Anthropogenic contaminants as tracers in fractured chalk aquifer: transport mechanisms and analytical modeling

摘要

One of the major problems encountered in characterizing fluid movement and solute transport in fractured media is how to deal with the high degree of complexity and heterogeneity in these media. To understand the physical transport mechanisms of contaminant in a single fracture, tracer tests were carried out using an artificially fractured chalk block (17 × 15 × 14 cm3). Sorptive and diffusive properties of chalk were evaluated by performing batch and diffusion experiments. The simulation of the breakthrough curves (BTCs) with the single fissure dispersion model could not give reasonable fit. A multi-channel dispersion model with the superposition of BTCs of at least two different flow paths resulted in an acceptable fit. Those simulations indicate the existence of flow channeling effects within the fracture. Comparing the BTCs of nitrate and eosin implied possible adsorption and/or degradation of nitrate within the fracture and the matrix. High dispersivity values are observed which might be explained as a result of spreading of the tracer pulses by diffusion into different flow channels and/or stagnant water zones. The description of the observed BTCs with a multi-channel model is still a theoretical suggestion and needs more investigations to be confirmed.