Groundwater pumping to remove a floating contaminant

Remediation stages

The main steps in remediating a contaminated site are as follows:
- Emergency measures to reduce an immediate hazard.
- Contamination analysis: geological and hydrological studies, historical review of industrial activities at the site, sample collection and analysis.
- Risk assessment: exposure scenarios (pathways, analysis of doses/effects).
- Identification of remediation goals and re-sources based on the preceding factors and the site's future use.
- Remediation and/or confinement.
- If necessary, monitoring and/or use restrictions.

Contamination analysis

Before remediation begins, it is important to fully understand the situation in the ground. This is not an easy task. The process is complex because soil is not uniform. It can contain a number of materials (clay, sand, rock, limestone, etc.) with very different characteristics, particularly as concerns permeability. Some sections of a site may be saturated, the concentration of pollution can vary throughout a site and the contaminants themselves can change over time.
All of this means that it is very difficult to obtain an accurate 3D map of a given site. This requires a detailed review of past activities and the types of contaminants that may have been present, as well as the collection and analysis of samples taken far below the surface. Contamination analysis is followed by risk assessment.

Remediation techniques

There are many complex remediation technologies to choose from.
To simplify, they are generally separated into two main groups: biological treatment and physicochemical treatment. The leading methods are described below.

Biological technologies
- Biodegradation*: this method, like most biological treatment solutions, uses microorganisms to break down organic contaminants (notably light hydrocarbons). Through the digestion process, bacteria transform the contaminants into water and carbon dioxide.
- Bioventing: this solution combines soil venting and biological treatment. Air circulating through the soil stimulates biodegradation. The resulting molecules produced by microorganisms are extracted along with vapors from the soil.
- Phytoremediation: living plants can be used to take up heavy metals into their leaves or roots. The plants are then pulled up and incinerated.
- Natural attenuation: the toxicity of certain contaminants declines naturally over time as they Biodegradation* (organic contaminants*) or are modified (inorganic contaminants). This type of treatment entails monitoring the site and checking that the proper conditions continue to exist throughout the attenuation process.

Physicochemical technologies
- Soil vapor extraction: extraction wells pull volatile contaminants out of the ground. The extracted vapors are condensed to liquids, adsorbed onto active charcoal or incinerated.
- Pumping: contaminated groundwater* is pumped and cleaned through biological or physicochemical treatment. This method keeps the contaminant plume from advancing. However, vast quantities of water must be pumped for many years.
- Skimming: floating contaminants can be skimmed from the surface of groundwater.
- In situ flushing: water and surfactants injected into the soil dissolve organic contaminants which can then be pumped to the surface and separated in a settling tank.
- Thermal desorption: soil contaminated by organic products* (even chlorinated) is roasted at a temperature of less than 500 °C. This vaporizes the contaminants without destroying the soil.
- Soil washing or retorting: these methods can be used to extract metals such as mercury and zinc.
- Solidification/stabilization: a binding agent such as cement or lime is added to the soil to keep the contaminant from spreading. The resulting product is left on site, used as a material or landfilled.
- Incineration: soil is excavated and then heated to very high temperatures to destroy all organic molecules.
- Confinement: a waterproof barrier is built around the contaminated area. It may be sunk down to a waterproof geological layer such as clay to keep contaminants away from unconfined aquifers.
- Permeable reactive barriers: to clean polluted groundwater, a permeable wall is built below ground downstream from the contaminant plume. Reactive materials in the wall destroy the contaminants or change them into harmless chemicals. Specific applications have been developed for several contaminants. For example, some reactive barriers free oxygen to stimulate aerobic biodegradation, while iron-containing barriers are used to remove chlorine.
- Landfilling.

Different methods for different sites

A number of factors have to be taken into account when selecting a remediation technology: the type of contaminant, the site's profile, cost issues, etc. Most often, different methods are combined over time or to treat different parts of the site.