Mass balance and decontamination times of Polycyclic Aromatic Hydrocarbons in rural nested catchments of an early industrialized region (Seine River basin, France)

Accumulation of Polycyclic Aromatic Hydrocarbons (PAHs) in soils and their subsequent release in rivers constitute a major environmental and public health problem in industrialized countries. In the Seine River basin (France), some PAHs exceed the target concentrations, and the objectives of good chemical status required by the European Water Framework Directive might not be achieved. This investigation was conducted in an upstream subcatchment where atmospheric fallout (n = 42), soil (n = 33), river water (n = 26) and sediment (n = 101) samples were collected during one entire hydrological year. PAH concentrations in atmospheric fallout appeared to vary seasonally and to depend on the distance to urban areas. They varied between 60 ng·L− 1 (in a remote site during autumn) and 2380 ng·L− 1 (in a built-up area during winter). PAH stocks in soils of the catchment were estimated based on land use, as mean PAH concentrations varied between 110 ng·g− 1 under woodland and 2120 ng·g− 1 in built-up areas. They ranged from 12 to 220 kg·km− 2. PAH contamination in the aqueous phase of rivers remained homogeneous across the catchment (72 ± 38 ng·L− 1). In contrast, contamination of suspended solid was heterogeneous depending on hydrological conditions and population density in the drainage area. Moreover, PAH concentrations appeared to be higher in sediment (230-9210 ng·g− 1) than in the nearby soils. Annual mass balance calculation conducted at the catchment scale showed that current PAH losses were mainly due to dissipation (biodegradation, photo-oxidation and volatilization) within the catchments (about 80%) whereas exports due to soil erosion and riverine transport appeared to be of minor importance. Based on the calculated fluxes, PAHs appeared to have long decontamination times in soils (40 to 1850 years) thereby compromising the achievement of legislative targets. Overall, the study highlighted the major role of legacy contamination that supplied the bulk of PAHs that are still found nowadays in the environment.