EPM Modeling of a Field‐Scale Tritium Tracer Experiment in Fractured, Weathered Shale

Abstract

A 2D equivalent porous media (EPM) model was used to simulate transport of tritium for a field‐scale tracer experiment in a fractured and highly weathered shale saprolite. The tritium plume was characterized by rapid migration of the leading edge of the plume (up to 0.4 m/day), slower movement of the center of mass of the tritium pulse (0.009 m/day) and very slow decline of concentrations in the “tail” of the breakthrough curve (which has persisted at the site for at least 16 years). The EPM model successfully described the shape of the plume and the breakthrough curves for a monitoring well 3.7 m downgradient of the injection well using a flow velocity of 0.01 m/day and longitudinal and transverse dispersivity values of 0.8 m. An unusually low ratio of longitudinal and transverse dispersivity (αL/αT= 1) was needed to fit the nearly circular shape of the plume, which is believed to be caused by the water‐table slope being perpendicular to the orientation of the prominent bedding plane fractures (which are expected to correspond to Kmax). Simulated values for concentrations in the long “tail” of the breakthrough curve observed in a downgradient well were especially sensitive to the value of longitudinal dispersivity used. The best‐fit simulation, based on data over a 5 year period, was extrapolated to the most recent data point (16 years after the start of the injection) and the simulated concentration was very close to the measured value. Model predictions made with a slightly different value of longitudinal dispersivity resulted in very large errors at late time, indicating that duration of monitoring data (which strongly influences the fitting of longitudinal dispersivity) is a critical factor in accurate prediction. The experiment and simulations show that contaminant plumes can persist for long periods of time in fractured porous materials, presumably due to diffusive exchange between the rapidly moving water in the fractures and the relatively immobile pore water in the fine‐grained matrix.

Hydrogeology and pedology of saprolite formed from sedimentary rock, eastern Tennessee, USA

“…Additional field tracer experiments, that involved adding solutes or colloids to wells in the deeper shale/ siltstone saprolite at Burial Ground 4 (Webster, 1996;McKay et al, 1997) and at West Bear Creek Valley McKay et al, 2000) and monitoring their down-slope movement, showed that once infiltration reached the water table, which typically occurs just above the saprolite-bedrock contact, it could also move very rapidly (b1 to 200 m/day). Both the stormflow zone and the upper portion of the water table zone (especially where it includes the saprolitebedrock transition zone) are considered as potential rapid contaminant migration pathways at the ORR.…”

“…The weathered material overlying the shale and siltstone is typically 1 to 10 m thick and retains a substantial amount of sedimentary layering and structure from the parent bedrock: Hence, it is referred to as saprolite (Dreier et al, 1987;Hatcher et al, 1992;Driese et al, 2001). Field studies in shale or siltstone saprolite at ORR include: pedologic and geologic characterization (Rothschild et al, 1984a,b;Dreier et al, 1987;Dorsch and Katsube, 1999;Driese et al, 2001); measurement of hydraulic properties of the solumn and upper C-horizon (Luxmoore et al, 1981b;Watson and Luxmoore, 1986;Wilson and Luxmoore, 1988;Wilson et al, 1989); lateral flow and transport of solute tracers in the upper 1-2 m of the solumn and saprolite (Wilson et al, 1993); vertical infiltration and solute transport in a 3 m deep undisturbed pedon (Jardine, personal communication); solute or colloid tracer experiments in the saturated zone just above the saprolite-bedrock contact Webster, 1996;Sanford et al, 1996;McKay et al, 1997McKay et al, , 2000; tracer experiments or monitoring of contaminant plumes in the partially weathered rock just below the saprolitebedrock contact (Olsen et al, 1983;Shevenell et al, 1994;McCarthy et al, 1998a,b;Jardine et al, 1999;Lenczewski et al, 2003).…”

Comparing Discrete Fracture and Continuum Models to Predict Contaminant Transport in Fractured Porous Media

“…Jørgensen et al (, ), McKay et al (), Helmke et al (), and Chambon et al (), among others, have shown that numerical models based on different conceptual models of transport in fractures can all be calibrated to reproduce equally well‐observed contaminant distributions in a porous medium that contains fractures or macropores. It can therefore be argued that the choice of a conceptual model to simulate transport in fractured porous media is mainly a question of preference or practicability, providing that transport in fractures and other macropores is accounted for (Helmke et al ; Harrar et al ).…”

“…Conditions that favor the suitability of the EPM and DP models are uniform and high macropore density compared to the scale of investigation, and rapid contaminant diffusion into the matrix blocks relative to advection along the fractures (Sudicky 1990;Jørgensen et al 1998;Helmke et al 2005). Jørgensen et al (1998Jørgensen et al ( , 2004a, McKay et al (1997), Helmke et al (2005), and Chambon et al (2011), among others, have shown that numerical models based on different conceptual models of transport in fractures can all be calibrated to reproduce equally well-observed contaminant distributions in a porous medium that contains fractures or macropores. It can therefore be argued that the choice of a conceptual model to simulate transport in fractured porous media is mainly a question of preference or practicability, providing that transport in fractures and other macropores is accounted for (Helmke et al 2005;Harrar et al 2007).…”

Aqueous flow and transport in analog systems of fractures embedded in permeable matrix

“…Preferential flow has been a subject of increasing interest particularly in contaminant hydrology. Investigations have shown that fractures and macropores provide shortcuts for the contaminant and significantly affect the hydraulic, dispersive, and hydrochemical characteristics of the porous medium [Keller et al, 1986;DMstous et al, 1989;JOrgensen and Fredericia, 1992;McKay et al, 1997;Villholth et al, 1998].…”