Lead contamination of soils
The diffuse pollution of soils with lead is basically related to airborne inputs originating from industrial and transport emissions. Above a certain threshold, lead is toxic to animals and humans and, in certain circumstances, to plants too. Lead contamination in animals and humans mainly occurs through inhalation and the oral ingestion of soil particles. Polluted playgrounds pose a particularly severe risk to children due to their low body weight and play behaviour. A high blood lead level can have serious health consequences, including irreversible neurological disorders (lead poisoning). As opposed to this, lead uptake by plants is low and it rarely reaches the food chain.
The average lead content in the top 20 cm of the soil recorded over the entire measurement period was considerably lower than the guide value specified in Annex 1, SoilPO (50 mg/kg TS). Hence the state may be assessed as good.
Lead levels remained relatively stable between the first (1985-89) and third (1995-99) sampling cycles and fell considerably thereafter. This reduction can be linked with the improvement in air quality: the reduction in the lead content of motor fuel from the 1970s and its complete elimination from 2000 contributed, among other things, to a reduction in atmospheric inputs. As opposed to this, lead is only soluble in highly acidic soils and is rarely introduced by percolating water or absorbed by vegetation: given that lead pollution persists, the element continues to accumulate in the topsoil and remains stored there indefinitely. The observed reduction in the lead concentration of the tested topsoil can be explained by dilution, that is a transfer of contaminated surface soil particles to the lower layers of the soil: this is made possible by the movement of soil organisms (in particular earthworms), which convey elements to lower levels, and by an increase in ploughing depth. Hence, the reduction in the lead content of the top 20 cm of the soil does not mean that the old lead residues have been eliminated; they were merely transferred to other parts of the soil. Given that the atmospheric input of lead into the soil have fallen due to the ban on leaded petrol and the lead content of the topsoil is declining, the trend is assessed as positive.
Every five years since 1985, a composite sample has been taken from the top 20 centimetres of the soil in each of the 103 permanent plots operated by the Swiss Soil Monitoring Network (NABO).
The testing of the lead content is carried out on the topsoil as this is where the lead contamination from atmospheric pollution accumulates.
The lead content is analysed in accordance with the method specified in the Soil Protection Ordinance (nitric acid extraction 2M HNO3). The results (in mg per kg of dry soil) are presented in the form of average lead content of the 75 non-forestry plots per measurement cycle.