# 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).