Endocrine-disrupting hormones may enter the environment via land application of livestock manure. With respect to both livestock production and soils, Iowa is the prototype for agriculture in the Midwest. Our hypothesis is that the risk of hormone transport can be better understood by knowledge of the mechanisms of sorption, desorption, and transport of colloid-hormone complexes. Objectives are to (1) determine the rate, intensity, and capacity for adsorption and desorption of estrogens by colloidal components of cattle manure and three Iowa soils, (2) quantify the impact of physical and chemical variables that regulate colloid-mediated transport of estrogens in soils, and (3) quantify colloid-mediated pathways in the profile-scale dissipation of manure-associated estrogens. The research plan focuses on: (1) adsorption-desorption processes that determine how much of a land-applied hormone is transferred from manure to the aqueous phase or to the soil, (2) leaching of colloid-associated estrogens through aggregated, structured soils, and (3) transport of colloid-associated estrogens with runoff. This scale-integrated research will strengthen the development of transport models that are indispensable for assessing the environmental risk of hormones in land-applied livestock waste. The work proposed and will address issues where current knowledge is critically incomplete. At the project’s conclusion, we will have identified key mechanisms by which estrogens interact with colloidal organics in cattle manure and in soil. We will also have identified how those mechanisms can be incorporated into predictive models of hormone transport in or over soils.