Taking an on-site approach to soil stabilization and remediation
Chemical stabilization of heavy metal contaminated soil unlocks greater opportunity for site redevelopment By Kevin Ignasziak, Stantec Redevelopment of contaminated sites, or brownfields, never stops. In fact, the US Environmental Protection Agency estimates that 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 metalscontaminated 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, just to name a few. Generally, soils are determined to be impacted via laboratory sampling performed under a site characterization or investigation. These steps may be required as part of a land acquisition, due to a public complaint, or a multitude of other reasons. Regardless of how the contamination is encountered or where it stems from, something must be done to address the issue prior to redevelopment.
• •
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: A remediation technology that reduces 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.
These options provide clients with alternatives for waste management and disposal options over the standard “hog and haul” approach favored in the past, whereby soil is excavated and removed via truck, resulting in higher greenhouse gas emissions. 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. 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 permitted to accept hazardous waste in accordance 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 non-hazardous waste. These designated landfills, sometimes referred to as RCRA Subtitle C facilities, are few and far between and are generally limited in capacity. It’s also important to note that switching landfills to accommodate the hazardous waste stream can have many consequences.
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 significantly over the past 20 years, including: • Thermal Desorption: Off-site remediation technology that utilizes heat to volatilize contaminants (volatile organic compounds) so that they can be separated from the soil particles and either collected and treated or incinerated. • Bioremediation: Broad term encompassing many on-site remediation technologies that are focused on the enhancement of naturally occurring degradation of
24 | The ROCHESTER ENGINEER APRIL 2021
In the simplest sense, this affects the project’s bottom line by increasing transportation and disposal costs. Many times, these costs can make up a large portion of the total project budget and are prohibitive for clients. Until recently, there was not much that could be done to avoid these costs, but over the past three years, our design and field teams have been incorporating the use of on-site chemical stabilization for soils impacted by hazardous heavy metals to reduce these hefty transportation and disposal costs.
A new approach to stabilization
Chemical stabilization via mixing an amendment into the soil has been successfully applied to soils impacted by a variety of metal contaminants. The process does not destroy or remove the metals, but instead decreases their mobility and may reduce their toxicity. Essentially, the process encapsulates feature article
