TRACER TEST IN FRACTURED CHALK .2. NUMERICAL-ANALYSIS

A two-well tracer test carried out in fractured chalk was analyzed usi ng a three-dimensional finite-difference moder for Bow and transport w hich, was constructed on the basis of the geological and hydraulic inf ormation collected at the field site. The model was developed as a dua l-porosity continuum model, in which advection was assumed to occur on ly in the fractures, and the water in the porous matrix was assumed to be static. The exchange of solute between the fractures (mobile phase ) and the porous matrix (immobile phase) was assumed to occur as a dif fusion process in response to the local concentration difference of so lute between the two phases. Simulations from the dual-porosity model reproduced the shape of the observed breakthrough curves, although som e portions of the tail were not accurately represented. The model was also applied as a single-porosity model for advection and dispersion i n the fractures with no solute exchange with the porous matrix. The si mulations from the single-porosity model greatly overestimated the obs erved lithium concentrations, and showed very little tailing effect in the falling limb. The study shows that, based on the given tracer tes t, solute transport in a fractured chalk cannot be represented by a si ngle-porosity approach and hence when dealing with contaminant transpo rt in such systems, both a fractured and a porous domain need to be co nsidered.