Modeling long-term uptake and re-volatilization of semi-volatile organic compounds (SVOCs) across the soil–atmosphere interface

Abstract

Soil-atmosphere exchange is important for the environmental fate and atmospheric transport of many semi-volatile organic compounds (SVOCs). This study focuses on modeling the vapor phase exchange of semi-volatile hydrophobic organic pollutants between soil and the atmosphere using the multicomponent reactive transport code MIN3P. MIN3P is typically applied to simulate aqueous and vapor phase transport and reaction processes in the subsurface. We extended the code to also include an atmospheric boundary layer where eddy diffusion takes place. The relevant processes and parameters affecting soil-atmosphere exchange were investigated in several 1-D model scenarios and at various time scales (from years to centuries). Phenanthrene was chosen as a model compound, but results apply for other hydrophobic organic compounds as well. Gaseous phenanthrene was assumed to be constantly supplied to the system during a pollution period and a subsequent regulation period (with a 50% decline in the emission rate). Our results indicate that long-term soil-atmosphere exchange of phenanthrene is controlled by the soil compartment - re-volatilization thus depends on soil properties. A sensitivity analysis showed that accumulation and transport in soils in the short term is dominated by diffusion, whereas in the long term groundwater recharge and biodegradation become relevant. As expected, sorption causes retardation and slows down transport and biodegradation. If atmospheric concentration is reduced (e.g. after environmental regulations), re-volatilization from soil to the atmosphere occurs only for a relatively short time period. Therefore, the model results demonstrate that soils generally are sinks for atmospheric pollutants. The atmospheric boundary layer is only relevant for time scales of less than one month. The extended MIN3P code can also be applied to simulate fluctuating concentrations in the atmosphere, for instance due to temperature changes in the topsoil.

Modeling short-term concentration fluctuations of semi-volatile pollutants in the soil–plant–atmosphere system

“…c 10 times lower organic carbon content is assumed in the subsoil than in the topsoil. d Implementation of the atmospheric boundary layer following Bao et al (2015) and Foken (2008). Fig.…”

“…Soils represent a terrestrial reservoir for legacy SVOCs and may (temporarily) act as secondary sources, re-volatilizing pollutants to the atmosphere due to changes in land use (Komprda et al, 2013), reduced anthropogenic emissions (Jones and de Voogt, 1999;Kurt-Karakus et al, 2006;Bao et al, 2015), and temperature changes. Re-volatilization happens due to changes in seasons and climate on the long term (Lamon et al, 2009;Ma and Cao, 2010;Ma et al, 2011;Komprda et al, 2013) or because of the diurnal cycle of solar radiation in the short term (Wallace and Hites, 1996;Lee et al, 1998;Gouin et al, 2002;Totten et al, 2002;Mandalakis et al, 2003;MacLeod et al, 2007;Gasic et al, 2009;Morselli et al, 2011); both are relevant for the environmental fate of SVOCs because they may influence volatilization rates from soils.…”

“…Commonly, numerical models are used to investigate temperaturedriven soil-plant-atmosphere exchange of SVOCs by simulating sorption and diffusion in soils, partitioning to plants, and the temperature dependence of these processes (Cousins et al, 1999;Prevedouros et al, 2000;Hung et al, 2001;Scholtz et al, 2002aScholtz et al, , 2002bDalla Valle et al, 2004;MacLeod and Mackay, 2004;van den Berg and Leistra, 2004;Wegmann et al, 2004;MacLeod et al, 2007;Gasic et al, 2009;Collins and Finnegan, 2010;Morselli et al, 2011;Garcia et al, 2014;Loizeau et al, 2014;Trapp, 2015;Bao et al, 2015;Lichiheb et al, 2016). For instance, several models (i.e.…”

“…For instance, several models (i.e. Cousins et al, 1999;Scholtz et al, 2002aScholtz et al, , 2002bGasic et al, 2009;Loizeau et al, 2014;Bao et al, 2015) have addressed soil-atmosphere exchange of SVOCs (two compartments; without plants) considering integrated physico-chemical parameters. Previous studies (i.e.…”

“…In this study, therefore, we extended a previous version of a numerical model MIN3P (Bao et al, 2015) to consider the impact of time-and depth-dependent temperature changes as well as photochemical oxidation in the atmosphere on short-term concentration fluctuations of SVOCs in the soil-plant-atmosphere system, using PCB-52 and phenanthrene as model compounds. We focus on a grassland or agriculturaltype system, and do not include a forest canopy.…”