Animal waste is a valuable resource, which, when managed properly, can reduce the need for commercial fertilizer. It can also improve the soil water holding capacity and tilth. Similarly, as a newly recognized water resource, nutrient-rich reclaimed wastewater supplies not only water, but also plant nutrients (especially nitrogen and phosphorus) that can benefit agricultural production. When animal waste or reclaimed wastewater is used for land applications or agricultural irrigation, the major concern is the possible spreading of pathogenic organisms in the soil and the possibility of groundwater contamination once the pathogenic organisms pass through the vadose zone and reach the groundwater table. Proper chemical treatment is required either to kill the pathogens or to enhance the retention of the pathogens in the subsurface soil to protect the groundwater from being contaminated. In this study, we have investigated the impact of lipopolysaccharide extraction on bacterial retention in a porous medium (silica sand) using column experiments. Three gram-negative bacterial strains of Escherichia coli, Pseudomonas fluorescens and Pseudomonas aeruginosa were used as model bacterial strains and their transport was described by the two-region (equilibrium/kinetic) model. After lipopolysaccharide extraction, all these three strains showed greater retention in the porous medium. Increase in retention after the lipopolysaccharide extraction was most pronounced for Pseudomonas fluorescens and least for E. coli. Bacterial retention in the porous medium was correlated with their interactions with the porous medium. (C) Koninklijke Brill NV, Leiden, 2008