Investigating the impact of advective and diffusive controls in solute transport on geoelectrical data

Water Resources Research

Dehkordy

et al. 2018

Self Cite

Quantifying coupled mobile/less‐mobile porosity dynamics is critical to the prediction of biogeochemical storage, release, and transformation processes in the zone where groundwater and surface water exchange. The recent development of fine‐scale geoelectrical monitoring paired with pore‐water sampling in groundwater systems enables direct characterization of hydrologic exchange between more‐ and less‐mobile porosity during tracer tests. We adapt this technique to sandy interface sediments at a groundwater flow‐through kettle lake. Tracer experiments were conducted within controlled‐head permeameters over a range of specified downward flow conditions over several days. Although the bed was predominantly composed of highly permeable sands and gravels, cobble inclusions created less‐mobile flow zones at the centimeter scale. Less‐mobile porosity fractions, residence times, and rates of exchange were inferred from paired bulk and fluid electrical conductivity data, without the need for inverse model calibration. The conservative solute experiments were paired with 15NO3− and other reactive amendments, revealing anaerobic processes occurring at shallow sediment depths where pore‐water sampling indicated bulk‐oxic conditions. The average less‐mobile porosity residence times as evaluated with the geoelectrical method were on 1‐hr timescales, which appear to be biogeochemically important in the context of creating anoxic microzones within less‐mobile porosity of sandy interface sediments.

Section: Introductionmentioning

confidence: 99%

Direct Observations of Hydrologic Exchange Occurring With Less‐Mobile Porosity and the Development of Anoxic Microzones in Sandy Lakebed Sediments

Briggs

Water Resources Research

Dehkordy

et al. 2018

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“…The advection–dispersion equation is the classical description of subsurface solute transport (Bencala, ), with the assumption of a Gaussian distribution for concentration breakthrough curves (BTCs; Baeumer, Benson, Meerschaert, & Wheatcraft, ; Wheaton & Singha, ). However, this paradigm cannot explain some anomalous transport behaviours, such as skewed BTCs in heterogeneous media reported both in field and laboratory‐scale experiments (Baeumer et al, ; Briggs et al, ; G. Liu, Zheng, & Gorelick, ; Wheaton & Singha, ).…”

Section: Introductionmentioning

confidence: 99%

“…In a heterogeneous medium, connectivity of pores will vary spatially. Well-connected pores form mobile, advection-dominated flow domains, whereas poorly connected pores form less-mobile domains where exchange may be dominated by either molecular diffusion or relatively slow advection (Berkowitz, Cortis, Dentz, & Scher, 2006; Day-Lewis, Linde, Haggerty, Singha, & Briggs, 2017;Feehley, Zheng, & Molz, 2000;Wheaton & Singha, 2010;Wondzell, 2015). These coupled but functionally different porosity domains result in a spectrum of solute residence times in the hyporheic zone that may show power-law distribution in channel return flow (Aubeneau, Hanrahan, Bolster, & Tank, 2014;Cardenas, 2015;Cardenas, Cook, Jiang, & Traykovski, 2008;Gooseff, McKnight, Runkel, & Vaughn, 2003;Haggerty, Wondzell, & Johnson, 2002).…”

Section: Introductionmentioning

confidence: 99%

“…However, this paradigm cannot explain some anomalous transport behaviours, such as skewed BTCs in heterogeneous media reported both in field and laboratory-scale experiments (Baeumer et al, 2001;Briggs et al, 2015;G. Liu, Zheng, & Gorelick, 2007;Wheaton & Singha, 2010). Mathematical models based on a dual-domain approach such as first-order mass transfer (van Genuchten & Wierenga, 1976), multi-rate mass transfer (Haggerty & Gorelick, 1995), and continuous-time random-walk (Berkowitz et al, 2006) have been suggested to describe such behavior.…”

Section: Introductionmentioning

confidence: 99%

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Simulation of less‐mobile porosity dynamics in contrasting sediment water interface porous media

Dehkordy

Briggs

et al. 2018

Hydrological Processes

Considering heterogeneity in porous media pore size and connectivity is essential to predicting reactive solute transport across interfaces. However, exchange with less‐mobile porosity is rarely considered in surface water/groundwater recharge studies. Previous research indicates that a combination of pore‐fluid sampling and geoelectrical measurements can be used to quantify less‐mobile porosity exchange dynamics using the time‐varying relation between fluid and bulk electrical conductivity. For this study, we use macro‐scale (10 s of cm) advection–dispersion solute transport models linked with electrical conduction in COMSOL Multiphysics to explore less‐mobile porosity dynamics in two different types of observed sediment water interface porous media. Modeled sediment textures contrast from strongly layered streambed deposits to poorly sorted lakebed sands and cobbles. During simulated ionic tracer perturbations, a lag between fluid and bulk electrical conductivity, and the resultant hysteresis, is observed for all simulations indicating differential loading of pore spaces with tracer. Less‐mobile exchange parameters are determined graphically from these tracer time series data without the need for inverse numerical model simulation. In both sediment types, effective less‐mobile porosity exchange parameters are variable in response to changes in flow direction and fluid flux. These observed flow‐dependent effects directly impact local less‐mobile residence times and associated contact time for biogeochemical reaction. The simulations indicate that for the sediment textures explored here, less‐mobile porosity exchange is dominated by variable rates of advection through the domain, rather than diffusion of solute, for typical low‐to‐moderate rate (approximately 3–40 cm/day) hyporheic fluid fluxes. Overall, our model‐based results show that less‐mobile porosity may be expected in a range of natural hyporheic sediments and that changes in flowpath orientation and magnitude will impact less‐mobile exchange parameters. These temporal dynamics can be assessed with the geoelectrical experimental tracer method applied at laboratory and field scales.

“…We demonstrate, for the first time, the synthesis of point‐scale fluid samples of fluid conductivity ( σ f , μS/cm) and electrical geophysical data to directly quantify anomalous mass transfer behavior in situ, and provide controlled laboratory evidence to show that the σ b ‐ σ f hysteresis observed in field data [ Singha et al , 2007] is a function of mass transfer between mobile and immobile domains. We collected electrical resistivity measurements made on controlled media (i.e., quartz sand and zeolites) in column experiments to isolate the effect of mass transfer from other factors such as heterogeneity or difference in support scale [ Wheaton and Singha , 2010]. Low‐field nuclear magnetic resonance (NMR) measurements are used to corroborate the pore size distribution and link the best‐fit estimates of mobile and immobile porosity obtained from time‐lapse fluid sample measurements.…”

Section: Introductionmentioning

confidence: 99%

Direct geoelectrical evidence of mass transfer at the laboratory scale

Swanson

Singha

Water Resources Research

et al. 2012

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[1] Previous field-scale experimental data and numerical modeling suggest that the dual-domain mass transfer (DDMT) of electrolytic tracers has an observable geoelectrical signature. Here we present controlled laboratory experiments confirming the electrical signature of DDMT and demonstrate the use of time-lapse electrical measurements in conjunction with concentration measurements to estimate the parameters controlling DDMT, i.e., the mobile and immobile porosity and rate at which solute exchanges between mobile and immobile domains. We conducted column tracer tests on unconsolidated quartz sand and a material with a high secondary porosity: the zeolite clinoptilolite. During NaCl tracer tests we collected nearly colocated bulk direct-current electrical conductivity ( b ) and fluid conductivity ( f ) measurements. Our results for the zeolite show (1) extensive tailing and (2) a hysteretic relation between f and b , thus providing evidence of mass transfer not observed within the quartz sand. To identify bestfit parameters and evaluate parameter sensitivity, we performed over 2700 simulations of f , varying the immobile and mobile domain and mass transfer rate. We emphasized the fit to late-time tailing by minimizing the Box-Cox power transformed root-mean square error between the observed and simulated f . Low-field proton nuclear magnetic resonance (NMR) measurements provide an independent quantification of the volumes of the mobile and immobile domains. The best-fit parameters based on f match the NMR measurements of the immobile and mobile domain porosities and provide the first direct electrical evidence for DDMT. Our results underscore the potential of using electrical measurements for DDMT parameter inference.