title: On the importance of diffusion and compound-specific mixing for groundwater transport: an investigation from pore to field scale.
abstract: Mixing processes significantly affect and limit contaminant transport and transformation rates in the subsurface. The correct quantification of mixing in groundwater systems must account for diffusion, local-scale dispersion and the flow variability in heterogeneous flow fields (e.g., flow-focusing in high-conductivity and de-focusing in low-conductivity zones). Recent results of multitracer laboratory experiments revealed the significant effect of compound-specific diffusive properties on the physical displacement of dissolved species across a representative range of groundwater flow velocities. The goal of this study is to investigate the role of diffusion and compound-specific mixing for solute transport across a range of scales including: (i) pore-scale (~10⁻² m), (ii) laboratory bench-scale (~10⁰ m) and (iii) field-scale (~10² m). We investigate both conservative and mixing-controlled reactive transport using pore-scale modeling, flow-through laboratory experiments and simulations, and field-scale numerical modeling of complex heterogeneous hydraulic conductivity fields with statistical properties similar to the ones reported for the extensively investigated Borden aquifer (Ontario, Canada) and Columbus aquifer (Mississippi, USA, also known as MADE site). We consider different steady-state and transient transport scenarios. For the conservative cases we use as a metric of mixing the exponential of the Shannon entropy to quantify solute dilution either in a given volume (dilution index) or in a given solute flux (flux-related dilution index). The decrease in the mass and the mass-flux of the contaminant plumes is evaluated to quantify reactive mixing. The results show that diffusive processes, occurring at the small-scale of a pore channel, strongly affect conservative and reactive solute transport at larger macroscopic scales. The outcomes of our study illustrate the need to consider and properly account for compound-specific diffusion and mixing limitations in order to accurately describe and predict conservative and reactive transport in porous media.
authors.affiliation: Department of Civil and Environmental Engineering, Stanford University, 473 Via Ortega, 94305 Stanford, CA, USA; Center for Applied Geosciences, University of Tübingen, Hoelderlinstrasse 12, D-72074 Tübingen, Germany. Electronic address: mrolle@stanford.edu.
authors.forename: Massimo
authors.initials: M
authors.lastname: Rolle
authors.forename: Gabriele
authors.initials: G
authors.lastname: Chiogna
authors.forename: David L
authors.initials: DL
authors.lastname: Hochstetler
authors.forename: Peter K
authors.initials: PK
authors.lastname: Kitanidis
mesh_terms: D004058:Diffusion; D060587:Groundwater / Q000737:chemistry*; D008962:Models, Theoretical; D016062:Porosity; D016958:Pseudomonas putida / Q000378:metabolism; D014050:Toluene / Q000737:chemistry / Q000378:metabolism; D014872:Water Movements; D014874:Water Pollutants, Chemical / Q000737:chemistry* / Q000378:metabolism
chemical_list: D014874:Water Pollutants, Chemical; D014050:Toluene
keywords: Diffusion; Dilution index; Groundwater transport; Mixing; Numerical modeling