Transport and Retention of Manure-borne E. coli in Saturated Well-Structured Soil

Manure is a source of several bacterial pathogens that can potentially contribute to surface and ground water contamination. We hypothesized that manure colloids could enhance bacteria survival, could compete for soil adsorption sites and serve as carriers. Colloid transport in soil is affected by soil structure and flow velocity, because only the pathways formed by large pores can serve as conduits for colloidal particles. Therefore, transport of manure-borne bacteria should be affected by flow velocity. To test these two hypotheses, undisturbed 20-cm soil columns of a silt loam soil were subjected to saturation. A pulse of 4% filtered bovine manure solution containing E. coli bacteria and KCl was passed through columns, preceded and followed by deionized water infiltration at 9 degrees C during 10 days. E. coli concentrations, chloride content and turbidity were measured in influent and in effluent. After the experiment, columns were cut into 2-cm layers to enumerate viable bacteria in pore solution and bacteria attached to the soil, and to measure bulk density and water content. Complementary batch experiments were carried out to measure attachment of E. coli to soil in presence of various amounts of manure colloids. Attachment of E. coli to soil was much smaller in presence of manure, and decreased with the increase in manure contents. The attachment isotherm was linear without manure, and convex in presence of manure. Maximum bacteria concentrations in leachate were observed before the first pore volume of soil solution has been displaced with the influent. Maximum breakthrough chloride concentrations were observed after the one pore volume of influent passed the column. Effluent turbidity peaked and then stabilized at low levels. Bacteria content in soils varied within two orders of magnitude after the breakthrough experiment. From 1% to 3% of the total applied bacteria were found in pore solution, and from 5% to 18% were attached to soil particles. Individual columns had different average water flow velocities ranging from 2.3 to 9.3 cm/day. E. coli and manure colloid transport was similar at low velocity during the whole experiment. At high flow rates, the E. coli transport was similar to the chloride transport until 0.5 volume of the pore solution was replaced with the influent, and was retarded after that. Bacteria and manure breakthrough curves had much longer tails compared with chloride. The E. coli attachment to soil in the fast-flow columns was similar to that in the batch experiment with 4% manure content. Attachment in the batch experiments with 0% and 2% manure bracketed the attachment observed in the slow-flow columns. Overall, slow manure colloid transport and high concentration in pore solution reduced attachment to soil and increased survival of E. coli. Increase in flow velocity decreased attachment and entrapment of manure and bacteria in pore space. Variability in flow velocity and its effect on E. coli and manure transport were probably caused by different macroporosity in individual columns of the same soil.