Diffusion

Pesticide in the dissolved phase is available for diffusion. Diffusion transfers pesticide between the water and sediment layers. The direction of movement is controlled by the pesticide concentration. Pesticide will move from areas of high concentration to areas of low concentration. The amount of pesticide that is transferred between the water and sediment by diffusion is:

$pst_{dif}=\mid\frac{v_d}{depth}*(F_{d,sed}*pst_{rchsed}-F_d*pst_{rchwtr})*TT\mid$ 7:4.2.11

where $pst_{dif}$ is the amount of pesticide transferred between the water and sediment by diffusion (mg pst), $v_d$ is the rate of diffusion or mixing velocity (m/day), $depth$ is the flow depth (m), $F_{d,sed}$ is the fraction of total sediment pesticide in the dissolved phase, $pst_{rchsed}$ is the amount of pesticide in the sediment (mg pst), $F_d$ is the fraction of total water layer pesticide in the dissolved phase, $pst_{rchwtr}$ is the amount of pesticide in the water (mg pst), and $TT$ is the flow duration (days). If $F_{d,sed}*pst_{rchsed}>F_d*pst_{rchwtr}$,$pst_{dif}$ is transferred from the sediment to the water layer. If $F_{d,sed}*pst_{rchsed}<F_d*pst_{rchwtr}$, $pst_{dif}$ is transferred from the water to the sediment layer.

The diffusive mixing velocity, $v_d$, can be estimated from the empirically derived formula (Chapra, 1997):

$v_d=\frac{69.35}{365}*\phi*MW^{-2/3}$ 7:4.2.12

where $v_d$ is the rate of diffusion or mixing velocity (m/day), $\phi$ is the sediment porosity, and $MW$ is the molecular weight of the pesticide compound.