Fire and subsequent rain mobilise the mercury in soils
José Pichel Andrés/DICYT Research by the Centre of Environmental and Marine Studies (Centro de Estudos do Ambiente e do Mar, CESAM) of the University of Aveiro, with the collaboration of the Portuguese Institute of the Sea and the Atmosphere, has shown that forest fires and subsequent precipitations play an important role in the redistribution of mercury, a highly contaminating metal that after these episodes accumulates in adjacent soils and water. The scientists have discovered that eucalypt forests contain more mercury than pine forests, the two cases included in the study.
“Mercury is a toxic metal and its behaviour in the environment is complex; it may persist for decades even when the sources of emission have been eliminated”, DiCYT was told by Isabel Campos, a scientist of the Earth Surface Processes Team.
Moreover, mercury accumulates in the food chain and may have a detrimental effect not only on the environment but also on human health. It is therefore “important to study and assess the transport, the mobilisation and the destination of this metal in ecosystems”.
The research, which has been published in the scientific journal CATENA, reveals that soils that have been affected by fire lose mercury because this metal is very volatile. When it rains part of the metal present in the ashes passes to the soil, but “this increase in the soil is less than the loss of mercury in the ashes, which means that the missing percentage may have passed to neighbouring areas or to aquatic systems”, the researcher points out. For this reason “forest fires and subsequent rain play an important role in the mobilisation of mercury in the environment”, he adds.
The most important point of these results is that the mobilisation of mercury after a fire and subsequent rain may be a source of contamination of the soil, of the groundwater, and of other aquatic habitats downstream of the burnt areas. In other words the fire may be a source of the contamination of the environment far beyond the areas affected by the flames. This research should therefore be used to “define strategies to mitigate the damage that fire causes to terrestrial and aquatic ecosystems”.
A relevant aspect of the research is the difference in mercury according to the type of tree encountered. The soils of eucalypt forests have higher levels of mercury than pine forests and this tendency is maintained in the ashes. Eucalypts accumulate more mercury and the soil therefore retains more of the metal. The eucalyptus is one of the most representative species of the forests of central Portugal despite the fact that it is not autochthonous.
Essential studies for forest recovery
In general forest fires are extremely destructive and scientists consider it "essential" to study the changes in the biological, physical, and chemical properties of soils as they constitute "the means of growth of the vegetation and the microbial biomass”. In order to recreate the conditions present before the fire it is important to find out the redistribution of elements such as carbon, sodium, potassium, calcium, iron, zinc, or copper, but it should also be taken into account that “the excess of non essential elements may have a harmful effect on the productivity and sustainability of ecosystems” and this includes metals such as mercury.
The research was carried out in areas that had recently been burnt in Ermida and São Pedro do Sul in central Portugal and in other unburnt areas that served as a control for the study and by means of experiments. The work involved a multidisciplinary team of experts on hydrology, chemistry, eco-toxicology, and ecology. In current studies to support the FIRETOX (Toxic effects of wildfires on aquatic systems) project the group is studying the possible effects of the transport of metals in the burnt areas on the water and the organisms that live in it.
Effects of wildfire on mercury mobilisation in eucalypt and pine forests. Isabel Campos, Carlos Vale, Nelson Abrantes, Jan Jacob Keizer, Patrícia Pereira. CATENA, Volume 131, August 2015, Pages 149–159. doi:10.1016/j.catena.2015.02.024