Glossary

Risk analysis aims to identify the effects on human health resulting from prolonged exposure to the action of substances present in contaminated environmental matrices. By calculating risk threshold concentrations (CSRs), it allows for the determination of whether a potentially polluted site can be classified as contaminated or non-contaminated.

Hydraulic barriers are one of the most commonly used systems for reducing aquifer contamination. This technology involves the use of extraction wells (or “barrier wells”), which draw in the contamination plume, preventing its migration into the surrounding environment. Hydraulic barriers are often paired with a system for treating the water extracted, hence the term Pump & Treat. Following treatment, the water can be discharged into the sewer system or a surface water body or reintroduced into the aquifer through reinjection wells. Unfortunately, as it is not possible to reduce the pollutant below a certain equilibrium value, which is often higher than the concentration limits, the effectiveness of the treatment diminishes after the initial stages. For this reason, the Pump & Treat system is generally used as a safety measure, in conjunction with other soil and aquifer remediation interventions.

Bioremediation is a remediation technology that exploits the natural ability of microorganisms present in the subsoil, or specifically introduced, to degrade organic pollutants. By ensuring optimal conditions for the growth of microbial colonies (pH, temperature, redox potential, oxygen levels, nutrients, etc.), they are used to convert organic compounds into carbon dioxide, water and/or biomass.  
 
Inorganic contaminants and metals on the other hand cannot be biodegraded, but the action of microorganisms can in any case alter their valence state and thus lead to their absorption, immobilisation in the soil, precipitation, accumulation, and concentration within the biomass, resulting in a reduction of concentrations in the affected environmental matrix.  
 
The most commonly applied active bioremediation strategy is enhanced bioremediation (biostimulation), a technique aiming to stimulate biodegradation processes by adding nutrients (nitrogen, phosphorus, etc.) and/or other reagents (oxygen, lactate, etc.). This provides the microorganisms with the energy necessary to grow and carry out the processes of degradation.  
 
The principal bioremediation technologies are:

• bio-venting, involves injecting air into unsaturated soils to increase oxygen levels and stimulate the biodegradation activity of indigenous bacteria;
• bio-sparging, employs indigenous microorganisms to biodegrade organic components in the saturated zone, through air injections;
• bio-slurping, combines elements of bio-venting and vacuum-enhanced free-product pumping, in order to recover it from the ground water while simultaneously stimulating the degradation processes of the indigenous microorganisms in the unsaturated zone;
• bioaugmentation, microorganisms selected based on indigenous populations or isolated in the laboratory for their ability to degrade specific contaminants (allochthonous bacteria) are introduced into the existing system to accelerate the biodegradation processes.

Remediation is an intervention to eliminate sources of pollution and pollutants present in environmental matrices, or to reduce their concentrations to values equal to or below the risk threshold concentrations.

Contamination threshold concentrations (CSCs) represent the levels of contamination in water and soil above which a site is considered potentially polluted, necessitating its characterisation. This is a series of investigative activities that enable the reconstruction of contamination phenomena.

Risk Threshold Concentrations (CSRs) represent the site-specific levels of acceptable contamination, based on its intended use, to be determined through risk analysis. Exceeding these values necessitates a safety and remediation intervention. The aim of the remediation intervention is to achieve the decontamination of the areas.

Thermal desorption exploits soil heating to cause the vaporisation of volatile and semi-volatile contaminants, facilitating their subsequent extraction and treatment. The soil is not destroyed by the treatment, but retains many of its physical properties. 
 
The soil can be heated using steam injections, microwave irradiation of soil, energisation with electromagnetic waves, electric heating elements and burners.  
 
Starting from the injection point, the heat front spreads throughout the area to be treated, causing the evaporation of water and the volatilisation of contaminants contained in the soil, which are then recovered through the extraction points and treated on the surface.  
 
The intervention area must be isolated using appropriate covers, in order to minimise heat loss, optimise the extraction of contaminants using recovery systems and prevent the infiltration of rainwater which would affect the propagation of the heat front.

Phytoremediation technologies are tools for environmental restoration that exploit the ability of herbaceous plants and tree species to absorb not only nutrients, but also other molecules that can be accumulated or metabolised by the plants themselves.  
 
The use of plants enables the extraction of pollutants from the subsoil and their accumulation in plant tissues, or their decomposition, or the modification of the characteristics of pollutants and the subsoil, creating a more favourable environment for the degradation of heavy metals, organic compounds, and radioactive substances.  
 
At the end of the treatment, the plant biomass is collected while the aerial part, rich in metals, is separated from the roots and disposed of in a landfill, or is sent for incineration. 
Some applications of phytotechnologies are: 

• phytobarriers, they mitigate the spread of contamination in the air, thanks to the filtering action of the aerial parts, or the surface ground water, thanks to the action of the roots; 
• phyto-dehydration, the transpiration of the plants achieves a gradual dehydration of the substrate, reducing the risk of contamination; 
• phytocapping, is a system that uses soil and vegetation as a surface barrier to control percolation. It is used in landfills as an alternative to traditional systems. 

Groundwater Circulation Wells are designed to remove contamination from ground water by generating a vertical recirculation flow. Ground water is withdrawn and re-injected into the ground after being treated, thanks to special screens. Contaminants are extracted in the vapour phase through stripping, in a closed-loop circuit without atmospheric emissions, while the liquid flow is treated with ultraviolet rays. This technology is highly effective on secondary, persistent, sources of contamination.

Measures to prevent the spread of contamination beyond the site while awaiting remediation or safety measures. They are carried out before or during the characterisation phase.

Interventions carried out at a site with ongoing activities to ensure an adequate level of safety for people and the environment, pending further permanent safety measures or remediation to be implemented upon cessation of the activity.

Interventions aimed at definitively isolating polluting sources from the surrounding environmental matrices and ensuring a high level of protection for health and the environment.

The conceptual model is a representation of a site's contamination scenario. It helps to understand how and where the contamination originated, identifying the sources, the most hazardous compounds, and the affected matrices (soil, ground water, surface water). Furthermore, it enables the reconstruction of contamination migration paths, identifying which targets may be exposed, based on land use.   

Replacement drilling is a technique that involves large diameter (2 m approx.) perforations, with the excavation advancing progressively under the protection of a temporary rigid covering, anchored in jointed elements. The activity is conducted in such a manner as to ensure the complete remediation of the land: after the drilling is completed, the extracted material is unloaded directly into the bed of a vehicle or a big bag, and the hole is filled with compliant and certified material. Extraction of the covering then follows.

Excavation involves the mechanical removal of contaminated soil and its subsequent disposal or treatment at a suitable authorised facility. Off-site disposal normally involves the creation of a temporary on-site storage facility and on-site characterisation, in order to adequately classify the excavated soil and assign the corresponding EWC (European Waste Catalogue) code. 
 
The excavation of contaminated soil generally follows these operational phases:  

• construction of temporary structures to support excavation faces (e.g. sheet piles);  
• excavation using a mechanical excavator, conducting a preliminary selection to enable a qualitative separation of the sections with signs of contamination from those without such signs;  
• storage of heaps in temporary storage areas within the site;  
• characterisation of the heaps through sampling and laboratory analysis, followed by assignment of EWC codes;  
• transport and disposal of waste and contaminated soil at a suitable authorised facility;  
• inspection of the walls and the bottom of the excavation;  
• restoration of the excavation with certified backfill material. 

The sources of contamination are categorised variously as:  
 

• primary, which have caused the contamination, such as waste mixed with soil, tanks, lagooning, heaps, etc.;  
• secondary, themselves impacted by contamination and through which contaminants migrate, such as surface or deep soil and aquifer, etc.