Organochlorine pesticides in the soils of the Acerra plain: concentration and distribution of DDT isomers and metabolites

Organochlorine pesticides (OCPs) are a group of synthetic molecules, consisting of organic structures containing at least one aromatic ring and one covalently bound chlorine atom, widely used for decades in agriculture (as insecticides and fungicides) and, subordinately, in the medical field.

The study area, corresponding to the Acerra-Marigliano conurbation, is located in the middle sector of the Campania Plain, a wide coastal belt roughly extending from the Garigliano River plain, in the northwest of the Campania region, to the Sarno River basin, southward of the volcanic complex of Mt. Somma-Vesuvius. Most of the study area is occupied by agricultural activities (crops, orchards and vineyard) and, subordinately, by industrial settlements and urbanized areas. A total of 33 surficial composite soil samples were collected across the study area, with an average density of 1 sample per 4 sqkm, to be analyzed for their OCPs content.

Compared to other synthetic organic pesticides, OCPs show greater environmental persistence and are generally characterized by a marked tendency towards bioaccumulation and biomagnification along trophic chains due to their lipophilic character. The covalent bond between carbon and chlorine in OCP molecules is very stable and resistant to microbial degradation, but some organisms and plants, together with some physical factors (including pH, solar radiation and humidity), can encourage their metabolic degradation.

The best-known OCP is certainly the dichloro-diphenyl-trichloroethane (DDT), which is one the OCPs defined as Persistent Organic Pollutants by the Stockholm Convention (2001), synthesized since 1873 and used as an insecticide and pesticide since the 1940s. DDT has been, and still is, used to combat malaria in some sensitive areas, such as Africa, India and South America, but its use has been banned in Italy since 1978. DDT can undergo degradation processes, including volatilization or photolysis, whose products are dichloro-diphenyl-dichloroethane and dichloro-diphenyl-dichloroethylene (DDD and DDE, respectively), two compounds with similar properties and, above all, highly persistent. The USEPA (2015) has classified DDT and its metabolites as probably carcinogenic substances for humans, also responsible for damage to the liver, reproductive system and nervous system.

This study aimed at investigating the local distribution, possible sources and contamination levels of DDT isomers and metabolites in the soils of the study area. In particular, we considered six compounds: p,p′-DDT; o,p′-DDT; p,p′-DDD; o,p′-DDD; p,p′-DDE and o,p′-DDE. Concentration of ΣDDTs (i.e. the sum of the six compounds) ranges from a minimum of 4.13 ng/g to a maximum of 734.75 ng/g.

The Italian decree 46/2019 establishes a guideline value for DDE, DDD and DDT concentrations in agricultural soils of 10 ng/g. This value is largely exceeded in the whole study area as regards p,p′-DDT and p,p′-DDE, for which the average concentration resulted equal to 70.37 ng/g and 75.94 ng/g, respectively.

O,p′-DDD and o,p′-DDE show very low concentrations throughout the study area with average concentrations of 1.44 ng/g and 0.75 ng/g, respectively; o,p′-DDT (mean = 5.67 ng/g) and p,p′-DDD (mean = 6.24 ng/g) overcome the guideline mostly in soils collected in the surrounding areas of the towns of Acerra, Brusciano and Marigliano.