Review of the empirical equations to estimate mobile or available Cu content in soils and applicability at the European scale for risk assessment
Résumé
Soil contamination by trace elements (TE) due to
anthropogenic activities is an ongoing issue. But knowledge of
the soil total contents in TE cannot describe the risk for
contamination of the different ecosystem compartments. Large
soil survey allows the measure of total TE but not TE in solution
or free TE values despite their better adequacy for environmental
purpose. From the knowledge of total TE contents, soil
parameters like organic matter (OM) content or pH can be used
in empirical equations to calculate either TE in solution (about to
reach freshwater and mimicking mobile forms) or free forms of
TE (taken as a proxi of bio-available TE). However, empirical
equations were rather provided at a small, local scale, and were
never tested to a larger scale despite their relevance for risk
assessment. Here, we focused on copper (Cu) as TE of particular
interest because largely used in the agricultural sector. We
collected from literature the empirical equations allowing
estimation of the amount of Cu in solution and of free Cu from
the knowledge of total soil Cu contents and of various soil
parameters. Then, based on a total soil Cu map over Europe and
the most accurate equation, we provided European maps of Cu in
solution and of free Cu to identify specific areas linked to risks
of Cu mobility or availability. Our results confirmed the strong
effect of soil pH and OM content to explain soil Cu availability
or mobility but showed a stronger pH effect to explain free Cu
compare to Cu in solution. Depending on the Cu estimated
forms, different areas were identified corresponding to potential
high risks. Italy, South East and West of France and South of
Spain were regions with higher Cu in solution associated to total
Cu concentrations while highest levels of free Cu were calculated
in west Iberia, East Europe, Scandinavia and central France
regions due to the combined effect of low pH and moderate total
Cu concentrations. Our results point out the usefulness of
empirical equations at the continental scale to define
environmental guidelines for mobility or availability of
contaminants.