> For the complete documentation index, see [llms.txt](https://swatplus.gitbook.io/io-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://swatplus.gitbook.io/io-docs/theoretical-documentation/section-7-main-channel-processes/in-stream-pesticide-transformations/mass-balance.md).

# Mass Balance

The processes described above can be combined into mass balance equations for the well-mixed reach segment and the well-mixed sediment layer:        $$$$$$\Delta pst\_{rchwtr}=pst\_{in}-(pst\_{sol,o}+pst\_{sorb,o})-pst\_{deg,wtr}-pst\_{vol,wtr}-pst\_{stl,wtr}+pst\_{rsp,wtr}\pm pst\_{dif}$$​                                                                                

                                                                                                                             7:4.3.1

$$\Delta pst\_{rchsed}=-pst\_{deg,sed}+pst\_{stl,wtr}-pst\_{rsp,wtr}-pst\_{bur}\pm pst\_{dif}$$         7:4.3.2

where $$\Delta pst\_{rchwtr}$$ is the change in pesticide mass in the water layer (mg pst),  $$\Delta pst\_{rchsed}$$ is the change in pesticide mass in the sediment layer (mg pst), $$pst\_{in}$$ is the pesticide added to the reach segment via inflow (mg pst), $$pst\_{sol,o}$$ is the amount of dissolved pesticide removed via outflow (mg pst), $$pst\_{sorb,o}$$ is the amount of particulate pesticide removed via outflow (mg pst), $$pst\_{deg,wtr}$$ is the amount of pesticide removed from the water via degradation (mg pst), $$pst\_{vol,wtr}$$ is the amount of pesticide removed via volatilization (mg pst), $$pst\_{stl,wtr}$$ is the amount of pesticide removed from the water due to settling (mg pst), $$pst\_{rsp,wtr}$$ is the amount of pesticide removed via resuspension (mg pst), $$pst\_{dif}$$ is the amount of pesticide transferred between the water and sediment by diffusion (mg pst),$$pst\_{deg,sed}$$ is the amount of pesticide removed from the sediment via degradation (mg pst), $$pst\_{bur}$$ is the amount of pesticide removed via burial (mg pst).