Soil Uptake and Emissions of Atmospheric Pollutants

Which factors and processes control accumulation, leaching and volatilisation of pollutants in top soils?

Since the last century, soils (and sediments) have accumulated agricultural and atmogenic pollutants such as organochlorine pesticides, polycyclic aromatic hydrocarbons (PAHs) and polychlorinatedbiphenyls (PCBs). These compounds have been distributed in the atmosphere at global scale, and long-term fate depends on interaction with the top soils. Whether soils are sinks or sources of such diffuse pollutants depends on the input history, sorption and transformation capacity of the soils as well as on the aqueous leaching and (re-)volatilization rates of the compounds of interest. While soils may be regarded as sources for these legacy pollutants in industrialized regions, they still act as sinks in remote areas and in general they are sinks for emerging compounds such as flame retardants. For many compounds, substantial data exist to parameterize specific processes such as sorption mechanisms/isotherms and sorption/desorption kinetics at the grain scale. However, the interplay of the physical and biogeochemical processes which determines the long-term fate of such compounds is not clear. Several of these compounds (e.g. phenanthrene) are easily degradable under aerobic conditions, so that the reason for their persistence in biologically highly active top-soils exposed to atmospheric oxygen is not clear. Especially the rates of exchange of these compounds between soils and the atmosphere are not well understood, even though it is crucial for long-term fate and atmospheric spreading of diffuse pollutants.

Fig. 1 Conceptual model of soil-atmosphere exchange of phenanthrene with regions identified. ① Linear sorption/ desorption only in the soil layer;② volatilization/precipitation;③ 1st order biodegradation only in the soil layer.

The conceptual model for the soil-atmosphere exchange of a model compound, phenanthrene, in shown in Figure 1, wherein there are several defined layers within the profile (bulk air and near-surface air, and sorptive soil layers). Using this conceptual model, this work aims to implement a 2-D numerical simulation of soil-atmosphere exchange using the geochemical model MIN3P, and including factors like sorption, biodegradation, volatilization, temperature variation, recharge rate, eddy diffusion and molecular diffusion which are estimated from literature data.

Special interest for the soil-atmosphere exchange of organic compounds includes: (1) understanding the physical and biogeochemical processes controlling the long-term fate of organic pollutants; and (2) estimating cross compartmental concentration gradients and flux rates under future scenarios of emissions or climate change. Future work aspires to link the model to field observations of this exchange process. In addition, the model developed could be extended to simulate the dynamics of carbon turnover (CO₂, O₂, etc) and weathering processes.

Publications

Bao, Z., Haberer, C., Maier, U., Amos, R. T., Blowes, D. W., Grathwohl, P. (2017): Modeling controls on the chemical weathering of marine mudrocks from the Middle Jurassic in Southern Germany. Chem. Geol., doi: 10.1016/j.chemgeo.2017.03.021
Bao, Z., Haberer, C., Maier, U., Beckingham, B., Amos, R. T., Grathwohl, P. (2016): Modelling short-term concentration fluctuations of semi-volatile pollutants in the soil-plant-atmosphere system. Sci. Total Environ. 569-570, 159-167, doi: 10.1016/j.scitotenv.2016.06.117
Bao, Z., Haberer, C., Maier, U., Beckingham, B., Amos, R. T., Grathwohl, P. (2015): Modelling long-term uptake and re-volatilization of semi-volatile organic compounds (SVOCs) across the soil-atmosphere interface. Sci. Total Environ. 538, 789-801, doi: 10.1016/j.scitotenv.2015.08.104

Which factors and processes control accumulation, leaching and volatilisation of pollutants in top soils?

PhD Thesis