Mapping background concentrations in soils of Wallonia

Vandeuren, Aubry [UCL] Pereira, Benoît [UCL] Sonnet, Philippe [UCL]

In Wallonia, as in many other regions of the world, government agencies rely on soil maps of trace metal background concentrations to implement soil protection policies. Here we present the result of a study funded by the Walloon Public Service which aims to establish trace metal background concentration maps in Walloon soils. Two databases containing georeferenced soil analyzes providing data on soil background concentrations were used for this study: the CAPASOL database (17,000 sampled agricultural soils) and the permanent inventory of woody resources (about 700 sampled forest soils). The methodology used for drawing the maps involves three steps. In the first step, we divided Wallonia (17,000 km²) into soil pedogeochemical units based on (1) the soil map of the Wallonia (1:20,000), (2) geological maps (1:40,000 and 1:25,000), and (3) literature data and survey results about soils and geology of Wallonia. In the second step, we produced a single reliable dataset by compiling the two available soil databases by a data selection and data leveling method which we designed for this purpose. On the last step, we drew the maps by using a geostatistical model based on the pedogeochemical soil units and the reliable dataset of soil analyzes produced in the previous step. The resulting maps reveal large variations in local background concentrations over short distances, mainly due to the high diversity of soil parent materials, and to the atmospheric deposition of pollutants around former base metal smelters. Wallonia has higher Ni background concentration in soil than average values in Europe (according to the values of Geochemical Atlas of Europe), principally because of high geogenic background levels. Also, higher Zn and Cd values are found, principally due to atmospheric deposition of pollutants originating from former base metal smelters.