Generally sites with radioactive contamination are also simultaneously polluted with many other different toxics, especially heavy metals. Besides the radioactivity, these wastes may also hold different amounts of chemicals, toxic pollutants and precipitates. The radionuclides released into the environment can give rise to human exposure by the transport through the atmosphere, aquatic systems or through soil sub-compartments. The exposure may result from direct inhalation of contaminated air or ingestion of contaminated water, or from a less direct pathway, the ingestion of contaminated food products. Contamination of the trophic chain by radionuclides released into the environment will be a component of human exposure to ionizing radiations by transferring the radionuclides into animal products that are components of the human diet. This can occur by first ingestion of contaminated pasture by animals and then by ingestion of animal products contaminated. The relevant incorporation of the radionuclides into cow's milk is usually due to the ingestion of contaminated pasture. This transfer process is often called the pasture-cow-milk exposure route. A compartment dynamic model is presented to describe mathematically the radium behaviour in the pasture-cow-milk exposure route and predict the activity concentration in each compartment. The dynamic model is defined by a system of linear differential equations with constant coefficients based in a mass balance concept. For each compartment a transient mass balance equation defines the relations between the inner transformations and the input and output fluxes. The concentration within each compartment is then transcribed to doses values based on a simplified exposure pathway and a pre-defined critical group.