A radial-flow tracer test in Chalk: Comparison of models and fitted parameters.
7 - 15.
The derivation of transport parameters from radial-flow tracer tests will depend strongly on the conceptual and mathematical model that is fitted to the data. This is illustrated by a test over 199 m in the English Chalk, which gave a unimodal, tailing breakthrough curve with about 25% tracer recovery. A laboratory study suggested that the tracer used, fluorescein, experienced little sorption on Chalk so it was assumed that the field results were unaffected by retardation. Three models were compared: a simple one-dimensional (1-D) advection-dispersion, radial-flow advection-dispersion, and radial flow in a double-porosity medium. The two advection-dispersion models failed to fit the overall shape of the breakthrough curve, but the double-porosity model was successful in this regard. No model accounted fully for the conservation of mass of tracer, but the double-porosity model was again superior. The estimation of transport parameters (porosity and dispersivity of the fracture network, porosity of the matrix) depends strongly on the model employed, with a six-fold variation in dispersivity between the lowest and highest values. If the results from such tracer tests are to be used in parameter selection for larger-scale models of contaminant transport, it is crucial that the tracer test is itself interpreted with an appropriate model.
|Title:||A radial-flow tracer test in Chalk: Comparison of models and fitted parameters|
|UCL classification:||UCL > School of BEAMS > Faculty of Maths and Physical Sciences > Earth Sciences|
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