What Is Solidification?
Solidification refers to a group of cleanup methods that prevent or slow the release of harmful chemicals from wastes, such as contaminated soil, sediment, and sludge. These methods usually do not destroy the contaminants, but instead keep them from “leaching” into the surrounding environment at unsafe levels.1

What is the Purpose of Solidification?
Leaching occurs when water from rain or other sources dissolves contaminants and carries them downward into groundwater or over land where it can seep into lakes and streams. So the purpose of solidification is to bind the waste in a solid block of material. With the contaminants trapped in place there is reduced chance of leaching into the environment since the block is less permeable to water than the waste contained within it.

Solidification is often used together with the Stabilization method to prevent people and wildlife from being exposed to harmful substances, particularly metals and radioactive contaminants. However, certain types of organic contaminants such as PCBs and pesticides can also be solidified.1

How Does Solidification Work?
The process involves mixing waste with a binding agent (substance that makes loose materials stick together). Common binding agents include cement, asphalt, fly ash, and clay. For binding to occur; most binding agents must be mixed with water. The mixture is allowed to dry and harden to form a solid block.

Additives can be mixed into the waste while the contaminants are still in the ground (often referred to as “in situ”). This usually involves drilling holes with cranes employing large mixers or augers that inject the additives underground and mix them with the waste. The number of holes needed depends on the size of the augers and the contaminated area. Dozens of holes may have to be drilled.1

When shallow enough, the contaminated soil is excavated and additives are mixed into the waste at an above ground location (often referred to as “ex-situ”). The mixing process occurs using either heavy earthmoving equipment (backhoes and front-end loaders) or by placing the contaminated soil into machines called “pug mills” which can grind and mix materials at the same time.

Is Solidification Safe?
The additives used in solidification are materials often used safely in construction and other activities. When properly handled, these materials do not pose a threat to workers or the community.1

To ensure safety, water or foam can be sprayed on the ground to make sure that dust and contaminants are not released into the air during the mixing process. If necessary, the waste can also be mixed inside tanks, or the mixing area can be covered to minimize dust and vapors.

At the end of the process the solidified product is tested to ensure that contaminants do not leach. The strength and durability of the solidified materials are also tested.

Is Solidification Effective?
Solidification provides a relatively quick and lower-cost way to prevent exposure to contaminants, particularly metals and radioactive contaminants.1 When contaminants are in high concentrations and relatively small areas, solidification and stabilization have been selected or are being used in cleanups at over 250 Superfund sites across the country.

  1. A Citizen’s Guide to Solidification and Stabilization, EPA,, retrieved October 16, 2014