A comparison of clay colloid and artificial microsphere transport in natural discrete fractures

Transport of monodispersed buoyant 1-μm latex microspheres, dense 1.34-μm montmorillonite particles, Li+ and Br− was investigated in a naturally fractured chalk core with an average equivalent hydraulic aperture of 183 μm. Studied parameters were: tracer arrival time, C/C0C/C0 values, mass recovery, size distribution and the impact of initial concentration. Breakthrough time of both colloidal tracers was faster than that of the soluble tracers. Significantly lower recovery and slightly slower breakthrough time were observed for the clay particles relative to the microspheres, apparently mainly due to the former's higher density, resulting in preferential gravitational settling of the clay particles. However, variable surface charge and nonuniform shape and size of the clay particles may also play a role in the observed differences. From the theoretical scale ratio, the time interval calculation seems to be a major factor in colloid recovery. Clay-particle size fractionation was observed (0.62 vs 1.34 μm at the outflow and inflow, respectively), and there was no significant influence of the initial concentration (100 and 500 mg/L) on transport properties. Our observations indicate that colloid density is a dominant property for their transport in fractures. This work emphasizes the need for caution when the results of studies in which buoyant colloids are used as tracers are extrapolated to natural systems in which clay colloids are present.

Graphical abstractColloid density is a dominant property for colloid transport in fractures. This work emphasizes the need for caution when the results of studies in which buoyant colloids are used as tracers are extrapolated to natural systems in which clay colloids are present. Note the significant differences between the BTC's of solutes vs both dense and buoyant colloids.Figure optionsDownload full-size imageDownload as PowerPoint slide