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Berlin Environmental Atlas

01.11 Criteria for the Evaluation of the Soil Functions (Editin 2018)

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01.11.10 Binding Strength for Heavy Metals

Description

Heavy metals are bound by adsorption to humic materials, clay minerals and sesquioxides. The solubility of heavy metals depends on their total content and the pH value of the soil solution. The solubility of heavy-metal compounds generally increases with increasing acidification. This is due to the fact that metals tend to form stable oxides at higher pH values, or they tend to enter into not easily dissoluble compounds, e.g. PbCaCO3, by precipitation.

The relative binding strength of heavy metals is one of the components used to evaluate the Filteration and Buffering Function (cf. map 01.12.3).

Different types of heavy metal are bound differently (DVWK, 1988). Cadmium dissolves relatively quickly. It is a widespread background pollutant in Berlin and is noteworthy regarding its harmfulness. Therefore, the binding strength of cadmium, which is easily soluble, is used here as a benchmark for the binding strength for heavy metals, following the method proposed by the Hamburg Ministry for Environment and Health (2003).

Methodology

Blume and Brümmer (1987, 1991) developed a concept to assess the sensitivity of soil to metal pollution, a concept that is now being implemented throughout Berlin. The assessment is based on the relative binding strength of individual metals depending on the pH value of the soil solution, assuming the conditions of a sandy soil with low sorption and low humus content. Higher humus, clay and iron hydroxide contents are reflected by increases or decreases. The calculation is carried out to a depth of 1 m. For this purpose, characteristic values for the topsoil and subsoil are determined gradually depending on the pH value, the humus content and the clay content. The sum of these yields the BSSM binding strength. This value is adjusted based on the proportion of coarse soil and the horizon thickness. It can range from 0 to 5, reflecting a binding strength of “none” to “very high” for heavy metals.

Tab. 1: Evaluation of the Relative Binding Power of Heavy Metals
Levels of Binding Strength for Heavy Metals Designation
0 none
1 very low
2 low
3 medium
4 high
5 very high
Tab. 1: Evaluation of the relative binding strength for heavy metals depending on the pH value, humus and clay content, the proportion of coarse soil and horizon thickness (according to Blume and Brümmer 1987, 1991)

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