Release Rates of Manure-Borne Coliform Bacteria from Data on Leaching through Stony Soil

Manures are sources of several human pathogens that can potentially contribute to surface and groundwater contamination. Microorganisms must first be released from the manure matrix before they can infiltrate into and leach through the vadose zone. The objective of this study was to estimate rates of rainfall-induced release of fecal coliform (FC) from surface-applied bovine manure. Simulated rainfall of 7.1 cm h(-1) was applied to the surface of 90-cm-long lysimeters filled with the undisturbed stony soil. When the steady state was reached, clumps of manure were placed on the surface. Rainfall was continued for about 5 h after application of manure, and 10-min leachate portions were analyzed for turbidity and FC. The convective-dispersive equation with linear adsorption-exclusion and the first-order removal-regrowth terms was used as a model of the coliform transport in soil. Asymptotic properties of the solution of this equation with the exponentially decreasing boundary concentration were used to infer the release rate constant from the FC breakthrough curves. A value of 0.0054 +/- 0.0015 min(-1) was found for the FC release rate constant. The regression line of reduced coliform concentrations on reduced turbidity values was not significantly different from the one-to-one line; R-2 was 0.807. Assuming that turbidity can be used as a measure of concentration of manure particulates in leachates, we found that average values for the release rate constants were not significantly different for FC and manure particulates. The average velocity of bacteria and manure particulates transport was about seven times larger than the average pore velocity. The proposed technique of estimating FC and manure release rates shows promise for use in further studies needed to elucidate and assess factors affecting release rate.