Transport of Escherichia coli through soil / by Bruce Rosa.

Abstract

Sorption is an important process in the retention of pathogens by clay barriers. Batch sorption experiments were conducted to investigate the sorption of nonpathogenic 'E. coli' to illite, kaolinite, and montmorillonite clays, a natural red clay, a silt and a sand. The Freundlich isotherm model provides the best fit for the sorption data. The sorptive capacity (SC) of each soil for 'E. coli' was calculated at an equilibrium ' E. coli' concentration of 1 x 108 CFU/mL. The SC values correlate strongly with the measured cation exchange capacity (having an R 2 of 0.92), as well as the weighted average particle size (having an R2 of 0.83), of the soils. Columns of compacted and normally consolidated high plastic clay and compacted non-plastic silt soils were exposed to cycles of freezing and thawing to simulate conditions of physical weathering, in turn creating a fracture network in the specimens. To determine bacterial transport properties, these columns, as well as intact sand, were permeated with a nonpathogenic E. coli strain suspended in a minimal salt medium in a constant head permeability mode. As a result of freezing and thawing, the permeability of the soil columns was found to increase by one order of magnitude for the silt and two to three orders of magnitude for the normally consolidated and compacted clay columns. Full (100%) breakthrough of the E coli was observed after three pore volumes of flow in the sand, 35 pore volumes of flow in the silt, and was not observed in both compacted and normally consolidated clay columns, even after as many as 55 pore volumes of flow. However, as many as 5 x 10[superscript 5] CFU/mL of E. coli were found to permeate through clay columns, indicating a possible public health risk from bacterial transport through freeze-fractured soil liners. Both sorption and filtration through soil were found to be important factors in determining bacterial transport through fine grained soils.