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Taking an on-site approach to soil stabilization and remediation

February 26, 2021

By Kevin Ignaszak

Chemical stabilization of heavy-metal-contaminated soil unlocks greater opportunity for site redevelopment

Redevelopment of contaminated sites never stops. In fact, the US Environmental Protection Agency estimates there are over 450,000 such sites—brownfields as they are more commonly known—in the country. With redevelopment comes the need for remediation and the careful consideration of factors such as ideal strategy, cost, and timeline.

In the Northeastern US, where many communities were previously home to industrial manufacturing, heavy-metals-contaminated brownfield sites are among the top focus areas for remediation by our team. This type of contamination comes from sources that are too numerous to count but it’s frequently from historic manufacturing and firing range operations.

Contaminants include lead, arsenic, and cadmium. Laboratory sampling performed for a site characterization or investigation reveals the impacted soils. These steps are often required as part of a land acquisition, due to a public complaint, or other reasons. Once discovered, redevelopment is on hold until the issue is resolved.

In the Northeastern US, many communities were previously home to industrial manufacturing.

Advancements in the field

Technologies relating to on-site treatment and stabilization of waste soil have advanced the past 20 years, including:

  • Thermal desorption: Off-site remediation technology that uses heat to volatilize contaminants (volatile organic compounds) so that they can be separated from the soil and either collected and treated or incinerated.
  • Bioremediation: Broad term including many on-site remediation technologies that focus on boosting the naturally occurring degradation of contaminants performed by soil microorganisms. This method is generally most effective on petroleum and chlorinated sites.
  • Physical treatment: Technologies that physically reduce the impacts of contaminants. These include soil washing, physical encapsulation of contamination within a defined area (slurry walls, sheet pile walls, injection walls, permeable reactive barriers), or physical transformation of soil to a glass-like substance in a process called vitrification.
  • Chemical fixation: Reducing the mobility of a contaminant in soil. For example, solidification of heavy metals onto soil particles to reduce leachability of metals into the surrounding soil.
Incorporating the use of on-site chemical stabilization for soils impacted by hazardous heavy metals reduces hefty transportation and disposal costs.

These options provide clients with alternatives for waste management and disposal options over the standard “hog and haul” approach—excavation and removal via truck, resulting in higher greenhouse gas emissions— favored in the past.

For many of these heavy-metals-contaminated sites, the previous best method was to excavate, sample, characterize, and dispose of contaminants in a permitted landfill. That’s about as simple as it gets. But what if these soils exceed the EPA’s hazardous threshold for a specific contaminant?

In many instances, this simply meant disposal at a different facility in harmony with the Resource Conservation and Recovery Act (RCRA) passed in 1976, albeit with a transportation and disposal fee that is many times higher (up to four times more per ton) than for nonhazardous waste. These designated landfills are few and far between and are generally limited in capacity. Switching landfills comes with consequences.

In the simplest sense, it affects the project’s bottom line by increasing transportation and disposal costs. Many times, these costs make up a large portion of the total project budget and are prohibitive for clients. Until recently, there were few options to avoid these costs. Over the past three years, our design and field teams have incorporated the use of on-site chemical stabilization for soils impacted by hazardous heavy metals to reduce the hefty transportation and disposal costs.

Chemical stabilization via mixing an amendment into the soil decreases the mobility of contaminants and may reduce their toxicity.

A new approach to stabilization

We have used chemical stabilization via mixing an amendment into the soil on a variety of metal contaminants. The process does not destroy or remove the metals but decreases their mobility and may reduce their toxicity. Essentially, the process captures the particles to prevent leaching in a landfill setting.

Stabilization chemicals include reagents such as cement, fly ash, slag, phosphorus-containing materials, clays, and other proprietary blends. Typical excavating and mixing equipment can mix the soil while blending in a reagent.

In instances where shallow soil contamination exists, we can mix the reagent using standard equipment such as a bucket or mechanical mixing head to the end of a tracked excavator. The mixing process creates a relatively homogenous soil matrix, reducing contaminants within the mixed zone and promoting contact with the treatment reagent. It also results in a volume increase of at least 10% to 15%.

Mixing the reagent into the soil while it is in place also has the potential to avoid licensing and other requirements that pertain to on-site treatment of hazardous waste. Toxicity leaching characteristic procedure (TCLP) sampling, multiple extraction method (MEM), and total metals sampling are performed on the stabilized soils to confirm hazardous thresholds.

This approach has few limitations when evaluating its effectiveness. That makes heavy-metals-contaminated brownfields a prime candidate for its use regardless of size or location.

Hazardous soils are uniformly mixed with a stabilization reagent in batches, until the color became uniformly lighter throughout—much like mixing cookie dough.

Putting it into practice

Our Northeast Environmental Services group recently completed a similar project for a confidential client in the Southern Tier of New York. The client was faced with a storm sewer pipe replacement. However, the pipe was buried in hazardous lead-impacted soils. The soil would require excavation, management, and off-site disposal.

We proposed the use of an on-site, soil-stabilization agent, which was approved by the New York State Department of Environmental Conservation (NYSDEC). Our teams observed the mixing process performed by the client’s environmental contractor. Hazardous soils were uniformly mixed with a stabilization reagent in a metal mixing container (like a standard roll-off container) using an excavator bucket. Soils were mixed in batches until the color became uniformly lighter throughout.

The stabilized soils were sampled for lead and the results were shared with NYSDEC and the proposed local, nonhazardous landfill. Prior to stabilization, lead concentrations were nearly three times the hazardous waste threshold. After the stabilization process, many of the collected samples had nondetectable levels of lead. MEM samples were collected and demonstrated that long-term leaching within an acidic environment was not an issue. Soils were disposed of at the local municipal nonhazardous landfill for less than a quarter of the price of the nearest hazardous landfill, resulting in an estimated cost savings to the client—from transportation and disposal alone—of $110,000.

By challenging the status quo and taking an innovative approach to decades-old techniques, extensive cost savings can be unlocked—and an even greater number of contaminated sites can be cleaned up.

Learn more about Stantec’s National Brownfields Group.

  • Kevin Ignaszak

    Kevin has more than 29 years of professional environmental engineering and remediation experience.

    Contact Kevin
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