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Title: Transport of E. coli D21g with runoff water under different solution chemistry conditions and surface slopes

Author
item Bradford, Scott
item Headd, Brendan
item ARYE, GILBOA - Ben Gurion University Of Negev
item SIMUNEK, JIRI - University Of California

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/20/2015
Publication Date: 4/27/2015
Citation: Bradford, S.A., Headd, B.J., Arye, G., Simunek, J. 2015. Transport of E. coli D21g with runoff water under different solution chemistry conditions and surface slopes. Journal of Hydrology. 525:760-768.

Interpretive Summary: Runoff has been reported to be the primary transport route for pathogen dissemination on hillslopes. Tracer and indicator microbe runoff experiments, and associated numerical simulations, were conducted to investigate the influence of solution chemistry on the transport, retention, and release of E. coli D21g. The runoff breakthrough curves (BTCs) for D21g were sensitive to the soil slope and the solution chemistry. Greater amounts of cell retention occurred for higher chamber slopes because of enhanced exchange with the soil, and at higher solution ionic strength (IS) because of an increase in the adhesive interaction with the soil. Retained cells were slowly released from the soil to the runoff water when the IS of the runoff water was reduced, especially at the highest chamber slope that had the greatest amount of retained cells. This information will be of use to scientists, engineers, regulators and health professionals in assessing the risks of microbial contamination to water resources.

Technical Abstract: Tracer and indicator microbe runoff experiments were conducted to investigate the influence of solution chemistry on the transport, retention, and release of Escherichia coli D21g. Experiments were conducted in a chamber (2.25 m long, 0.15 m wide, and 0.16 m high) packed with ultrapure quartz sand (to a depth of 0.10 m) that was placed on a metal frame at slopes of 5.6%, 8.6%, or 11.8%. Runoff studies were initiated by adding a step pulse of salt tracer or D21g suspension at a steady flow rate to the top side of the chamber and then monitoring the runoff effluent concentrations. The runoff breakthrough curves (BTCs) were asymmetric and exhibited significant amounts of concentration tailing. The peak concentration levels were lower and the concentration tailing was higher with increasing chamber slope because of greater amounts of exchange with the sand and/or extents of physical nonequilibrium (e.g., water flow in rills and incomplete mixing) in the runoff layer. Lower amounts of tailing in the runoff BTC and enhanced D21g retention in the sand occurred when the solution ionic strength (IS) was 100 mM NaCl compared with 1 mM NaCl, due to compression of the double layer thickness which eliminated the energy barrier to attachment. Retained cells were slowly released to the runoff water when the IS of the runoff water was reduced to deionized water. The amount and rate of cell release was greatest at the highest chamber slope, which controlled the amount of exchange with the sand and/or the extent of physical nonequilibrium in the runoff layer, and the amount of retained cells. The observed runoff BTCs were well described using a transient storage model, but fitted parameters were not always physically realistic. A model that accounted for the full coupling between flow and transport in the runoff and sand layers provided useful information on exchange processes at the sand surface, but did not accurately describe the runoff BTCs which were influenced by physical nonequilibrium in the runoff layer.