4.4 Methods for Calculating Selected Parameters in the Mass Loss Expressions

The first order decay/degradation coefficient is related to radioactive decay and degradation.  The RAAS and MEPAS databases contain decay half-lives for radionuclides and degradation half-lives for some non-radioactive contaminants.  If values of half life are available, the decay coefficient is calculated by



For all volatilization related calculations, the module uses the modified Henry's Law constant (unitless).  The Henry's Law constant is obtained from the contaminant database and multiplied to obtain the unitless version by



where

H_i  is the Henry's Law constant for contaminant i (press./concen. basis) (atm m³ mol^-1) br> R  is the Universal gas constant (atm L mol^-1 K^-1)
 T  is the absolute temperature (K).

The overall mass transfer coefficient, K_li, in Equations 4.13 and 4.14 is calculated by the source-term release module.  This calculation is (Hwang 1982)



where

k_il  is the liquid-phase mass transfer coefficient for contaminant i at a pond surface (cm yr^-1)
k_ig  is the gas-phase mass transfer coefficient for contaminant i at a pond surface (cm yr^-1).

These mass transfer coefficients are calculated as follows (Hwang 1982):