Final Report Summary - DILREACT (Compound-dependent dilution and reactive processes in groundwater)
The protection and the sustainable use of groundwater resources is one of the most important environmental issues of our time. Groundwater serves as a primary source of drinking water throughout the world and is also used in many agricultural and industrial activities. Many sources of pollution, both of anthropogenic and geogenic origins, threaten the quality of groundwater. The focus of the DILREACT project is the detailed study of dilution and mixing processes in porous media and in particular in aquifer systems. These processes are of crucial importance since they are typically slow and often represent the limiting factor for the overall degradation of contaminants released in groundwater systems. Therefore, their characterization and correct description is fundamental for the understanding of contaminant natural attenuation in the subsurface, for the application of remedial actions and for the implementation of effective groundwater management strategies.
The research methodology of DILREACT is based on an integrated approach combining high-resolution laboratory and field data with multi-scale numerical modeling. Figure 1, modified from Rolle et al. JCH 2013, illustrates the scales of investigation of the DILREACT project.
Fig. 1. Scales of DILREACT investigation
High-resolution multi-tracer flow-through experiments and pore-scale modeling allowed capturing the importance of processes at the sub-continuum scale, such as incomplete mixing and the development of compound-specific concentration gradients in the pore channels and quantifying their effect in a proposed non-linear and compound-specific parameterization of dispersion in porous media. Such behavior has been shown for both conservative (Rolle et al. TiPM 2012) and reactive (Hochstetler et al. AWR 2013) transport as well as in 2-D and 3-D setups (Ye et al. JCH 2014, accepted).
A modeling approach was adopted to improve the description of mixing and solute transport at different scales (Fig. 1) and, in particular, in heterogeneous aquifers at the field-scale. We selected complex heterogeneous hydraulic conductivity domains with statistical properties similar to the ones reported for the extensively investigated Borden aquifer (Ontario, Canada) and Columbus aquifer (Mississippi, USA). For the quantification of mixing, we proposed metrics based on the Shannon entropy to accurately evaluate dilution either in a given volume (dilution index) or in a given solute flux (flux-related dilution index). Our results for conservative and reactive transport demonstrate that although compound-specific diffusive processes occur at the small pore scale, they persist and manifest themselves at larger macroscopic scales relevant in practical applications in both mildly and highly heterogeneous aquifers (Rolle et al. JCH 2013). The study of plume dilution was also extended to transient transport (Rolle and Kitanidis AWR 2014) and in particular to solute and dilution breakthrough (Fig. 2).
Fig. 2. Transient transport: solute and dilution breakthrough.
Furthermore, the approach of DILREACT was extended to the evaluation of isotopic signatures during transport and transformation of organic contaminants (Jin et al. ES&T 2013 and 2014) and was applied to interpret the high-resolution data collected in a shallow aquifer contaminated by chlorinated compounds (Van Breukelen and Rolle ES&T 2012).
An additional topic tackled during DILREACT was the transport of charged species. We demonstrated the existence of Coulombic effects in advection-dominated transport of electrolytes in porous media and the need of a multicomponent ionic dispersion formulation to properly capture such electrostatically coupling effects in both homogeneous (Rolle et al. GCA 2013) and heterogeneous (Muniruzzaman et al. GCA 2014) porous media.
The novel results of DILREACT have broad implications for further theoretical and applied research as well as for practical applications. Direct fields of application include the management of groundwater resources, risk assessment and the implementation of natural and/or engineered remediation of contaminated sites. Such applications have an important impact on the civil society since the protection of groundwater as drinking water reservoir and as environmental valuable resource counts among the most important priority of European Environmental Legislation. Specifically, the results of this research are of interest for environmental policy makers as well for public and private consultants dealing with water quality issues and management and remediation of contaminated sites.
Besides the applications and associated socio-economic impact in the environmental field, the fundamental and physically-based outcomes of the project are relevant for a broad spectrum of disciplines dealing with flow and transport in porous media, such as reservoir engineering, carbon sequestration, chemical engineering, geology and geochemistry.
The research methodology of DILREACT is based on an integrated approach combining high-resolution laboratory and field data with multi-scale numerical modeling. Figure 1, modified from Rolle et al. JCH 2013, illustrates the scales of investigation of the DILREACT project.
Fig. 1. Scales of DILREACT investigation
High-resolution multi-tracer flow-through experiments and pore-scale modeling allowed capturing the importance of processes at the sub-continuum scale, such as incomplete mixing and the development of compound-specific concentration gradients in the pore channels and quantifying their effect in a proposed non-linear and compound-specific parameterization of dispersion in porous media. Such behavior has been shown for both conservative (Rolle et al. TiPM 2012) and reactive (Hochstetler et al. AWR 2013) transport as well as in 2-D and 3-D setups (Ye et al. JCH 2014, accepted).
A modeling approach was adopted to improve the description of mixing and solute transport at different scales (Fig. 1) and, in particular, in heterogeneous aquifers at the field-scale. We selected complex heterogeneous hydraulic conductivity domains with statistical properties similar to the ones reported for the extensively investigated Borden aquifer (Ontario, Canada) and Columbus aquifer (Mississippi, USA). For the quantification of mixing, we proposed metrics based on the Shannon entropy to accurately evaluate dilution either in a given volume (dilution index) or in a given solute flux (flux-related dilution index). Our results for conservative and reactive transport demonstrate that although compound-specific diffusive processes occur at the small pore scale, they persist and manifest themselves at larger macroscopic scales relevant in practical applications in both mildly and highly heterogeneous aquifers (Rolle et al. JCH 2013). The study of plume dilution was also extended to transient transport (Rolle and Kitanidis AWR 2014) and in particular to solute and dilution breakthrough (Fig. 2).
Fig. 2. Transient transport: solute and dilution breakthrough.
Furthermore, the approach of DILREACT was extended to the evaluation of isotopic signatures during transport and transformation of organic contaminants (Jin et al. ES&T 2013 and 2014) and was applied to interpret the high-resolution data collected in a shallow aquifer contaminated by chlorinated compounds (Van Breukelen and Rolle ES&T 2012).
An additional topic tackled during DILREACT was the transport of charged species. We demonstrated the existence of Coulombic effects in advection-dominated transport of electrolytes in porous media and the need of a multicomponent ionic dispersion formulation to properly capture such electrostatically coupling effects in both homogeneous (Rolle et al. GCA 2013) and heterogeneous (Muniruzzaman et al. GCA 2014) porous media.
The novel results of DILREACT have broad implications for further theoretical and applied research as well as for practical applications. Direct fields of application include the management of groundwater resources, risk assessment and the implementation of natural and/or engineered remediation of contaminated sites. Such applications have an important impact on the civil society since the protection of groundwater as drinking water reservoir and as environmental valuable resource counts among the most important priority of European Environmental Legislation. Specifically, the results of this research are of interest for environmental policy makers as well for public and private consultants dealing with water quality issues and management and remediation of contaminated sites.
Besides the applications and associated socio-economic impact in the environmental field, the fundamental and physically-based outcomes of the project are relevant for a broad spectrum of disciplines dealing with flow and transport in porous media, such as reservoir engineering, carbon sequestration, chemical engineering, geology and geochemistry.