Solid-Liquid Partitioning

Pesticides will partition into particulate and dissolved forms. The fraction of pesticide in each phase is a function of the pesticide’s partition coefficient and the reach segment’s suspended solid concentration:

$F_d=\frac{1}{1+K_d*conc_{sed}}$ 7:4.1.1

$F_p=\frac{K_d*conc_{sed}}{1+ K_d*conc_{sed}}=1-F_d$ 7:4.1.2

where $F_d$ is the fraction of total pesticide in the dissolved phase, $F_p$ is the fraction of total pesticide in the particulate phase, $K_d$ is the pesticide partition coefficient (m$^3$/g), and $conc_{sed}$ is the concentration of suspended solids in the water (g/m$^3$).

The pesticide partition coefficient can be estimated from the octanol-water partition coefficient (Chapra, 1997):

7:4.1.3

where is the pesticide partition coefficient (m/g) and is the pesticide’s octanol-water partition coefficient (mg m(mg m)). Values for the octanol-water partition coefficient have been published for many chemicals. If a published value cannot be found, it can be estimated from solubility (Chapra, 1997):

7:4.1.4

where is the pesticide solubility (moles/L). The solubility in these units is calculated:

7:4.1.5

where is the pesticide solubility (moles/L), is the pesticide solubility (mg/L) and is the molecular weight (g/mole).