Exploring the effects of aperture size, aperture variability and matrix properties on biocolloid transport and retention in a single saturated fracture

Abstract

<p>To increase the understanding of contaminant transport, specifically biocolloid transport in fractured media, a series of experiments were conducted on single saturated fractures. Hydraulic and solute tracer tests were used to characterize three separate fractures: one natural fracture and two synthetic fractures. Zeta potentials are reported showing the high negative electric charge of the synthetic fractures relative to the natural fractures in the phosphate buffer solution (PBS) used during the biocolloid tracer tests.</p> <p><em>E. coli</em> RS2-GFP tracer tests were conducted on all three fractures at specific discharges of 5 m/d, 10 m/d and 30 m/d. Lower <em>E. coli</em> recovery was consistently observed in the natural fracture, due to 1) attachment because of the lower negative charge of the natural fracture relative to the synthetic fracture; and 2) the presence of dead end fractures within the fracture matrix. In the synthetic fractures, where surface charges were equal, in the larger, more variable fracture aperture, lower recoveries were found when compared to the smaller, less variable fracture aperture, which was not expected. This indicates that aperture variability plays a larger role than fracture aperture size in the retention of biocolloids in fractures.</p> <p>Differential transport was consistently observed in all three fractures, but was more prominent in the synthetic fractures. This indicates that charge exclusion plays a more dominant role in the differential transport of colloids than size exclusion, though size exclusion cannot be eliminated as a retention mechanism based on these experiments. Differential transport was also heavily influenced by specific discharge as the difference in arrival times between the bromide and <em>E. coli</em> increased in all three fractures as the specific discharge decreased.</p> <p>Visualization tests were completed on the synthetic fractures showing the location of multiple preferential flow paths, as well as areas with low flow.</p>