Pesticide in the dissolved phase is available for volatilization. The amount of pesticide removed from the water via volatilization is:
The volatilization mass-transfer coefficient can be calculated based on Whitman’s two-film or two-resistance theory (Whitman, 1923; Lewis and Whitman, 1924 as described in Chapra, 1997). While the main body of the gas and liquid phases are assumed to be well-mixed and homogenous, the two-film theory assumes that a substance moving between the two phases encounters maximum resistance in two laminar boundary layers where transfer is a function of molecular diffusion. In this type of system the transfer coefficient or velocity is:
For lakes, the transfer coefficients are estimated using a stagnant film approach:
Alternatively, the transfer coefficients can be estimated with the equations: