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):

$K_d=3.085*10^{-8}*K_{ow}$ 7:4.1.3

where $K_d$ is the pesticide partition coefficient (m$^3$/g) and $K_{ow}$ is the pesticide’s octanol-water partition coefficient (mg m$^{-3}_{octanol}$(mg m$^{-3}_{water}$)$^{-1}$). 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):

$log(K_{ow})=5.00-0.670*log(pst'_{sol})$ 7:4.1.4

where $pst'_{sol}$ is the pesticide solubility ($\mu$moles/L). The solubility in these units is calculated:

$pst'_{sol}=\frac{pst_{sol}}{MW}*10^3$ 7:4.1.5

where $pst'_{sol}$ is the pesticide solubility ($\mu$moles/L), $pst_{sol}$ is the pesticide solubility (mg/L) and$MW$ is the molecular weight (g/mole).