Heavy metals: impacts and future effects on the forest

Summary

Atmospheric contamination by trace metals

Trace metals (TMEs) are elements originating from the Earth's crust that are found in minute concentrations in the natural environment. They can be dispersed into the atmosphere through a number of natural processes such as forest fires, volcanic eruptions or marine aerosol emissions. However, since ancient times, man has also contributed to their dispersal by exploiting metals for industrial activities. This anthropic influence exploded in the 20th century in the northern hemisphere with the Industrial Revolution and the development of mechanised transportation. Natural TME cycles were altered at the global scale and their dispersal into the atmosphere over long distances contaminated areas that would normally have been free of this type of pollution.

Today, cadmium and lead emissions are decreasing in Europe, in particular thanks to the 2000 ban on lead additives in petrol and to improved industrial emissions-reduction techniques. This may not be the case for all metals, however. Some emissions, for example copper and nickel, seem to have stabilised while others are increasing (antimony, for example). Furthermore, though emissions are mostly decreasing in Europe, they are globally increasing at the planetary scale due to industrial development in Asia.

Monitoring atmospheric deposits

The EMEP Programme monitors and assesses the extent of long-distance TME atmospheric deposits in Europe. Deposition models are built from measurements taken at sites throughout the territory. In France, only two stations are equipped to monitor TME atmospheric deposits and both are part of the MERA Network. However, the BRAMM Network, which classifies mosses as TME bio-accumulators, covers the entire territory with its 528 sampling sites. Whenever possible, BRAMM sites are set up near RENECOFOR sites. This proximity makes it possible to compare mosses and lichens to evaluate their role as bio-indicators of TME deposits and to study how forest cover influences the recording of these deposits.

Indeed, forests are particularly sensitive to atmospheric pollution by metals. The canopy acts as an exchange surface for reactions with TMEs. Dry deposits accumulate there. The exudation of organic molecules through the canopy leaves can totally or partially dissolve this dry deposit, which can then create a solution with rainwater or other forms of precipitation. The canopy can also assimilate the elements, whether they are deposited in dry or wet form. These processes change the TME flux reaching the forest floor. Thanks to the CATAENAT Network, this effect can be directly observed by comparing deposit data recorded in the open air and under forest cover. For example, on average lead fluxes are five times as heavy under forest cover, while zinc fluxes are halved.

Metal dynamics in forest ecosystems

What happens to TMEs in the forest depends on which ecosystem compartment they are transferred to (soil, water, vegetation) - as shown in the figure below. Their mobility is linked at least partially to their potential role in the biosphere (some are important micro-nutrients like copper, cobalt, chromium, nickel or zinc; others like antimony, cadmium or lead play no known biological role). Their chemical properties also influence their mobility. In particular, their affinity with organic matter (both solid and dissolved) and their pH sensitivity control to a large extent how they are transferred within the soil layers and into the biosphere, and whether or not they accumulate in the forest soil. On an acidic plot, like SP 57 for example, trace metals will not accumulate in the soils because they will dissolve and be carried away in the water runoff whereas on a plot with base soils, like SP11, they will tend to accumulate.

Flow diagram of ETM in forest ecosystems

The impact of metals on forest ecosystems

Soil analyses on the RENECOFOR plots reveal the presence of metals dispersed through human activity, notably in eastern and northern France. However, contamination levels remain low. Current TME atmospheric deposits in French forests are moderate, and are representative of long-distance deposit levels in rural Europe. Most are of anthropic origin, but remain below critical charge thresholds - the maximum admissible quantities an ecosystem can support, according to current knowledge.

Summary

Chronic low-level contamination and long time-scales are primordial criteria to consider when dealing with risks due to activities in the nuclear sector, including the storage of radioactive material.

The wide variety of contexts and the complex interactions among different environmental compartments (atmosphere, soil, water, flora and fauna) make it difficult to model the transfer of chemical elements into the biosphere; choices must be made concerning which concepts, parameters and hypotheses will best reflect the degree of accuracy desired. To avoid under-estimating the long-term risks involved with toxic radioactive pollutants in the environment, existing models (classically applied to the food chain and in environmental impact studies for radioactive waste storage sites) have adopted a simplified view of how contaminants are transferred into the environment: they presuppose a stylized, balanced environmental system and apply a precautionary strategy with a pessimistic bias on parameter values. This conservative approach is above all dictated by regulations and criteria for radiation protection. However, it is not well adapted when precise scientific information is the goal: for example, accurately describing the dynamics of how these pollutants accumulate in the soil or in vegetation, or determining speciation, whether they can be absorbed into the biosphere or how toxic they actually are. As a complement to the classic predictive models mentioned above, researchers need a more mechanistic description of contaminants' bio-geochemical cycle in our typical natural ecosystems to develop more rational dynamic models capable of handling uncertainty and leading to appropriate questioning.

Today forest ecosystems are often found near zones where radioactive waste has been stocked. In many different climatic zones, forests are the natural ecosystems that dominate areas where man does not occupy the territory. The long time-scales (100 to 10,000 years) required to evaluate the risks associated with nuclear waste storage are similar in length to the time-scales involved in forest ecosystem functioning. The long-lasting nature of forest ecosystems therefore makes them well-adapted to the study of the long-term behaviour of environmental contaminants. More precisely, this includes:

• Describing how contaminants are redistributed between the soil and the vegetation, and the time-scales involved under environmental conditions representative of future climates

• Comparing the impact of potential contaminant releases with the impacts of the historic contaminant footprint (natural vs. artificial) in the environment

• Specifying the risks of accumulation by modelling different vectors of contamination (atmospheric deposits vs. underground release)

• Checking the validity of the simplified generic transfer models currently being used.

The main priority concerns stable or radioactive isotopes of the elements Cl, I, Se, Cs, C, B, As, Hg, Cs ..., the focus of the Andra programme, which are detectable in the natural background. Future models will aim for ecological realism, rather than complexity, and will be based on data such as recorded concentrations, stocks and fluxes (retention, transformation, volatilisation,...) collected from densely equipped sites or sites where access to a long time series of measurements is authorised.

In this context, the RENECOFOR Network was approached to furnish various samples (water, soil, biomass) representative of contrasted environmental conditions. For example, RENECOFOR sample collections were used to better quantify chlorine (inorganic vs. organic) transformation fluxes in the soil column, thus providing more explicit chlorine cycle models. Other studies are underway to identify the role of environmental factors influencing the distribution, speciation or residence time of other elements (chorine-36, iodine, selenium, caesium).

Summary

Polycyclic aromatic hydrocarbons (PAHs) are a family of persistent organic pollutants (POPs) originating essentially from the incomplete combustion of organic matter. The European Union has singled out PAHs for priority action due to their toxicity and generalised presence throughout the environment (they are often dispersed through the atmosphere and deposited far from their point source).

Within the framework of a doctoral thesis, PAH concentrations in tree leaves and in forest humus and soil were studied based on samples preserved in the soil collection taken from 1993 to 2011 on 14 RENECOFOR plots.

High PAH levels were occasionally found at certain sites near local emission point sources (forest fire sites, nearby industrial or urban areas). When these particular situations were excluded, however, PAH concentrations in the leaves and needles (sampled each year) regularly decreased throughout the study period (see figure below).

Evolution of the concentrations sum of the PAHs analyzed in leaf samples from 14 plots of the RENECOFOR network

This decrease was concomitant with a decrease in the atmospheric PAH emissions estimated for the same period. This shows that analysing historic leaf samples can reveal changing trends in atmospheric HAP levels, at least to a certain degree. Based on measured concentration levels, an estimated 2 to 5% of global PAH emissions were fixed in the leaves of French forests. These results are far below the 44±18% initially proposed by Simonich and Hites (1994) in Indiana (USA), and indicate that forests may have less of a filtering effect on atmospheric pollutants than is often suggested.

In the humus, PAH concentrations seem to be mostly dependent on processes related to mineralisation and the incorporation of organic matter into the soil. Indeed, higher concentrations were (found in the OH horizon of the thickest humus types (mor-humus formation). Nevertheless, the lightest PAHs were likely mobilised via the water flux.

Below the humus, very high PAH stocks were found in the soil mineral layers compared to incoming fluxes, thus indicating a very long residence time. The build-up of PAHs in French forest soils could reflect a very long period of accumulation (several hundred years).

In this study, the RENECOFOR Network proved to be extremely pertinent for retro-active environmental observations thanks not only to its network of permanent plots, but also to its sample collections (soil collection, leaf archives).