Transport and retention of manure-borne coliforms in soil

Manure is a source of several bacterial pathogens that can potentially contribute to surface and groundwater contamination. Results from most bacterial transport studies in soils are only partially applicable to manure-borne bacteria because microorganisms are released along with manure particulates as manure dissolves. The objective of this study was to compare transport of chloride ion, Escherichia coli (E. coli) and manure colloids in undisturbed soil columns with a well-developed structure. Breakthrough column experiments were conducted with undisturbed, 20-cm long Tyler soil columns from the A horizon. A pulse of 4% filtered bovine manure solution with E. coli and KCl was passed through the columns. Escherichia coli concentrations, chloride content, and turbidity were measured in influent and in effluent. Columns were cut into 2-cm layers after the experiment to measure: (i) viable bacterial concentrations in pore solution and attached to soil; (ii) bulk density; (iii) water content. Companion batch experiments were performed to measure attachment of E. coli to soil in the presence of various amounts of manure. Escherichia coli attachment to soil decreased with increased manure content due to increased competition for attachment sites. Flow velocity affected E. coli transport and attachment to soil; there was relatively more attachment at slower flow velocity than at higher flow velocity. Escherichia coli attachment to soil was 18, 5, and 9% at flow velocities of 2.3, 8.4, and 9.3 cm d-1, respectively. Spatial variability in soil structure may result in large variations of pore water velocity and consequent differences in transport of manure particulates and bacteria under ponded infiltration.