3.1.1  First-Order Decay

 The radioactive decay/degradation process is assumed to follow first-order kinetics, which means that the rate of loss of mass at any given time is directly proportional to the mass present at that time.  The mass flux equation for loss from the source zone by decay/degradation is then given by



where    l_i is the first-order decay/degradation coefficient for contaminant i (yr^-1)

This equation applies if decay/degradation is the only loss process, and also if the horizontal leaching process is occurring simultaneously.

 Each contaminant is assumed to have a unique overall first-order decay coefficient  l_i.  Note that Equation 3.5 is based on the assumption that the first-order decay coefficient is independent of which phase (aqueous, solid-sorbent, or NAPL) the contaminant resides in.