Assessment of turbulence effects on effective solute diffusivity close to a sediment-free fluid interface
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ORIGINAL PAPER
Assessment of turbulence effects on effective solute diffusivity close to a sediment-free fluid interface E. Baioni1,2 • G. M. Porta2 • M. Mousavi Nezhad1
•
A. Guadagnini2
Accepted: 5 September 2020 Ó The Author(s) 2020
Abstract Our work is focused on the analysis of solute mixing under the influence of turbulent flow propagating in a porous system across the interface with a free fluid. Such a scenario is representative of solute transport and chemical mixing in the hyporheic zone. The study is motivated by recent experimental results (Chandler et al. Water Res Res 52(5):3493–3509, 2016) which suggested that the effective diffusion parameter is characterized by an exponentially decreasing trend with depth below the sediment-water interface. This result has been recently employed to model numerically downstream solute transport and mixing in streams. Our study provides a quantification of the uncertainty associated with the interpretation of the available experimental data. Our probabilistic analysis relies on a Bayesian inverse modeling approach implemented through an acceptance/rejection algorithm. The stochastic inversion workflow yields depth-resolved posterior (i.e., conditional on solute breakthrough data) probability distributions of the effective diffusion coefficient and enables one to assess the impact on these of (a) the characteristic grain size of the solid matrix associated with the porous medium and (b) the turbulence level at the water-sediment interface. Our results provide quantitative estimates of the uncertainty associated with spatially variable diffusion coefficients. Finally, we discuss possible limitations about the generality of the conclusions one can draw from the considered dataset. Keywords Effective diffusion Sediment-water interface Stochastic model calibration hyporheic region Uncertainty quantification
1 Introduction The hyporheic region is a major component driving functioning of river ecosystems through regulation of processes associated with, e.g., attenuation of pollutants by biodegradation or adsorption and mixing. All of these & M. Mousavi Nezhad [email protected] E. Baioni [email protected] G. M. Porta [email protected] A. Guadagnini [email protected] 1
School of Engineering, University of Warwick, Warwick CV47AL, UK
2
Dipartimento di Ingegneria Civile e Ambientale, Politecnico di Milano, Milano, Italy
processes are critically affected by chemical residence time and mixing rate within the hyporheic zone, these being in turn controlled by turbulent flow patterns which are documented to propagate across the interface between river flow and the hyporheic region (Buss et al. 2009). Major research findings on stream-subsurface solute exchanges and feedbacks are summarized in Boano et al. (2014) and Rode et al. (2015). Dissolved chemicals transported through stream flow are typically subject to a temporary storage within the hyporheic zone beneath and/or alongside the stream bed followed by subsequent release into
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