to clean up petroleum contaminated sites,” said UST branch manager Dawn Baase. “This project was such a huge undertaking, but based on site characteristics and the depth of the contamination, it made sense to proceed with the project under West’s supervision.” 

Once the corrective action strategy was approved by the branch, the team went to work. 

The prime contractor, Southern Environmental Services, removed 7,000 tons of clean topsoil and stockpiled it on site. It became known as Sammy’s Mountain. 

Over the course of three months, from November 2019 to January 2020, they divided the site into twelve, 27.5-square-foot cells and worked to remove the top 15 feet of soil, adding and mixing the carbon-based amendment to the contaminated soil underneath.  

Mixing the bottom two feet of dirt, they would add carbon-based amendment to each cell, mix and add two more feet of dirt, repeating the process over and over to ensure contact between the tainted soil and the added product. The repeated mixing also aerated the soil at the same time. 

The goal is for the petroleum to bind with the activated carbon while the native bacteria, enhanced with the additional microbes, continue to reproduce, “eating” the contaminants in the new environment. 

Division of Waste Management drone pilot Belle Thomas documented the work, flying once a week to track progress and help create a time-lapse video

Blueprint for the Future

If soil-mixing proves successful at Sammy’s, the UST Branch can consider using it again in the future at sites with favorable conditions.  

“We’re always interested in upgrading our tools and adding new ones,” Johnson said. “Soil mixing is definitely a tool we want to add to our toolbox and improve over time.”  

An aerial photo of the work site taken on Feb. 6, 2020.

Initial laboratory results have been positive, showing a significant reduction in contamination at a number of the locations tested. However, additional sampling will be done over the next year. The team installed monitoring wells and will be sampling soil and groundwater to check long-term success.  

“We literally turned the site upside down,” Johnson said. “So we’ll give the site time to stabilize and return to some sort of equilibrium before making any conclusions.” 

The hope is to return the restored property to the bank without the risk of exposure to contamination and with no limitations on future land use. 

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