Soil Pollution Assessment and Remediation: State of the Science, Trends, and New Techniques

 

    The UN’s FAO and UNEP write in their 2021 Global Assessment of Soil Pollution that: “Soil pollution is one of the main threats affecting soil health worldwide. However, soil pollution is {more} unique than other threats such as erosion or salinization: it is difficult to perceive with the naked eye, and its effects are only visible when the level of pollution causes acute effects on the environment and human health. Soil has the ability to filter, buffer, retain and degrade contaminants, in which components such as soil organic carbon, organisms, pH or type of clays and other mineral complexes play a key role.” That UNEP/FAO global assessment is my main source for this post which I summarize and paraphrase quite a bit here. It is also the source of all the graphics. The first graphic shows a macro model for the fate and transport of contaminants. 

     Soil pollution can be compounded over time in the same area which can result in ‘cocktails’ composed of multiple contaminants. Soil pollution can affect humans, flora, fauna, and ecosystems. There are both natural and anthropogenic sources of soil pollution. Soil pollution is often diffuse pollution rather than point-source pollution, makes it more difficult and more expensive to remediate. The graphic below shows a chemical classification of common soil contaminants. Not listed are emerging contaminants such as PFAS and microplastics. Microplastics are classified as incidental nanomaterials. Other nanomaterials are naturally occurring (such as volcanic ash) or manufactured (such as paints & pigments).

 

     Inorganic heavy metal trace element pollution in soil can be significant and can affect human, animal, plant, and ecosystem health. These pollutants are often persistent as they do not break down easily into non-harmful components. Their reactions in living cells are what cause harm. Radionuclides, either naturally occurring or from anthropogenic activity such as uranium mining, cause harm directly due to the radiation. Organic, or carbon-based contaminants, are mostly anthropogenic synthetic chemicals. These include pesticides, industrial chemicals, household chemicals, and refined hydrocarbons. Some are naturally occurring such as those deposited by wildfire smoke residue and volcanic eruptions. Naturally occurring human-extracted hydrocarbons like oil and natural gas liquids are included as well. Many of these substances, whether synthetic or natural, are or produce volatile organic compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs). Organic contaminants tend to be persistent in the environment and are toxic at varying levels. The most persistent ones are known simply as persistent organic pollutants (POPs).

     The FAO/UNEP report lists the variables of soil pollution as follows:

“The extent and duration of soil pollution and the risk of harm of a specific contaminant or contaminant class depends on several determinants:

- the chemical nature of the contaminant; that is, inorganic compounds versus organic compounds;

- the inherent physico-chemical properties, for example, chemical structure, volatility2, water solubility, lipophilicity3, lipophobicity;

- the inherent toxicity of the compounds in their various forms;

- parameters describing the interaction and transformation of the compound with environmental media and biological organisms such as solution partition coefficients, decay rate of radionuclides, metabolization (degradation) rates;

- the source of origin and pathway to the environment, for example, geogenic versus anthropogenic, direct release or disposal in soil versus atmospheric deposition;

- place and time of emission of contaminants;

- emission quantities;

- soil concentration and analytical detection values;

- bioavailability and/or toxicity of contaminants, as well as species (e.g. chromiumIII vs chromiumVI) or transformation products (e.g., DDT vs DDE), depending on environmental conditions (e.g. soil pH, moisture, weathering conditions);

- land use of and sensitivity to receptors in affected areas; for example, agricultural and forestry activities, recreational, residential or industrial areas.”

 

Effects of Soil Properties on Contaminants

     Soil properties affect the fate and transport of contaminants. Contaminants can accumulate, transform, or degrade as they move through the soil matrix. Soil can buffer and filter contaminants to a certain extent, which can trap and dilute them adequately before they reach groundwater. Both chemical and biological processes occur of many different kinds. Soil texture, structure, pH, organic matter, mineralogy, moisture, temperature, and biodiversity all affect the fate and transport of contaminants. Plants and animals can also affect contaminant fate and transport by taking up contaminants or subjecting them to biological processes.

 

Sources of Soil Contamination

     Soil pollution types are often a result of land use. Pesticides are often a major contaminant in agricultural areas. Asbestos contamination may occur in soils near old buildings. Sewage systems carry biological contaminants and household chemicals in abundance. The use of treated or non-treated municipal wastewater in many places can be a major source of contamination. Polyethylene plastic films used for mulching can contaminate agricultural areas with microplastics. Urban pesticides are a contaminant source in urban areas. Combustion particulate products that land and accumulate nearby are contaminants in those areas. They often contain concentrated amounts of heavy metals. Highly concentrated forms of combustion products like massive piles of coal ash from coal-fired plants can highly contaminate nearby soil, surface water, and groundwater. Raw sewage is a major contaminant in some places. Erosion and runoff can spread contamination but also dilutes it. Municipal solid waste from landfills is mixed with soil and layers of barriers to decompose into biogas which is now often collected and utilized but there are also toxic liquids that leach from landfills. Landfill leachate can be highly toxic and monitoring wells around landfills look for it and attempt to quantify it and model its transport. That leachate is also often collected for further sequestering. Health-care waste has its own dangers and protocols. Electronic waste is a newer form of waste that has issues as well. Some toxins leach out. Recycling is done but is not economically viable in most circumstances. Industrial waste streams can be quite large and can affect soil. Waste from oil and gas, mining, and manufacturing is quite common and widespread. Leaking wells can add hydrocarbons and saltwater to soil. Mine tailing piles can mix concentrated heavy metals and radionuclides into the soil. Industrial effluents can leach into the soil. As the graphic below shows, pollutants are also subject to global processes which can transport them and even accumulate them quite far from their source areas. Soil dust can be carried by the wind all over the globe.

 

Soil Pollution Remediation

     Since water tables often reach into soil and since groundwater moves contaminants it can increase contaminant dispersal within soil. Therefore, groundwater and soil remediation often overlap. The goal of remediation is to eliminate or thoroughly isolate the source of the contamination. This is in contrast to contaminant management and adaptation, which seek to remove contamination by cutting off exposure pathways or by removing the receptor. As the graphic below shows, the costs of remediation per volume of contaminant released rise the longer a release is left un-remediated as the contaminant spreads and saturates the local soil.

 

Once contamination is discovered and assessed there is a workflow to achieve the goals of management and remediation. This may include stopping further contaminants from being released, covering the area with an impermeable layer to prevent further contaminant migration through the soil with rainwater and vapors from getting too heavy in the air, danger warnings, signage, fencing, and restriction of food and water production in the polluted zone. UNEP/FAO note that most frameworks on soil remediation have the following guidelines: Define the remediation objectives. Design the remediation strategy. Implement the remediation strategy. Finalize including monitoring and long-term aftercare. Defining the objectives is the goal of site assessment and site investigation. These may be done with techniques such as soil vapor probes and groundwater monitoring wells. Who is liable and responsible for the contamination and who pays needs to be determined at this stage. Remediation strategy design is dependent on many factors determined through assessment. All regulatory considerations and available technologies should be evaluated. Procurement, on-site health and safety, waste disposal, and monitoring need to be considered in the design. Close monitoring and management during the implementation phase of remediation is required. Finalization depends on the goals of remediation, whether they were to eliminate contamination and return of the site for normal use or they were to isolate the contamination. Isolated contamination will require more and longer monitoring. The graphic below shows the stages of remediation of a contaminated site. 

 

Soil Remediation Technologies

    Soil remediation technologies are classified in two ways. One is that they will be either biological treatment methods or physical/chemical treatment methods. Remediation technologies may also be in situ or ex situ methods. The soil remains in the ground for in situ methods but is excavated and stored elsewhere for ex situ methods. A recent example of ex situ remediation is the February 2023 Ohio train derailment and chemical spill where contaminated soil was excavated and transported to special hazardous waste landfills. Where applicable, biological-based in situ methods can be the cheapest and most effective soil remediation technologies. However, in situ treatment can take longer to remediate and bioremediation is only applicable to certain contaminants. The table below compares in situ and ex situ advantages and disadvantages.

 

 

 

In Situ Biological Treatment

 

     There are many methods of biological-based or nature-based remediation in situ remediation. Microorganisms, soil macroorganisms (such as earthworms), or plants may be utilized. Nature-based techniques can successfully remediate soils contaminated with petroleum hydrocarbons, chlorinated solvents, polycyclic aromatic hydrocarbons, pesticides, and trace elements. Bioremediation typically refers to a nature-based in situ method that utilizes microorganisms to break down and degrade organic pollutants. Aerobic or anaerobic bacteria may be used depending on the pollutants and circumstances. Aerobic bacteria are used to bioremediate non-chlorinated or slightly chlorinated hydrocarbons while anaerobic bacteria are used for more heavily chlorinated hydrocarbons. Bioremediation may include biostimulation, where the soils microorganisms are stimulated to better degrade the pollutants, or bioaugmentation, or enhanced bioremediation, where other microbes are introduced to degrade the pollutants. Soil properties can be enhanced by nutrients, pH adjustments, and temperature control to make bioremediation more effective. Bioventing and biosparging work by adding air to the soil to enhance aerobic decomposition reactions. Bioventing is air injection into the vadose zone above the water table. Biosparging is air injection into the saturated zone below the water table. These are used to remediate less volatile hydrocarbons such as refinery spills and leaking underground storage tanks. They may require filtration and vapor extraction in addition. Phytoremediation is an in-situ nature-based method that utilizes plants. It has been used extensively with trace element pollution and has had some success with organic pollution as well. The graphic below shows the different processes that occur during phytoremediation. Chelating agents may be applied to increase trace element mobility. Soil pH adjustments can increase or decrease the mobility of trace elements. Electokinetics has also been used to enhance the mobility and uptake of trace elements. Phytostabilization utilizes contaminant-tolerant plants to consolidate and immobilize polluted soils to prevent the spreading of the contaminants by wind and water erosion. Phytoextraction involves plants that hyperaccumulate certain trace elements, often heavy metals. These non-food chain plants can then be further processed as biofuel, further composted, or even phytomined or agromined, where the metals are extracted for use. This is known as farming “metal crops.” Phytoextraction is also used to desalinate oversalinated soils. It is also used to mitigate acid mine drainage from iron mines. Phytovolatilization involves specialized plant enzymes that can alter and volatize inorganic or organic contaminants in soil. This method has been used to volatize mercury into the atmosphere where it is less concentrated than in soil. Rhizodegradation is the breakdown of organic contaminants in soils by fungal and microorganism activity associated with the root zone. This method has been used to remediate PAHs. Vermiremediation utilizes earthworms. Earthworms can be tolerant to soil chemicals and have the ability through time to remove trace elements, pesticides and lipophilic organic contaminants, including polycyclic aromatic hydrocarbons (PAHs).

 

 

Ex Situ Biological Treatment

     Ex situ biological treatment requires excavation of the soil and treatment either on-site or at a special soil treatment facility. Bioremediation processes are similar to those of in situ treatments, but they can be better controlled and monitored and can be faster and more effective. Biopiling is a technique of taking a chunk of excavated soil, homogenizing it and optimizing its bioremediation. Bioleaching, according to UNEP/FAO “is an extractive technology that uses a solution inoculated with microorganisms to leach trace elements from polluted soils. It is also a process that is used in the mining industry as an alternative to cyanide to extract metals from low-grade ores or mine wastes.” The inoculated liquid is tricked over a pile of excavated soil. The leachate may be collected and taken for disposal. The method is efficient and cost-effective. Composting is an aerobic biological ex situ process that is often effective. Churning, addition of nutrients, and aerating all help to optimize composting. Landfarming involves transporting excavated polluted soil to a landfarming site and spreading it in a thin layer onto biologically active land or an impermeable surface. The polluted soil is ploughed into the soil surface. Sometimes organic amendments are added to enhance the process. Bioreactors can speed up the process of bioremediation with additives, temperature control, aeration, and flocculation. They can be used in aerobic and anaerobic decomposition methods. Both landfarming and bioreactors have successfully remediated hydrocarbon-contaminated soils. Ex situ remediation is shown in the model below.

 

In Situ Physical Treatment

     Physical and chemical treatments are generally more aggressive and work faster than biological treatments. Electrokinetic separation is a remediation method that “uses electrical fields and electrochemical processes to enhance the migration of polar inorganic and organic molecules out of soil. It is useful to increase the extraction rates for contaminants in low permeability soils.” It has been used in conjunction with biological methods such as phytoextraction. Soil vapor extraction and air sparging are similar to bioventing and biosparging but rely only on air, sometimes heated air, to volatilize contaminants in order to decrease their concentration in the soil. These methods are most effective in soils with high porosity and are ineffective in clays. Extraction pumps are used to provide a vacuum that draws air through the soil. In air sparging as in biosparging, air is injected below the groundwater table and volatiles are released as bubbles. In essence the volatiles are vacuumed out as shown in the graphic below.  Multiphase extraction is a variation of the process where the vacuum extracts air, vapors, liquid contaminants, and polluted groundwater.

 

In Situ Chemical Treatment

     Chemical treatment seeks to eliminate contaminants or to change their redox status. Making them more readily able to be eliminated or stabilized. Solidification/stabilization is one method to reduce contaminant mobility through precipitation, complexation, and/or adsorption reactions. Inorganic stabilization agents include soluble silicates, zeolites, lime, phosphates, and sulfur-based binders. Organo-clays are used to stabilize organic chemicals. Cementitious processes using lime or cement may be used to bind inorganic contaminants with high concentrations. Activated carbon and biochar can also be used in solidification/stabilization to reduce the bioavailability of trace elements and organic contaminants and can assist in returning polluted soil to agricultural use. Biochar can immobilize trace metals by neutralizing acids and increasing in soil cation exchange capacity. The carbon is plowed into the soil. Oxidation involves injecting oxidizing agents into polluted soil or to making a permeable barrier to control contaminant migration. This technique has been very successful with organic contaminants such as trichloroethylene and benzene. It also works very quickly. Local soil geology influences effectiveness. In situations where contaminants are confined by impermeable zones adjacent to permeable zones, the contaminants can collect along the impermeable zone so that the oxidizing agents can achieve ideal access. One downside of this method is that in some circumstances the contaminant levels can ‘rebound’ by re-occupying permeable zones. Reduction refers to a series of chemical reactions that commonly occur in saturated soils in a sequence. Dechlorination of organochlorine compounds and conversion of trace elements to a less toxic state are useful methods that rely on reduction reactions. As in oxidation, the reducing agents are injected into the soil. Dechlorination and converting hexavalent chromium to the less toxic and less mobile trivalent state are two of the main uses of reduction as an in situ chemical treatment method.

 

In Situ Thermal Treatment

     In situ thermal treatment can involve several different heat sources and ranges including electrical resistance heating, steam injection and extraction, conductive heating, radio-frequency heating, and vitrification. The main uses are to enhance biological and/or chemical degradation activities in the soil. Vitrification is a special method to turn the polluted soil into a stable form of glass through high heat. Thermal treatments enhance vapor releases, so covering the soil and vapor extraction are often employed. In pile thermal desorption involves the construction of a sealed treatment area at a location close to the polluted soil. The polluted soil is excavated and placed in the pile along with the heating and extraction network. Steam enhanced extraction is used with injection and extraction wells. Steam mobilizes contaminants as liquid and vapor that can then be extracted. Conductive heating utilizes electric heaters and may be used where contaminants are very close to the surface. Radio frequency heating, a high frequency AC method, may also be utilized but requires sufficient soil moisture. Vitrification is a special method of thermal treatment for radioactive waste. It is expensive due to the energy requirements. Electrical resistance or plasma arc technologies may provide the heat. It can be done in situ in cells or ex situ.

 

Nanoremediation

      Reactive nanomaterials can enable both chemical reduction and catalysis to mitigate contaminants. The ability of nanomaterials to pervade small spaces renders them suitable for in situ remediation. Nanomaterials utilized include nanoscale zeolites, metal oxides, carbon nanotubes and fibres, enzymes, various noble metals, mainly as bimetallic nanoparticles (BNPs), and titanium dioxide. Nanoscale zerovalent iron (nZVI) is currently the most widely used nanomaterial for remediation.

 

Ex Situ Physical Treatment, or Separation

     This type of treatment may separate soil particles by grain size to enhance remediation. Smaller particles such as clays require more work to remediate. The cleaned larger particles such as sand can be returned to the site. Gravity separation takes advantage of the different settling velocities of particles. Coagulation and flocculation are often used to enhance differential settling. Magnetic separation is used to attract magnetic particles from a slurry. It is used for uranium and plutonium contamination. Sieving, or physical separation involves using different screen sizes to filter out larger particles. Soil washing is a means to flush out contaminants with water. Soil washing may be physical or chemical. Physical methods involve grain size and settling velocity separation with minerals processing equipment. Chemical methods involve the use of aqueous solutions of acids, alkalis, complexing agents, other solvents and surfactants. Soil washing is often used as a first step in ex situ treatments. Solidification/stabilization is used in ex situ treatments as in in situ treatments, but the treated soil often requires disposal in special hazardous waste disposal sites.

 

Ex Situ Chemical Treatment

    This technique often involves chemical extraction by an extractant that is added to the soil washing recipe.  Chemical extraction is often used to further separate contaminants following their concentration by physical separation. The extraction methods include dissolution in strong inorganic acids, forming complexes with chelating agents, and dissolution in organic solvents. Chemical reduction/oxidation are used ex situ as they are in situ. Dehalogenation involves processing and heating the soil, followed by decomposition and/or volatilization of contaminants.

 

Mechanical-Chemical Treatment

     This may involve the use of ball mills to treat soil contaminated with organochlorine contaminants and other POPs. One example of pesticide remediation utilized “a vibratory mill with two horizontally mounted cylinders containing a grinding medium. The grinding medium provided the mechanical impact energy required to drive the chemical reaction.” The technique can also be used as a means of dechlorination.

 

Ex Situ Thermal Treatment

     Ex situ thermal treatment thermal desorption is a common method as it is in in situ thermal treatment. In pile thermal desorption “involves the construction of a sealed treatment area on a location close to the polluted soil. The polluted soil is excavated and placed in the treatment area along with the heating and extraction network. The pile is sealed with an extraction cover with the extracted contaminants recovered, filtered or destroyed in a catalytic oxidizer.” Thermal desorption plants are rotary heaters that are continuously fed with polluted soil. The heat drives off the volatiles. These plants are operated anaerobically under a vacuum or with an inert carrier gas to ensure that the volatilized gases do not combust in the desorption unit. The volatiles can be collected or combusted depending on the contaminants. Thermal desorption units can be set up on-site to avoid transporting contaminated soil. In pile thermal desorption involves construction of a sealed treatment area near the polluted soil. Cement kilns may be utilized as an ex-situ thermal treatment where organic contaminants are oxidized, and the non-volatile trace elements and soil minerals are incorporated into the cement. Soil is typically pre-homogenized and pre-processed. The volatiles are burned with the other fuels at the cement kiln so that the combustion products are more diluted. Only certain soils are suitable. High-temperature Incineration involves burning the soil at high temperatures and has been long used to destroy toxic waste. Pre-treatment for incineration often involves physical separation and thermal desorption.

 

Sequestration

     Sequestration of polluted soil involves transporting it to a specially engineered landfill suitable for holding hazardous waste. This ‘dig-and-dump’ approach is used with radioactive contamination. Pre-treatment in the form of stabilization is often a requirement. The landfills are specially designed to prevent leachate from escaping except via the leachate collection system.

 

Soil Pollution Management

     Longer-term containment measures at some sites may include liners, capping with non-contaminated soil, and phytomanagement where agricultural crops are banned on previously contaminated sites, are common methods of management. Non-agricultural use only designations are common at so-called ‘brownfield’ sites. Another method has been to use brownfield sites as sites for renewable energy deployment. Capped landfills or abandoned mine works can be used for solar farms.  

 

Role of the U.S. EPA

Since its inception, the U.S. EPA has been involved with cleanup up brownfield sites. The EPA Superfund is a fund set aside for the cleanup of the most toxic polluted sites which are often expensive to remediate. The EPA keeps track of new remediation technologies, including so-called green remediation technologies. All applicable regulations such as the Resource Conservation and Recovery Act (RCRA), which regulates hazardous and non-hazardous waste and the Comprehensive Environmental Response, Compensation, and Lability Act of 1980 (CERCLA) are administered in the U.S by the EPA. Thus, regulatory oversight of all soil remediation is required.

 

Sustainable Remediation

      Industrialized countries tend to have the most brownfield sites. As mentioned, the EPA promotes green remediation. A 2023 paper in Nature Reviews Earth & Environment observed that “conventional remediation strategies, such as dig and haul, or pump and treat, ignore secondary environmental burdens and socioeconomic impacts; over their life cycle, some strategies are more detrimental than taking no action. Sustainable remediation technologies, such as sustainable immobilization, low-impact bioremediation, new forms of in-situ chemical treatment and innovative passive barriers, can substantially reduce the environmental footprint of remediation and maximize overall net benefits.” They also note that sustainable methods reduce greenhouse gas emissions and can be integrated with nature-based redevelopment and sustainable energy systems. The graphic below from the paper shows a socioeconomic and financial comparison model of different remediation technologies and their relative environmental impacts.

Socioeconomic and Financial Model of Remediation Technologies and Their Impacts. Source: Sustainable remediation and redevelopment of brownfield sites. Deyi Hou, Abir Al-Tabbaa, David O’Connor, Qing Hu, Yong-Guan Zhu, Liuwei Wang, Niall Kirkwood, Yong Sik Ok, Daniel C. W. Tsang, Nanthi S. Bolan & Jörg  Rinklebe. Nature Reviews Earth & Environment volume 4, pages271–286 (2023). Sustainable remediation and redevelopment of brownfield sites | Nature Reviews Earth & Environment

 

A New Thermal Remediation Method Involves Electrical Pulses Which Superheat Soil

     Rice University researchers in conjunction with U.S. Army engineers have been researching a method using electrical pulses that heat contaminated soil to very high temperatures of between 1,832- and 5,432-degrees Fahrenheit. Non-toxic chemicals are added to propel the electrical pulses. An interesting side effect of the new tech is that soil fertility increases of 20-30% are being reported post-treatment. This method can be done in situ or ex situ and can remediate very quickly. The process does not use water and can mitigate different types of pollutants. The researchers describe this tech as ‘very promising.’

FAO/UNEP Report Reccomendations: 

• "Harmonise standard operating procedures for laboratory methods of soil contaminants analysis and develop standardized threshold levels of soil pollution.
• Promote the inclusion of soil pollution into conventional soil surveys, and the inclusion of data and information on soil pollution into national and global soil information systems.
• Promote the establishment of the Global Soil Pollution Information and Monitoring System.
• Increase the investment in targeted research and innovation on emerging contaminants: detection, fate in the environment, risks assessment and remediation.
• Develop and strengthen the inventory and monitoring of point-source and diffuse soil pollution at national, regional and global levels.
• Establish and strengthen national biomonitoring and epidemiological surveillance systems to identify, assess, and monitor damage and diseases attributable to soil pollution and support preventive actions."

 

References:

Global assessment of soil pollution: Report. FAO and UNEP, 2021. About this Publication (fao.org)

Scientists develop technology to ‘zap’ pollutants out of soil rapidly: ‘An incredibly promising technique’. Rick Kazmer. The Cool Down. November 11, 2023. Scientists develop technology to ‘zap’ pollutants out of soil rapidly: ‘An incredibly promising technique’ (msn.com)

Sustainable remediation and redevelopment of brownfield sites. Deyi Hou, Abir Al-Tabbaa, David O’Connor, Qing Hu, Yong-Guan Zhu, Liuwei Wang, Niall Kirkwood, Yong Sik Ok, Daniel C. W. Tsang, Nanthi S. Bolan & Jörg  Rinklebe. Nature Reviews Earth & Environment volume 4, pages271–286 (2023). Sustainable remediation and redevelopment of brownfield sites | Nature Reviews Earth & Environment

Remediation Technologies for Cleaning Up Contaminated Sites. U.S. EPA. Remediation Technologies for Cleaning Up Contaminated Sites | US EPA