Decontamination of soil and groundwater: what is it and how is it done?

The remediation of contaminated soils is applied to facilities with levels of contaminants that are likely to constitute a risk to health and the environment, in order to revert the contamination to acceptable levels and return the damaged resources to their former state.

Soil and groundwater remediation is a specialised technical process consisting of several consecutive steps and, if necessary, ending with the decontamination of the facility. The process involves identifying and characterising contaminated soils using a standardised and technically rigorous methodology.

1. Analytical characterisation of soil and groundwater

In this initial phase we carry out a detailed analysis of the site to determine whether it is contaminated and, if so, what type of contamination affects it, as contaminating substances are classified according to their danger to human health and ecosystems. The sampling strategy determines the location of the sampling points, their depth and the analytical parameters. After sampling and analysis in the laboratory, we will write a report with the assessment of the results and the conclusions of the characterisation. 

2. Quantitative analysis of health and environmental risks (QRA)

When the analytical characterisation shows that the reference limits established by the legislation are exceeded, we will carry out a quantitative analysis of the risks that the contaminants found may pose. 

 

Using a conceptual model that takes into account pollutants, physical environment, exposure pathways and receptors, we will propose the various risk scenarios and the quantification of this risk, to determine whether it is acceptable or unacceptable and, where appropriate, the target values for remediation.

3. Remediation project
 

The remediation project is prepared on the basis of the outcome of the RCA, taking into account the contaminants found, the type of facility and the decontamination target values established in the previous phase. 
 

The project details the remediation techniques to be used, which may be:

 

  • Ex situ: 
    • Selective soil stripping
    • Ex situ bioremediation
  • In situ:
    • Two-phase (gas/liquid) extraction - high vacuum
    • Chemical oxidation
    • Subsurface vapour extraction: Air sparging and bio-slurping
    • Land-farming, Bioremediation and biopiles
    • Phytoremediation
    • Monitored Natural Attenuation

 

Preferably, we will focus land remediation on eliminating contamination hotspots and reducing the concentration of contaminants and prioritise on-site treatment techniques that avoid the generation, transfer and disposal of waste.

4. Remediation of contaminated soils and groundwater
 

In the actual decontamination, specific techniques are applied in each case. We are going to see some of the techniques we have applied in real cases: 

 

Selective soil removal (ex situ decontamination).

 

Removal of contaminated soil, including extraction and management or treatment of affected groundwater and ensuring on-site selection and segregation between non-impacted, non-hazardous and hazardous materials, in order to maximise economic resources by using the most appropriate method according to the type of soil. 

 

On-site measurement devices allow us to assess soil quality, shortening decision-making times.

Two-phase extraction (gas/liquid) - high vacuum (in-situ decontamination)

 

Gas/liquid extraction by vacuum systems, water treatment by primary decantation and phase separation and gas phase treatment by activated carbon filters or air stripping.

 

Extraction of vapours from underground

 

Extraction of volatile organic compound vapours by subsoil suction systems using vacuum (high vacuum - depressants). The extracted compounds are retained in activated carbon modules or treated by tower stripping. 
 

 

In situ chemical oxidation (ISCO)

 

Injection of selected chemicals into groundwater to oxidise the pollutant and degrade it to harmless compounds.

Land-farming, bioremediation and biopiles

 

Promotion of the degradation of pollutants through the action of degrading micro-organisms. The proliferation of resident organisms in the soil itself is stimulated or specific micro-organisms (bacterial consortia) are added to the soil. In addition, other actives are added to help the process, such as nutrients, oxygen (in case aerobic degradation is more effective), metals, etc.

 

The aim is to promote the degradation of the pollutant without the need for costly excavation and without having to extract the affected groundwater.

 

Phytoremediation

 

Use of the capacity of plants to reduce or eliminate pollutants from the soil. The system is useful in the final stages of remediation, in soils or even in surface water (green filters). It acts on metallic elements or on specific organic pollutants, in this case as agents favouring the proliferation of degrading organisms.

 

Soil flushing

 

Washing of the unsaturated zone of the soil by injection of surfactants.

 

More information on soil and groundwater decontamination

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