Lead image: The New Hope Soil Remediation Project in Cleveland, OH. ReImagining Cleveland - Image Archive, CC BY NC ND

Soil remediation involves cleaning up polluted soils. There are many different ways to reduce exposure to contaminated soils, and remediation methods vary in how effective they are and how much they actually reduce the problem or simply move the problem from one place to another.

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Approaches for remediating contaminated soil

Remediation approaches are tailored to each site and situation. Different approaches could be combined at the same time or used one after another. Some factors that will affect which remediation approaches are most appropriate for a site:

• What contaminants are present: Inorganic contaminants like heavy metals (e.g., lead, arsenic)? Organic compounds like petroleum products?
• The level of contamination: Low, moderate, or high concentrations of contaminants?
• The size and depth of the area: A small part of a residential property? A community garden? A larger plot of land?
• The intended land use of the area: For growing food? A play area for children?
• And other factors, like the environmental conditions of the area (e.g., if you want to use plants for phytoremediation techniques, will the plants have enough sunlight and water to grow?), and the amount of time and money you have available for remediation.

Bioremediation

How bioremediation methods work:

Bioremediation methods use living things, like plants, fungi, and bacteria, to help reduce soil contamination. Some plants grown in contaminated soil can take up heavy metals into their roots and leaves, removing them from the soil. Some fungi and bacteria can break down organic contaminants in the soil into less toxic compounds.

For a comprehensive introduction to bioremediation, check out this excellent Bioremediation for Urban Gardeners factsheet created by @DanielleS with the Healing City Soils program out of BC, Canada. The factsheet covers: what bioremediation is, kinds of bioremediation and how they work on different contaminants, a list of plants for phytoremediation (bioremediation using plants), and a decision tree to determine if phytoremediation is right for your site. Plus much more. Most of the information in this section is informed by this fantastic factsheet.

Some different strategies:

Phytoremediation with living plants

Image: Diagram of a plant taking up heavy metals. Rona.fawzy19, CC BY SA

Phytoremediation involves growing certain plants in soils containing heavy metals. The plants take up the metals into their tissues as they grow, removing them from the soil over time. This particular process is phytoextraction.

• Ease of use: relatively accessible because it uses tools and skills that gardeners and farmers already have. It’s a slow process that requires plant maintenance.
• Relative cost: lower than conventional approaches to remediation.
• Suitability: varies for each site, but in general phytoremediation is more suitable for smaller scale areas (home gardens, small farms) with low to moderate heavy metal contamination at shallow depths (where plant roots can reach). Not as suitable for large sites, highly contaminated sites, or organic contaminants.

(From the Bioremediation for Urban Gardeners factsheet)

Mycoremediation with fungi/mushrooms and bioremediation with bacteria

Involves growing certain fungi or bacteria in soils contaminated with organic compounds, like petroleum products and polycyclic aromatic hydrocarbons (PAHs). The fungi and bacteria break down the organic compounds into less toxic ones.

“...more research is needed so that protocols for community-scale myco- and bioremediation can be applied safely and effectively to repair contaminated soils.”

(From the Bioremediation for Urban Gardeners factsheet)

Examples of projects:

• BKBioReactor: here’s a project involving microbial remediation of sediment in the Gowanus Canal in Brooklyn.
• Phytotechnology Project Profiles: a US EPA database of phytoremediation projects, detailing “information about relevant site background, the types of contaminants treated, the type of vegetation used, the mechanisms of phytotechnology, planting date, project size, location, cost, monitoring and performance results, and points of contact and references.”
• "Minnesota Develops Method To Recycle Petroleum-Contaminated Soil": a report from the Minnesota Department of Transportation describing a project where bioremediation followed up excavation of petroleum-contaminated soils, and cost significantly less than typical remediation methods.

Additional resources:

• Bioremediation: a brief introduction to bioremediation plus more resource links from the Environmental Inquiry program
• Citizen’s Guide to Phytoremediation: a factsheet from the US EPA describing different types of phytoremediation, plus examples of cleanup sites across the US using various plants for phytoremediation on different contaminants.
• Watson, N.M. Mycoremediation: Studies in mycelium as an agent of bioremediation. In Environmental ScienceBites Volume 2 by Lower B.H., Shaul T.R., Shaul K.A., and Weaver E.M.
• Rhodes, C.J. 2014. Mycoremediation (bioremediation with fungi) – growing mushrooms to clean the earth. Chemical Speciation & Bioavailability, 26:196-198.

Physical methods for soil remediation

How physical methods work:

Physical methods reduce exposure to contaminants at a site by removing soil, broadly blocking access to it, or physically separating or trapping contaminants.

Some different strategies:

Excavation

Excavation involves digging up contaminated soil and moving it to a landfill or treating it offsite.

• Ease of use: less accessible when it requires large equipment to dig up and transport soil.
• Relative cost: high cost to dig up and dispose of contaminated soils. Up to $260 USD per cubic meter of soil for excavation alone at a residential site.
• Suitability: may be used for large, highly contaminated sites, e.g., Superfund sites.

Capping / Encapsulation

This method involves placing a physical barrier over contaminated soil to contain the contamination.

• Ease of use: varies depending on the material used. Capping materials include concrete/asphalt, plastic or fabric, or clean fill/topsoil.
• Relative cost: installation costs may be relatively low for a small site using a single layer of barrier fabric, and relatively high for a large site requiring a more complex system of barriers.
• Suitability: may be suitable for highly contaminated areas that need immediate containment; but this method doesn’t remove contaminants from the area.

Thermal desorption

This method removes hydrocarbon contamination (like petroleum products) by heating soil in a specialized low-temperature dryer. The heat evaporates the hydrocarbons off the soil, and then they’re collected and treated separately.

• Ease of use: not accessible to most people as it requires specialized heating equipment at an offsite facility or in a portable unit for onsite use.
• Relative cost: high cost at up to $80 per cubic meter.
• Suitability: for high levels of hydrocarbon contamination. Its cost might limit this method to small volumes of soil.

Examples of projects:

• Reported in the Chicago Tribune, “EPA crews start digging out soil contaminated with lead in East Chicago.”

Additional resources:

• Citizen’s Guide to Excavation of Contaminated Soil: factsheet from the US EPA
• Landfill and Soil Capping

Chemical treatments

How chemical methods work:

Uses specific chemicals added to the soil to reduce contamination. The chemicals bind to contaminants so they’re stabilized in place, potentially making removal easier. Or, the added chemicals break down contaminants into less toxic substances.

Some different strategies:

Chemical fixation

This chemical method treats soils contaminated with organic compounds or heavy metals using additives that attach to the contaminants. For metals, these chemical additives may be “chelating agents.”

• Ease of use: relatively advanced as it requires comfort purchasing and using chemical additives and reagents, and knowing how they might negatively react with other compounds in the soil.
• Relative cost: $2700 for 700 kg of this commercial brand.
• Suitability: may be suitable for low to moderate contamination with heavy metals or organic compounds.

Chemical oxidation

Specific chemicals are added to the soil, where they react with pollutants and turn the pollutants into less toxic compounds.

• Ease of use: relatively advanced as it requires comfort purchasing and using chemical additives and reagents, knowing how they might negatively react with other compounds in the soil, and specialized equipment to apply the oxidizing chemicals.
• Relative cost: relatively high up front.
• Suitability: may be suitable for sites where it’s difficult to dig up soil and treatment needs to be done in place.

Examples of projects:

• This In Situ Oxidation: Application page from the US EPA provides links to compiled case studies

Additional resources:

• Overview of In Situ Chemical Oxidation (ISCO): from the US EPA, providing background information plus resources including A Citizen’s Guide to In Situ Chemical Oxidation and factsheets on when and how to use chemical oxidation.
• A paper describing how a chelating agent was used along with phytoremediation to treat soil contaminated with lead: Attinti R., Barrett K.R., Datta R., and Sarkar D. 2017. Ethylenediaminedisuccinic acid (EDDS) enhances phytoextraction of lead by vetiver grass from contaminated residential soils in a panel study in the field. Environmental Pollution, 225: 524-533.

How do you know remediation is working?

@DanielleS explains in a comment here that there are different ways to evaluate if remediation is working:

• Lab testing: Sampling the soil before and after remediation and getting the samples tested at a lab to see if contaminant concentrations have gone down. Lab tests can run around $30 to $300 USD per sample, depending on the contaminants you're testing for.
• Bioassays: An approach that uses living things, like plant seeds, to assess the toxicity of a site. You can also do before and after remediation to see if toxicity is reduced. You can learn more about bioassays on this wiki page. Bioassays can be more affordable than multiple lab tests.

Further reading and resources

Here are some more general resources, in addition to the ones listed above for different kinds of remediation methods

• Lombi E., and Hamon R.E. 2005. Remediation of Polluted Soils. In Encyclopedia of Soils in the Environment, pp. 379-385. LINK to PDF.
  • Includes a decision tree of different methods to treat polluted soil, sorted by containment vs. clean-up approaches and onsite vs. offsite methods.
• Remediation of Metals-Contaminated Soils and Groundwater. Pg. 15 has a table of different remediation methods for several heavy metals (lead, chromium, arsenic, zinc, cadmium, copper, mercury) and how those methods compare in relative cost, long-term effectiveness, suitability for high contamination levels, plus other factors.