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 (α(L)/α(T) = 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 K(max)). 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.