Rainfall Intensity-Dependent Release of Indicator Bacteria from Dairy Cattle Manure Applied on Vegetated Soil.

Simulating the release of manure-borne microorganisms during rainfall is essential for making predictions about their environmental fate and transport. The objectives of this work were to determine the effect of rainfall intensity on the release of E. coli, enterococci, and total coliforms from dairy cattle manure that was applied in soil boxes. The boxes (100 x 35 x 25 cm) were packed with sandy loam soil and vegetated with Kentucky blue grass. Three levels of rainfall intensity (3, 6, and 9 cm hr^-1) were used and runoff and leachate samples were collected at gradually increasing time intervals for one hour of release. Soil was sampled at 0, 2, 5, and 10 cm depths after rainfall. The runoff release data were fit with the one-parametric exponential release model and the two-parametric power-rational release model. Concentrations of bacteria in the initial runoff were approximately one order of magnitude below their effective concentrations in manure. The release kinetics began with a precipitous log-linear increase in cumulative mass released during the first 1 cm of rainfall, which was followed by a much slower release for the remainder of the rainfall event. Release kinetics for the three bacteria types did not significantly differ. While the rate-constant parameters in both models were not significantly affected by rainfall intensity, the concentrations of bacteria remaining at different soil depths following rainfall depended on it. The release and leaching of enterococci appeared to be more affected by rainfall intensity compared with other bacteria. Based on the root-mean-squared-error and the Akaike information criterion, the power-rational model is recommended for simulating the total relative mass of manure-bacteria release as a function of rainfall depth. These results will be used for improvement of microbial release and contaminant hydrology models that are used in risk assessment of microbial contamination in the environment.