EFFECTS OF WATER IONIC STRENGTH ON VIRUS AND VIRUS-SIZED PARTICLE TRANSPORT IN A VARIABLE-APERTURE DOLOMITE ROCK FRACTURE

Experiments were conducted with a laboratory scale variable-aperture dolomite rock fracture to evaluate effects of water ionic strength and divalent cation on transport of viruses and virus-sized particles. Tracer tests were conducted using bromide, virus-sized carboxylate-modified latex microspheres of two sizes (0.02 and 0.2 µm diameter), and two bacteriophages (MS2 and PR772) as pathogenic virus surrogates. These tests were conducted at three ionic strengths and compositions (3, 5, and 12 mM using sodium chloride and calcium chloride) at the water pH of 8.2 ± 0.3 and temperature of 21.3 ± 1.0 degree Celsius. The test conditions provided an unfavorable attachment environment for the microspheres and bacteriophages to attach on the fracture surfaces. Retention of microspheres and bacteriophages in the fracture increased with increasing water ionic strength. Higher retention was also observed with calcium chloride, compared to sodium chloride for similar ionic strengths. The smaller bacteriophage MS2 was retained to a greater degree in the fracture than the larger bacteriophage PR772. A similar trend was observed in case of microspheres retention. A one-dimensional advection-dispersion transport model with a first-order particle removal term could simulate most of the breakthrough curve except for the tailing behavior observed. Microsphere and bacteriophage diffusion into the rock matrix/micro-fissures connected to the fracture, preferential flow paths in the fracture, and presence of kinetic adsorption sites on the fracture surface likely contributed to these deviations between model and experiment. Application of a transport model incorporating matrix diffusion, particle attachment to kinetic adsorption sites, particle detachment, and decay is being conducted to provide a better understanding of virus and virus-sized particle transport in fractures.