Estimations of soil metal accumulation or leaching potentials under climate change scenarios: the example of copper on a European scale

Contaminant inputs to soil are highly dependent on anthropogenic activities, while contaminant retention, mobility, and availability are highly dependent on soil properties. The knowledge of partitioning between soil solid and solution phases is necessary to estimate whether deposited amounts of contaminants will be either transported with runoff or accumulated. Besides, runoff is expected to change during the next century due to changes in climate and in rainfall patterns. In this study, we aimed to estimate at the European scale the areas with a potential risk due to contaminant leaching (LP). We also defined, in the same way, the surface areas where limited copper (Cu) leaching occurred, leading to potential accumulation (AP) areas. We focused on Cu widely used in agriculture either in a mineral form or in association with organic fertilizers, resulting in high spatial variations in the deposited and incorporated amounts in soils, as well as in European policies of application. We developed a method using both Cu partition coefficients ( K f ) between total and dissolved Cu forms and runoff simulation results for historical and future climates. The calculation of K f with pedo-transfer functions allowed us to avoid any uncertainties due to past management or future depositions that may affect total Cu concentrations. Areas with a high potential risk of leaching or of accumulation were estimated over the 21st century by comparing K f and runoff to their respective European medians. Thus, at three distinct times, we considered a grid cell to be at risk of LP if its K f was low compared to the European median and if its runoff was high compared to the European median of the time. Similarly, a grid cell was considered to be at risk of AP if its K f was high and its runoff was low compared to its respective European median of the time. To deal with uncertainties in climate change scenarios and the associated model prediction, we performed our study with two atmospheric greenhouse gas representative concentration pathways (RCPs), defined according to climate change associated with a large set of socio-economic scenarios found in the literature. We used two land surface models (ORCHIDEE and LPJmL, given soil hydrologic properties) and two global circulation models (ESM2m and CM5a, given rainfall forecasts). Our results show that, for historical scenarios, 6.4 +/- 0.1 % (median, median deviation) and 6.7 +/- 1.1 % of the grid cells of the European land surfaces experience LP and AP, respectively. Interestingly, we simulate a constant surface area with LP and AP for around 13 % of the grid cells, which is consistent with an increase in AP and a decrease in LP. Despite large variations in LP and AP extents, depending on the land surface model used for estimations, the two trends were more pronounced with RCP 6.0 than with RCP 2.6, highlighting the global risk of combined climate change and contamination and the need for more local and seasonal assessments. Results are discussed to highlight the points requiring improvement to refine predictions.