PHT3D-UZF: A Reactive Transport Model for Variably-Saturated Porous Media by Ming Zhi Wu1,2,3 , Vincent E.A. Post4,5 , S. Ursula Salmon1,2 , Eric D. Morway6 , and Henning Prommer1,2,7
Abstract A modified version of the MODFLOW/MT3DMS-based reactive transport model PHT3D was developed to extend current reactive transport capabilities to the variably-saturated component of the subsurface system and incorporate diffusive reactive transport of gaseous species. Referred to as PHT3D-UZF, this code incorporates flux terms calculated by MODFLOW’s unsaturated-zone flow (UZF1) package. A volume-averaged approach similar to the method used in UZF-MT3DMS was adopted. The PHREEQC-based computation of chemical processes within PHT3D-UZF in combination with the analytical solution method of UZF1 allows for comprehensive reactive transport investigations (i.e., biogeochemical transformations) that jointly involve saturated and unsaturated zone processes. Intended for regional-scale applications, UZF1 simulates downward-only flux within the unsaturated zone. The model was tested by comparing simulation results with those of existing numerical models. The comparison was performed for several benchmark problems that cover a range of important hydrological and reactive transport processes. A 2D simulation scenario was defined to illustrate the geochemical evolution following dewatering in a sandy acid sulfate soil environment. Other potential applications include the simulation of biogeochemical processes in variably-saturated systems that track the transport and fate of agricultural pollutants, nutrients, natural and xenobiotic organic compounds and micropollutants such as pharmaceuticals, as well as the evolution of isotope patterns.
Introduction Investigation of biogeochemical processes in the unsaturated subsurface zone is essential for understanding the fate and transport of dissolved chemical species therein, and the chemical composition of groundwater recharge. Environmental issues in unsaturated systems include the transport and fate of heavy metals (e.g., Jacques et al. 2008), inorganic chemicals and pesticides introduced by agricultural activities (e.g., B¨ohlke 2002; Smiles and Smith 2004a; Bailey et al. 2013a), 1 National Centre for Groundwater Research and Training (NCGRT), University of Western Australia node, Private Bag 5, Wembley, WA 6913, Australia. 2 School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. 3 Corresponding author: Department of Civil and Environmental Engineering, University of Western Ontario, London, Ontario, Canada N6A 5B9; e-mail: [email protected] 4 School of the Environment, Flinders University, GPO Box 2100, Adelaide, SA 5001, Australia. 5 National Centre for Groundwater Research and Training, Flinders University, Adelaide, SA 5001, Australia. 6 U.S. Geological Survey, Nevada Water Science Centre, 2730 N. Deer Run Road, Carson City, NV 89701, USA. 7 CSIRO Land and Water, Private Bag No. 5, Wembley, WA 6913, Australia. Received July 2014, accepted December 2014. © 2015, National Ground Water Association. doi: 10.1111/gwat.12318
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and pharmaceuticals and personal care products (e.g., G¨arden¨as et al. 2006; Steiner et al. 2010). Contaminants can also be generated within the unsaturated zone, such as from the oxidation of sulfide minerals in mine spoils, tailing impoundments (e.g., Mayer et al. 2002) and in acid sulfate soils (e.g., Appleyard and Cook 2009). Some previous unsaturated-zone studies have also investigated the fate of greenhouse gas emissions (Blagodatsky and Smith 2012) and chemical tracers and isotopic signals to infer historical recharge conditions (e.g., Edmunds and Tyler 2002; Edmunds et al. 2002). Gaseous species may also play an important role in the biogeochemical behavior of variably-saturated aquifers. For example, the transport of oxygen in the unsaturated zone has been shown to be the limiting factor for the overall rate of pyrite oxidation in mine waste and acid sulfate soils (e.g., Salmon et al. 2014). Other environmental studies involving gaseous transport include the release of various gases from landfills (e.g., Molins et al. 2008), the dissolution, volatilization and attenuation of petroleum hydrocarbons (e.g., Molins and Mayer 2007; Davis et al. 2009; Molins et al. 2010), the generation and attenuation of acid mine drainage (e.g., Brookfield et al. 2006; Binning et al. 2007), wastewater-derived nitrogen and carbon transformation (e.g., MacQuarrie et al. 2001), and the effect of inorganic and organic carbon reactions and atmospheric exchange on the 14 C dating of groundwater (e.g., Gillon et al. 2012). Diffusion is
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generally considered to be the dominant transport mechanism through the unsaturated zone (e.g., Thorstenson and Pollock 1989; Kohfahl et al. 2007). Molins and Mayer (2007) showed that neglecting the gaseous advective term in the modeling of pyrite oxidation in mill tailings underestimated the oxygen flux by
Abstract A modified version of the MODFLOW/MT3DMS-based reactive transport model PHT3D was developed to extend current reactive transport capabilities to the variably-saturated component of the subsurface system and incorporate diffusive reactive transport of gaseous species. Referred to as PHT3D-UZF, this code incorporates flux terms calculated by MODFLOW’s unsaturated-zone flow (UZF1) package. A volume-averaged approach similar to the method used in UZF-MT3DMS was adopted. The PHREEQC-based computation of chemical processes within PHT3D-UZF in combination with the analytical solution method of UZF1 allows for comprehensive reactive transport investigations (i.e., biogeochemical transformations) that jointly involve saturated and unsaturated zone processes. Intended for regional-scale applications, UZF1 simulates downward-only flux within the unsaturated zone. The model was tested by comparing simulation results with those of existing numerical models. The comparison was performed for several benchmark problems that cover a range of important hydrological and reactive transport processes. A 2D simulation scenario was defined to illustrate the geochemical evolution following dewatering in a sandy acid sulfate soil environment. Other potential applications include the simulation of biogeochemical processes in variably-saturated systems that track the transport and fate of agricultural pollutants, nutrients, natural and xenobiotic organic compounds and micropollutants such as pharmaceuticals, as well as the evolution of isotope patterns.
Introduction Investigation of biogeochemical processes in the unsaturated subsurface zone is essential for understanding the fate and transport of dissolved chemical species therein, and the chemical composition of groundwater recharge. Environmental issues in unsaturated systems include the transport and fate of heavy metals (e.g., Jacques et al. 2008), inorganic chemicals and pesticides introduced by agricultural activities (e.g., B¨ohlke 2002; Smiles and Smith 2004a; Bailey et al. 2013a), 1 National Centre for Groundwater Research and Training (NCGRT), University of Western Australia node, Private Bag 5, Wembley, WA 6913, Australia. 2 School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. 3 Corresponding author: Department of Civil and Environmental Engineering, University of Western Ontario, London, Ontario, Canada N6A 5B9; e-mail: [email protected] 4 School of the Environment, Flinders University, GPO Box 2100, Adelaide, SA 5001, Australia. 5 National Centre for Groundwater Research and Training, Flinders University, Adelaide, SA 5001, Australia. 6 U.S. Geological Survey, Nevada Water Science Centre, 2730 N. Deer Run Road, Carson City, NV 89701, USA. 7 CSIRO Land and Water, Private Bag No. 5, Wembley, WA 6913, Australia. Received July 2014, accepted December 2014. © 2015, National Ground Water Association. doi: 10.1111/gwat.12318
NGWA.org
and pharmaceuticals and personal care products (e.g., G¨arden¨as et al. 2006; Steiner et al. 2010). Contaminants can also be generated within the unsaturated zone, such as from the oxidation of sulfide minerals in mine spoils, tailing impoundments (e.g., Mayer et al. 2002) and in acid sulfate soils (e.g., Appleyard and Cook 2009). Some previous unsaturated-zone studies have also investigated the fate of greenhouse gas emissions (Blagodatsky and Smith 2012) and chemical tracers and isotopic signals to infer historical recharge conditions (e.g., Edmunds and Tyler 2002; Edmunds et al. 2002). Gaseous species may also play an important role in the biogeochemical behavior of variably-saturated aquifers. For example, the transport of oxygen in the unsaturated zone has been shown to be the limiting factor for the overall rate of pyrite oxidation in mine waste and acid sulfate soils (e.g., Salmon et al. 2014). Other environmental studies involving gaseous transport include the release of various gases from landfills (e.g., Molins et al. 2008), the dissolution, volatilization and attenuation of petroleum hydrocarbons (e.g., Molins and Mayer 2007; Davis et al. 2009; Molins et al. 2010), the generation and attenuation of acid mine drainage (e.g., Brookfield et al. 2006; Binning et al. 2007), wastewater-derived nitrogen and carbon transformation (e.g., MacQuarrie et al. 2001), and the effect of inorganic and organic carbon reactions and atmospheric exchange on the 14 C dating of groundwater (e.g., Gillon et al. 2012). Diffusion is
Vol. 54, No. 1–Groundwater–January-February 2016 (pages 23–34)
23
generally considered to be the dominant transport mechanism through the unsaturated zone (e.g., Thorstenson and Pollock 1989; Kohfahl et al. 2007). Molins and Mayer (2007) showed that neglecting the gaseous advective term in the modeling of pyrite oxidation in mill tailings underestimated the oxygen flux by
