Influence of Specific Surface Area on Transport of Sorbing Solutes in Fractures: An Experimental Analysis

Experimental evidence is presented on the influence of specific surface area on the transport of the sorbing tracer strontium in fractures with uniform, but differing aperture. Specific surface area is defined as the ratio of the fracture surface area to the volume of mobile water in the fracture. Static sorption experiments on granite coupons suggest hysteresis in the sorption process, showing higher surface distribution coefficients for desorption than for sorption. Strontium was subject to significantly greater dispersion than the nonreactive tracer tritium. This enhanced dispersion is believed to be the result of chemical heterogeneity at the mineral grain scale, hysteresis in sorption, and limited transverse mixing across the fracture aperture. The influence of fracture aperture on retardation is much greater than predicted by the commonly used definition of the surface retardation factor. Strontium retardation was approximately an order of magnitude greater in a smaller-aperture fracture (450 µm, Ra ∼ 45) than in a large-aperture fracture (780 µm, Ra ∼ 3.5). We hypothesize that hysteresis in sorption, in conjunction with limited transverse mixing across the aperture, caused the apparent increase in sorption strength (Ka) with a decrease in fracture aperture.