The role of soil organic carbon in the global cycling of persistent organic pollutants (POPs): interpreting and modelling field data

Soil is an important global reservoir for persistent organic pollutants (POPs). The interaction between air (which often receives the majority of emissions) and soil plays a key role in the long term environmental cycling and fate of these chemicals. Soil surveys have been carried out to try and estimate regional and global distribution/inventory of POPs. A correlation between soil POPs concentration and soil organic carbon (SOC) has been observed in background soils [Meijer et al., 2003. Global distribution and budget of PCBs and HCB in baqckground surface soils: implications for sources and environmental processes. Environ. Sci. Technol., 37, 667], provoking discussion about whether POPs will approach steady-state (or equilibrium) between air and SOC, on a global scale. This manuscript investigates this relationship and in particular how soil concentrations can be influenced by factors such as temperature, SOC content and physicochemical properties. A simple two box model designed to investigate parameters that are likely to affect air-soil exchange revealed that more volatile chemicals such as HCB are likely to achieve steady-state conditions between air and soil relatively quickly whilst relatively involatile chemicals, such as heavy PCBs, may take considerably longer and other compounds (e.g. OCDD) may never achieve it. These model calculations provide an insight into which fate processes (e.g. volatilisation or degradation) may control a chemicals fate in the terrestrial environment. A different modelling exercise was used to explore the complex interaction of environmental parameters, representative of 'real world' conditions to study their potential influence on POPs cycling at the European scale. Results from the model suggested that compound degradation rates in soil (linked to SOC content), temperature, vegetation cover and ecosystem C turnover are all likely to significantly influence POP air-soil exchange and fate.