Animal waste is a valuable resource, which can add organic matter, nitrogen, phosphorous and potassium as well as other nutrients needed for plant growth when it is used in the agricultural field. When applied to an agricultural field, infectious agents or pathogenic organisms in the animal waste are usually retained in the soil. At the same time, surfactants are popularly utilized in pesticide applications to aid pesticide solubility and mobility. Consequently, the retained pathogenic strains might be flushed out of the soil matrices with a possibility of contaminating the groundwater. In this research, we investigated desorption of E. coli O157:H7 pre-deposited in silica sand and its subsequent release and transport in the presence of rhamnolipid biosurfactant in unsaturated laboratory columns. Based on the experimental observations, it was discovered that retained E. coli O157:H7 was released during flushing with rhamnolipid biosurfactant solutions. E. coli O157:H7 release rate coefficient increased both with the increase of rhamnolipid biosurfactant concentration and the increase of water saturation. With the increase of rhamnolipid biosurfactant concentration, the air-water surface tension decreased. With the increase of water saturation, the air-water interfacial area decreased. Both the decreased air-water surface tension and air-water interfacial area favored E. coli O157:H7 release. Increase of E. coli O157:H7 release rate coefficient with the decrease of air-water surface tension was more pronounced for low air-water interfacial area (i.e., high water saturation). Similarly, increase of E. coli O157:H7 release rate coefficient with the decrease of air-water interfacial area was more pronounced for low air-water surface tension (i.e., high rhamnolipid biosurfactant concentration). E. coli O157:H7 retention in unsaturated systems was found to be controlled by the capillary force, which was greatly impacted by water saturation and air-water surface tension. Increase of water saturation and rhamnolipid biosurfactant concentration both contributed to the decrease of the capillary force, resulting in increased E. coli O157:H7 release rate. (C) Koninklijke Brill NV, Leiden, 2011