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item Bradford, Scott

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/7/2006
Publication Date: 11/18/2006
Citation: Bradford, S.A., Simunek, J., Walker, S.L. 2006. Transport and straining of E. coli O157:H7 in saturated porous media. Water Resources Research. VOL 42, W12S12

Interpretive Summary: The transport and retention of a disease causing bacteria (Escherichia coli O157:H7) was studied in several sandy soils. The amount and location of bacteria retained in the soil was strongly dependent on the size of the soil particles and on the velocity of the flowing water through this system. The bacteria were transported in greater numbers when the water flow rate was higher and the sand grains were larger in size. Microscopic observations of E. coli retention in these sandy soils indicated that large numbers of bacteria could were retained in small soil pores. Results from this study strongly indicate that retention of bacteria in soil pores can be strongly influenced by chemical interactions between bacteria and the water flow rate. A computer model was adapted and successfully used to describe the observed bacteria transport behavior.

Technical Abstract: The transport and deposition behavior of pathogenic Escherichia coli O157:H7 was studied in saturated quartz sands of various sizes (710, 360, 240, and 150 mm) and at several flow rates. At a given velocity, column effluent concentration curves for E. coli tended to decrease in magnitude and become more asymmetric with decreasing sand size. In a given sand, experiments conducted at a higher velocity tended to produce higher effluent concentrations, especially for finer (240 and 150 mm) textured sands. The shape of the deposition profiles for E. coli were also highly dependent on the sand size and velocity. Coarser textured sands and higher flow rates were associated with less deposition and gradually decreasing concentrations with depth. Conversely, finer textured sands and lower flow rates tended to produce greater deposition and nonmonotonic deposition profiles that exhibited a peak in retained concentration. This deposition peak occurred nearer to the column inlet for finer textured sands and at low flow rates. Microscopic observations of E. coli retention in these sands indicated that straining was the dominant mechanism of deposition in the finer textured sands. Batch experiments also indicated that little E. coli attachment occurred for the selected sands and solution chemistry. A conceptual and numerical model was developed and successfully used to describe the observed E. coli transport and deposition data. Our conceptual model assumes that E. coli can aggregate when large numbers of monodispersed E. coli are deposited in straining sites. When the deposited E. coli reach a critical concentration in the straining site, the aggregated E. coli O157:H7 can be released into aqueous solution as a result of hydrodynamic shearing forces.