September 18, 2006

Environmental Solutions

Solidification solutions for contaminated soil

PETER KENTER

correspondent

When developers encounter contaminated soil, they choose to either cart the soil away and replace it with fresh soil, or treat it on site. One of the most popular methods of on site treatment is solidification/stabilization (S/S) — the addition of Portland cement powder to the soil to physically and chemically alter and bind contaminants.

S/S protects human health and the environment by immobilizing contaminants within the treated material. The resulting grainy soil can be covered with fresh top soil or used in construction as a base layer.

This treatment has been used for decades, but is now more popular than ever, particularly with the development of urban brownfields.

“The differences today are in how sophisticated people have become with mixing the material, and the wider range of materials being treated,” says Chuck Wilk, program manager for Waste Management with the Portland Cement Association in Skokie, Illinois.

Contaminated soil can be stabilized as follows:

• Ex situ, in which contaminated soil is excavated and passed through a pug-mill or other mixer where cement is added to it. The treated soil is then returned to where it was excavated or re-used off site.

• In situ, in which cement is mixed into contaminated soil or sediment while the material remains in place. Heavy construction equipment fitted with augurs can treat soil down to over 25 feet.

“One of the goals is to change the physical properties of the soil,” says Wilk. “S/S treatment can make treated material less permeable to groundwater and surface water, preventing the water from moving through it, eliminating the treated material as a source of contamination.”

Contaminants are also changed chemically. “Lead bound with cement, for example, changes chemically into lead hydroxide which has less solubility than elemental lead,” he says. “The new composition reduces how much of the lead dissolves in groundwater.”

S/S technologies were originally used almost exclusively to treat radioactive waste materials from the production of atomic bombs.

“It all depends on how you look at the end product,” says Jeff Westeinde, president of Quantum Environmental Group, an environmental remediation and hazardous waste materials management company. “Asphalt is oil-contaminated soil but happens to be a beneficial product, similar to stabilized soil.”

Westeinde’s company also employs solidification — encapsulating the material, which becomes part of the mix in construction concrete. The work is performed by specialized contractors.

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