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Some contaminant species experience radioactive decay or biological degradation and the concentration of these contaminants may decay as a function of time. The rate of radioactive decay is very predictable and is controlled by the half-life of the contaminant species. Whereas, the rate of biological decay is a function of several factors, including the presence of the appropriate bacteria, the presence of a suitable substrate, and the temperature. Both types of decay are often modelled by first order decay, with the controlling parameter being the half-life of the species.

 

The decay can take place in the source, the deposit, or the base. First order (exponential) decay is used for both radioactive and biological decay, eg.

 

c(t) = c(0) e- λ* t

 

where,

c(t) = concentration at time t,

c(0) = initial concentration,

λ = decay constant = .693147/half life.