PAH Analysis: The complete guide to understanding, detecting and treating soil pollution by polycyclic aromatic hydrocarbons

PAHs , or Polycyclic Aromatic Hydrocarbons, are organic chemical compounds formed by the incomplete combustion of fossil or organic materials (oil, coal, gas, wood, etc.).

They can be found in particular in:

• exhaust fumes,
• fuel and hydrocarbon leaks,
• industrial waste,
• atmospheric deposition,
• and sometimes in common products such as tars or certain pesticides.

PAHs are much more than just an industrial residue: they are persistent pollutants capable of transforming land into a veritable ecological time bomb. Why?

1. Proven toxicity : several PAHs, such as benzo[a]pyrene, are classified as carcinogenic (IARC).
2. Persistence : they degrade very slowly and accumulate in soils and groundwater.
3. Bioaccumulation : they can end up in the food chain via plants and animals.
4. Resource contamination : rendering soil unsuitable for construction, agriculture, or even industrial use without prior treatment.

In other words, without PAH analysis , it is impossible to have a clear view of the extent of pollution by hydrocarbons and other soil pollutants present on a site.

One might think that this diagnostic only concerns large industrial companies, but that's wrong. PAH analysis is essential in many cases:

• Selling or buying land : to avoid buying polluted land that will be expensive to clean up.
• Construction site : to ensure that the ground is sound before housing or offices are built.
• Rehabilitation of industrial brownfield sites : often heavily contaminated with hydrocarbons and heavy metals.
• Regulatory control : to comply with environmental obligations.
• Accidental pollution : after a fuel spill, for example.
• Vegetable garden and cultivation: to ensure there is no risk of contamination.

Beyond the legal obligation, it is primarily a strategic decision-making tool . Knowing the level of contamination helps avoid enormous additional costs for unforeseen remediation work. It also prevents any health risks to humans and living beings.

In France, as in Europe, the monitoring of PAHs is governed by several standards and directives. Among the most important are:

• Directive 2004/107/EC : limit values ​​for benzo[a]pyrene in air.
• AFNOR and ISO standards : for the sampling and analysis of soils, sediments and water.
• BASOL and BASIAS plans : databases that list polluted sites and require regular monitoring.

The permissible limit values ​​for PAH concentration in soil vary according to the intended use of the land, due to the different levels of health and environmental risk associated with it.

• For agricultural or residential land , the standards are very strict. For example, the maximum permissible concentration of benzo[a]pyrene, a particularly toxic PAH, can be set at around 0.1 mg/kg of dry soil . Any exceedance requires immediate remediation or containment action to protect the health of residents and crops.
• For industrial sites , the thresholds are generally higher because direct human exposure is less frequent. For example, the limit may be set at 1 mg/kg of dry soil for benzo[a]pyrene. However, these sites remain subject to regular monitoring to limit the dispersion of pollutants and environmental risks.

Wondering how experts detect the presence of PAHs in soil? As with total hydrocarbon analysis , PAH analysis follows a precise protocol to guarantee reliable results. Here are the four key steps, from site investigation to concrete solutions.

Before carrying out any sampling campaign, it is essential to conduct a preliminary study of the site .

This step aims to:

• Collect the history of the land : know the past uses (e.g. agricultural operation, presence of a former garage, industrial or craft site, illegal dumps, etc.).
• Identify potential sources of pollution : buried fuel oil tanks, hydrocarbon storage areas, presence of soils with an unusual appearance (blackened areas, odors, abnormal vegetation).
• Analyze the topography and hydrogeology of the site : slope, proximity to watercourses, groundwater or wells that may be impacted.
• Determine the area to be investigated and segment the area according to the use or identified risks (parking area, storage area, agricultural area, etc.).

Based on these elements, a sampling plan is established, specifying:

• The number of sampling points depending on the surface area (for example, a systematic grid or points targeted on suspicious areas).
• The sampling depth (e.g., surface horizon 0-30 cm for agricultural pollutants, or up to 1-2 m for industrial pollution).
• The type of equipment to use (manual auger, mechanical shovel, probe).
• Safety and traceability conditions (sampling form, geolocation of points).

Soil samples must be collected in accordance with the sampling plan, in order to accurately represent the condition of the site .

Best practices:

• Take at least 4 samples at different locations in the area to be analyzed in order to have good representativeness.
• Use a strict sampling protocol to avoid cross-contamination (systematic cleaning of equipment between each point, wearing clean gloves, use of sterile containers).
• Identify and label each sample correctly (date, depth, GPS location).
• Store the samples under appropriate conditions (temperature, protection from light) before sending them to the laboratory.

Once the soil samples have been taken, they are analyzed in an accredited laboratory to determine the presence and concentration of polycyclic aromatic hydrocarbons (PAHs) .

This step includes several technical phases:

3.1. Extraction of PAHs

• PAHs are extracted from the solid matrix (soil) using appropriate organic solvents (such as hexane or dichloromethane).
• Modern microwave-assisted extraction (MAE) or Soxhlet extraction techniques are often used to improve efficiency and reduce extraction time.
• The extract obtained is then concentrated and purified (removal of interferences, filtration) to prepare for instrumental analysis.

3.2. Quantification of PAHs

• The concentration of PAHs is measured using chromatography techniques coupled with sensitive detectors , primarily:
  • GC-MS (Gas Chromatography coupled with mass spectrometry) : ideal for volatile and semi-volatile compounds.
  • HPLC (High-performance liquid chromatography) with UV or fluorescence detector: suitable for heavier PAHs.
• Multi-level calibrations are performed to guarantee accuracy (calibration curves with certified standards).

3.3. Identification of the 16 priority PAHs (US EPA)

The laboratory specifically targets the 16 PAHs considered priorities by the US Environmental Protection Agency (US EPA), including:

• Naphthalene (C₁₀H₈)

• Chrysene (C₁₈H₁₂)

• Benzo[a]pyrene (C₂₀H₁₂, known for its carcinogenic properties)

  These compounds are being sought because they present significant health and environmental risks .

3.4: Report and recommendations

Following the analyses, the raw and interpreted results ( concentrations of each PAH in mg/kg of dry soil) are compared to national regulatory threshold values ​​(e.g., thresholds of the French decree on polluted sites and soils) or, failing that, to international guideline values ​​(US EPA, WHO).

If concentrations exceed critical values, a remediation plan is recommended (excavation and treatment of soil, thermal desorption, bioremediation).

If the cleanup is too complex, a containment and monitoring plan can be put in place (geomembrane, cover, regular monitoring of groundwater).

The final report must include :

• The analytical methods used and their validation (ISO standards, limits of detection).
• The results are presented in tables and concentration maps .
• Concrete recommendations for the project owner (remediation, monitoring, risk management measures).