Organic pollutants in soil

Dioxins and furans (PCDD/PCDFs), polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) are organic pollutants that can persist in the environment and bioaccumulate, thus constituting a threat to human health and the environment. These substances are organic compounds mostly of anthropogenic origin, characterised by high lipo-affinity, semi-volatile and resistant to degradation. These characteristics make them extremely persistent in the environment and capable of being transported over long distances.

These substances tend to bio-concentrate and exhibit a bio-magnification process, thus reaching potentially toxicologically relevant concentrations. Bio-magnification, i.e. the accumulation of increasing quantities as they pass from prey to predators, occurs via the food chain, most readily for decidedly lipophilic compounds and is most evident in terminal predators.

These organic pollutants are released into the environment from numerous sources, show some mobility between different environmental matrices, have a stable chemical structure and a considerable half-life. They can cause persistent, almost ubiquitous pollution and accumulate during particular events (ARPAV, 2016).

Since agricultural soils represent a particularly ‘undisturbed’ environment, they can serve as a basis for establishing reference values against which to compare situations subjected to greater impact (urban soils, industrial areas, areas near incinerators and/or cement plants, etc.) in accordance with national legislation (Legislative Decree no. 152/2006 as amended by Ministerial Decree 46/19).

Since 2017, the Geology, Soils and Seismic Area of the Emilia-Romagna Region, in collaboration with the Energy, Waste and Contaminated Sites Observatory of the Regional Enviroment Agency (ARPAE), has been carrying out the project ‘Acquisition of a cognitive framework of the background levels of some organic compounds in the soils of the Emilia-Romagna plain’ aimed at acquiring and implementing knowledge on the quality of the soils of the regional plain, with specific reference to the content of dioxins, furans, PAHs, PCBs in the soils. The aim of the project was to identify reference values for organic micropollutants in agricultural soils, linked to diffuse sources of pollution as defined by the ISO19258:2018 standard, namely:

• atmospheric fallout;
• spreading of sludge, compost and other organic soil improvers;
• spreading of sediments from flooding or excavation of watercourses.

Sampling Network

Sampling was carried out in rural areas adjacent to air quality monitoring stations (ARPAE), to ensure that the background levels were representative. Eight stations, each with 10 sampling points, were investigated during the summer and winter seasons, resulting in a total of 162 samples analysed by the ARPAE Laboratory in Ravenna. The selected sites are located within different soil-landscape units in the plain and hills.

Figure 1. Rural stations of the air monitoring network and sampling points

Table 1. List of sampling areas and soil types surveyed

Every group of organic compounds was statistically processed on three different levels:

• Level 1: Analysis of the entire dataset with only one level of clustering (typically, by CLASS or analyte);
• Level 2: Analysis of dataset by applying two levels of clustering (typically, by CLASS and then by one of the two main factors ‘Season’ or ‘Grouping’);
• Level 3: Analysis of dataset by applying three levels of clustering (typically, by CLASS and then the combination of the two main factors ‘Season’ and ‘Grouping’) in order to obtain the greatest possible meaningful detail of information.

PAHs and PCBs: values below the limit of quantification

For PAHs and PCBs, the vast majority of values are below the limit of quantification (LoQ). Statistical analysis reveals no significant correlations with soil type, season or geographical location. The levels detected are several orders of magnitude below the regulatory CSCs.

Dioxins and furans (PCDD/F): sources, distribution and soil factors

Concentrations and regulatory comparison

Concentrations of OCDD (the dominant congener) in the investigated soils range from 17.9 to 60.6 pg/g (group median). The calculated I-TEQ values are all well below the statutory thresholds (10 ng I-TEQ/kg for green/residential land), confirming the nature of widespread rural background contamination not attributable to point sources.

Data analysis (NNLS-CMB)

The 4-source NNLS-CMB (Non-Negative Least Squares — Chemical Mass Balance) analysis was used to identify the origin of PCDD/Fs in the soil. The method compares the fingerprint measured at each site with the reference fingerprints of four known sources, determining the optimal mixing ‘recipe’. The four sources analysed are:

1. Historic preserved PCP — original fingerprint of technical pentachlorophenol (PCP) – (source: Brambilla et al., 2006);
2. Historic degraded PCP — fingerprint of PCP following microbial transformation (dechlorination) – (source: Masunaga et al., 2001);
3. Vehicular traffic — Cleverly 1997 profile (60% petrol / 40% diesel) – (source: Cleverly et al., 1997);
4. Municipal solid waste incinerators — (source: Brambilla et al. 2006).

Main finding: In all 16 group×season combinations, historical pentachlorophenol (PCP) (preserved + degraded) is the dominant source, accounting for between 71% and 100%. This is consistent with the widespread historical use of PCP in agriculture to treat vineyard posts and agricultural timber, as well as its use as a fungicide, herbicide and insecticide until the European ban (Council Directive 91/173/EEC).

Soil characteristics influencing the PCDD/F profile

Various soil parameters were studied to understand their impact on the dioxin fingerprint revealed by the analysis. There are three main factors governing the extent of PCP transformation in soils:

1. Total limestone — the primary discriminating factor. The strongest statistical correlation is between total limestone and the weathering of PCP. The effect is not continuous but acts as a threshold close to zero: when limestone is practically absent (<1%), microbial dechlorination proceeds uninhibited.

2. Organic matter — the most generalised driver. Organic matter is the parameter with the strongest correlation, present even in sub-alkaline soils. It provides the energy substrate for anaerobic dechlorinating bacteria.

3. Soil texture and hydraulic conductivity — a seasonal amplifier. At sites with high clay content and very low hydraulic conductivity (Ksat), winter waterlogging creates anaerobic conditions that amplify weathering.

Contribution from municipal solid waste incinerators

The 4-source model attributes a detectable contribution to municipal solid waste incinerators (Inc.RSU) in only two cases: Molinella in summer (6%) and Alfonsine in winter (8%), which are geographically consistent with their proximity to Hera Bologna FEA plant (24 km) and Hera Forlì plant (33 km). At all other sites, the contribution is ≤1.4%. The levels detected do not indicate a significant impact of regional waste-to-energy plants on the background levels of dioxins in agricultural soils.

Conclusions

Agricultural soils in the Emilia-Romagna plain contain dioxins in quantities that are normal for the European context. The source is a legacy pesticide that has been banned for over 30 years, which produces dioxins of very low toxicity. There is no identifiable health risk.

Reports (italian language)

• Acquisizione di un quadro conoscitivo dei livelli di fondo di alcune molecole organiche nei suoli della pianura emiliano-romagnola (2026)

• Diossine, furani, PCB e IPA nei suoli del Veneto (2016)