Volatilization

Pesticide in the dissolved phase is available for volatilization. The amount of pesticide removed from the water via volatilization is:

$pst_{vol,wtr}=v_v*SA*\frac{F_d*pst_{lkwtr}}{V}$ 8:4.1.8

where $pst_{vol,wtr}$ is the amount of pesticide removed via volatilization (mg pst), $v_v$ is the volatilization mass-transfer coefficient (m/day), $SA$ is the surface area of the water body (m$^2$), $F_d$ is the fraction of total pesticide in the dissolved phase, $pst_{lkwtr}$ is the amount of pesticide in the water (mg pst), and V is the volume of water in the water body(m$^3$ H$_2$O).

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:

8:4.1.9

where is the volatilization mass-transfer coefficient (m/day), is the mass-transfer velocity in the liquid laminar layer (m/day), is the mass-transfer velocity in the gaseous laminar layer (m/day), is Henry’s constant (atm m mole), is the universal gas constant (8.206 10 atm m (K mole)), and is the temperature ().

For lakes, the transfer coefficients are estimated using a stagnant film approach:

8:4.1.10

where is the mass-transfer velocity in the liquid laminar layer (m/day), is the mass-transfer velocity in the gaseous laminar layer (m/day), is the liquid molecular diffusion coefficient (m/day), is the gas molecular diffusion coefficient (m/day), is the thickness of the liquid film (m), and is the thickness of the gas film (m).

Alternatively, the transfer coefficients can be estimated with the equations:

8:4.1.11

8:4.1.12

where is the mass-transfer velocity in the liquid laminar layer (m/day), is the mass-transfer velocity in the gaseous laminar layer (m/day), is the oxygen transfer coefficient (m/day), is the molecular weight of the compound, and is the wind speed (m/s). Chapra (1997) lists several different equations that can be used to calculate .