How do heavy metals get into the soil?

Heavy metals are often already present in parent rock from which the soil has developed (lithogenic). There are also anthropogenic causes of heavy metal pollution e.g. atmospheric emissions, fertilizers or contaminated water. Emissions from point sources such as metal processing industrial complexes can locally produce high levels of pollution. Some of these emissions however, are distributed further by air. Not only industrial activities but also other human activities, such as energy production, waste disposal and transport, have led to wide ranging heavy metal soil pollution on a low level.

The soil analyses made clear that most of the heavy metal deposits in the tested forest soils come from parent rock and that the level can be very high, e.g. chromium and nickel concentrations in the soil on serpentine rock near Davos. In most samples lead from atmospheric deposition was also present. This probably originated largely from former fuel additives.

Impact on organisms

Organisms require some heavy metals in small amounts, copper or zinc for instance. There is no known organism however, requiring cadmium or lead for survival. In large amounts all heavy metals are poisonous. The critical levels and the effects vary depending on the organism. Soil organisms such as bacteria and fungi are especially vulnerable. These play an important ecological role and therefore heavy metals deposits can impair nutrient availability or soil aeration restricting the soil as a growing place for plants. Root growth and vitality as well as the function of leaves or needles of sensitive plants can also be directly affected. High heavy metal concentration in plants and fungi could also impair the health of humans through the food chain. Children especially tend to put soil directly into their mouth.

Regardless of the source the Swiss ordinance relating to impacts on the soil pollution define as critical levels which once exceeded would endanger the “long term fertility” of the soil. The levels in around one third of the tested samples had been exceeded by at least one heavy metal in one layer of soil. Critical levels leading to a toxic effect on micro organisms are exceeded in half of the topsoils. Chromium appears to be the most critical metal. Critical levels are exceeded most frequently on the southern side of the Alps which is exposed to high atmospheric pollution from the Po area.

Ground water pollution

Heavy metals can be washed out of the soil and into ground water or surface water along with rain water seepage. Whilst the potential risk of groundwater pollution in the Jura or the central plateau with mainly carbonate containing parent rock is mainly small, we consider many sites on crystalline rock in the central Alps and in southern Switzerland to be more critical because of the acid underground.

The risk is not the same from all heavy metals because they do not all possess the same degree of mobility in soil. Nickel and copper are generally the most mobile of the examined heavy metals, the latter being especially mobile due to the effect of soluble organic substances in soil water. These also make lead much more mobile than once assumed. On the other hand zinc seems to be less mobile than its chemical properties would suggest. This is probably due to the fact that zinc is absorbed by plants as a micro nutrient and is transported with the litter back to the topsoil.

According to a survey heavy metals are not yet a problem for the quality of ground water in Switzerland. However, out of the 44 stations where heavy metals were measured only 9 are under forests and 2 of those in areas with acid rock. A comparison of heavy metal concentration in forest soils with ground water measurements in chosen areas on crystalline rock is urgently required.

Translation: Dawn Meister, Stallikon