5.4.1 First-Order Decay

     The degradation or radioactive decay 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 alone is then given by



Each contaminant is assumed to have a unique overall first-order decay coefficient l_i.

     Note that Equation 5.57 is based on the assumption that the first-order decay coefficient is independent of in which phase (aqueous, solid-sorbent, vapor, NAPL) the contaminant resides.