Contaminated land: Risk assess or remediate?

contaminated lands

By Sajjad Din, M.Sc., P.Geo.

Generally speaking, remediation or reclamation is a process in which consultants, contractors, construction managers, engineers and scientists conduct a series of steps to return contaminated land back to its original pre-human activity state, in terms of concentrations of various compounds in the soil and groundwater.

A risk assessment scientifically assesses the potential risk that exists for humans, plants, wildlife and the natural environment from exposure to a contaminant. The purpose of a risk assessment is to develop site specific standards that will allow uses, such as residential, that are being proposed to take place on the property.

Standards and Regulations

In Ontario, remediation may only require clean up to certain standards as outlined in the Soil, Ground Water and Sediment Standards for Use under Part XV.1 of the Environmental Protection Act. In many circumstances, achieving cleanup to these standards may be physically or chemically difficult due to various factors such as existing infrastructure, current land use, depth of impacts, soil types or shallow bedrock. For example, impacted groundwater may not be treatable due to it being at depths greater than 20 metres within the bedrock. Cleanup cost for said groundwater may be astronomical and hence beyond the financial reach of the owner or interested parties.

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Prior to completing a remediation program, remedial options feasibility assessment should be carried out, in order to assess what method or methods of cleanup would be most suitable and effective. This may also be called a comparative analysis, in order to prepare a Remediation Action Plan.

As part of this process, the options of natural attenuation and/or a risk assessment should be considered. Natural attenuation considers the allowance for the natural breakdown and reduction in contaminants over time. This is verifiable by monitoring, i.e., testing of samples collected in the field. Risk assessments consider existing natural barriers or placing artificial administrative or engineered barriers between human occupants of a property and the contaminants present.

Municipalities want to curb urban sprawl and brownfields redevelopment is one way to do this. The site assessment process is a significant part of urban brownfield development. However, once Phase I and II site assessments are complete and the aerial and vertical extent of impacts have been defined, depth to bedrock determined, soil characteristics verified, hydraulic conductivity and groundwater flow direction ascertained, it still is not always clear as to what remedial approach would be best suited to a site. This is because there may still be uncertainties regarding subsurface biochemical oxygen demand and other soil and groundwater characteristics.


A major factor in determining remedial methods is the type of pollutant and its potential mobility and reactivity. Most contaminants can be broken down into volatile organic compounds, petroleum hydrocarbons, metals and inorganics, pesticides/herbicides and polycyclic aromatic hydrocarbons. The type of contaminants found should be considered when deciding on whether to simply risk assess. This is because the mobility and the fate and transport of them will determine the level of risk to the local human and ecological populations.

Remediation Cost

With remediation one can aspire to end up with a property that meets applicable standards. Ideally, all impacted material is removed or treated. This allows the site owner or developer to carry out any type of land use they wish. Also, lending institutions are more willing to front the funds to purchase such a property.

However, remediation work may cost millions, depending on the level and extent of contaminants of concern on site, soil types, existing buildings, infrastructure and groundwater conditions. The remedial approach may involve both ex situ and in situ treatment methods. Additionally, there would likely be disruption to site activities.

Though certain costs are inherently a part of any risk assessment process, this route may be far more cost-effective and time efficient. With remediation projects, specifically in situ, there is an inherent uncertainty of the amount of time that would be required for completion. The process would require study of which site specific standards should be developed to allow contaminants to remain on site in higher concentrations then would otherwise be permitted under the generic Ministry standards.

For risk assessment, a team of professionals with diverse skills in science and engineering would develop specific standards based on the current and proposed site use. This would involve an assessment of site conditions such as geology, concentrations of contaminants, human occupancy, building structures (existing and proposed) and infrastructure. Based on these parameters, acceptable levels of contaminants left in place are determined. Additionally, certain engineering controls such as sub-slab ventilation, concrete barriers, solidification/stabilization, Waterloo Barriers and underground slurry walls are introduced into the assessment calculations.

In the long run, a wiser use of both remedial and risk assessment options will be required for greater urban redevelopment and revitalization. Too many contaminated properties lie unused that could be redeveloped if a risk assessment is conducted and the risk posed by on site contamination is deemed to be acceptable.

Sajjad Din, M.Sc., P.Geo., is a part time professor at Seneca and Centennial Colleges and also a consultant with Toronto Inspection Ltd. (References are available upon request)


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  1. This is a great list of the steps taken in remediation for a specific site. The extent of it all can be confusing and daunting! It helps to know that this is pretty standard stuff when it comes to contaminated site remediation.

  2. Most of the time good engineering consultants will give you a detailed outline prior to deciding on next steps. They should also give you options that include costs and other information that factor into your decision making.

  3. I agree that there is a lot of contaminated land out there that could be redeveloped if a proper assessment is done. In our state, I’ve noticed some sites that could have been a prime location if the land was okay. I hope there comes a time when they would be redeveloped and used. Thanks for sharing your knowledge about land remediation.

  4. One of the first major benefits that come from environmental remediation is the ability to improve the environmental state of land that’s been previously developed.


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