The Transport of Escherichia coli Through Freeze-Fractured Clay Soil

Abstract

Little is known about the transport of microorganisms through freeze-fractured clay soils. Normally consolidated clay (NCC) and compacted clay (CC) columns (representing a natural clay barrier and a compacted barrier, respectively) were exposed to six consecutive freeze–thaw cycles and permeated for 21 days with an Escherichia coli cell suspension (approximately 1 × 10⁷ colony forming units (CFU)/mL) containing a 2.1-mM bromide tracer. An unfractured sand column was also examined for comparison with the clay columns. While no E. coli was detected in the effluent of both untreated NCC and CC control clay columns, a relatively low density of E. coli (between 228 and 270 CFU/mL compared to 1 × 10⁷ CFU/mL in the influent) was first detected in the effluent of the freeze-fractured NCC and CC columns at 0.29 and 0.31 pore volumes (or at 5.4 and 4.1 h), respectively. It took 11 min for a full breakthrough of E. coli through the sand column, but only about 0.1% of the influent E. coli density was detected in the effluents of the freeze-fractured NCC and CC columns at day 21. These observations show that despite the high bacterial retention capacity of the freeze-fractured clay columns, the fractures were large enough for the E. coli to flow through. Based on batch sorption tests and the permeation data, it is estimated that 18%, 7%, and 84% of the freeze-fractured NCC, CC, and sand columns would be exposed to the influent, respectively, under a full E. coli breakthrough condition. Our data show that the high bacterial retention capacity of clay barriers can be compromised by freeze–thaw conditions.