Modelling of the natural chlorine cycling in a coniferous stand: implications for chlorine-36 behaviour in a contaminated forest environment

Abstract

Considered as one of the most available radionuclide in soil-plant system, 36 Cl is of potential concern for long-term management of radioactive wastes, due to its high mobility and its long half-life. To evaluate the risk of dispersion and accumulation of 36 Cl in the biosphere as a consequence of a potential contamination, there is a need for an appropriate understanding of the chlorine cycling dynamics in the ecosystems. To date, a small number of studies have investigated the chlorine transfer in the ecosystem including the transformation of chloride to organic chlorine but, to our knowledge, none have modelled this cycle. In this study, a model involving inorganic as well as organic pools in soils has been developed and parameterised to describe the biogeochemical fate of chlorine in a pine forest. The model has been evaluated for stable chlorine by performing a range of sensitivity analyses and by comparing the simulated to the observed values. Finally a range of contamination scenarios, which di↵er in terms of external supply, exposure time and source, have been simulated to estimate the possible accumulation of 36 Cl within the di↵erent compartments of the coniferous stand. The sensitivity study supports the relevancy of the model and its compartments and has highlighted the chlorine transfers a↵ecting the most the residence time of chlorine in the stand. Compared to observations, the model simulates realistic values for the chlorine content within the di↵erent forest compartments.

Experimental Evidence of Large Changes in Terrestrial Chlorine Cycling Following Altered Tree Species Composition

“…Longer residence time of 36 Cl in soil means increased radiation exposure times to humans, plants and soil-systems in case of contamination. 20 In spite of these implications we still lack fundamental knowledge about the spatial and temporal variability and the regulation of the processes involved in Cl cycling. Even very fundamental spatial patterns of Cl distribution remain unexplained, and local levels of Cl in soils can deviate substantially from the expected large scale input from wet and dry atmospheric deposition of Cl.…”

“…Longer residence time of 36 Cl in soil means increased radiation exposure times to humans, plants and soil-systems in case of contamination. 20 In spite of these implications we still lack fundamental knowledge about the spatial and temporal variability and the regulation of the processes involved in Cl cycling. Even very fundamental spatial patterns of Cl Previous attempts to study what controls Cl abundance and transformation rates have been based on small-scale laboratory experiments detached from full ecosystems, or correlations between Cl levels and monitored environmental variables where cause-effect relationships are unclear.…”

“…The fact that Cl are physiologically important for humans and other organisms, for example, in cellular ion balances, and the potentially rapid uptake of 36 Cl by plants and in the end humans, makes it desirable to account for in 36 Cl radioactive risk assessment models. Longer residence time of 36 Cl in soil means increased radiation exposure times to humans, plants and soil-systems in case of contamination . In spite of these implications we still lack fundamental knowledge about the spatial and temporal variability and the regulation of the processes involved in Cl cycling.…”

Chlorine Distribution in Soil and Vegetation in Boreal Habitats along a Moisture Gradient from Upland Forest to Lake Margin Wetlands

“…The concentrations in the herb layer were even higher than those in tree foliage that previously has been shown to accumulate high Cl concentrations. 16 , 20 , 37 Vegetation with annual above-ground plant parts, e.g., grass and herbs species, show high total Cl concentrations while plants with perennial plant parts, such as Vaccinium vitis-idaea , show lower concentrations ( Figure S1 ). We also noted that the Cl concentrations of Vaccinium myrtillus and Myrica gale (which has a perennial stem) were higher than those in Vaccinium vitis-idaea having perennial parts only.…”

“…Thus, to understand landscape Cl cycling and residence times, it is necessary to characterize Cl – and Cl org uptake and storage in common habitats and vegetation communities along gradients. This is of fundamental importance for the use of Cl – for assessing subsurface water movement and catchment hydrology, , and for risk assessment modeling associated with 36 Cl in nuclear waste , and other pollutants carried by groundwater and soil water. − …”