A newly constructed food processing plant is scheduled to begin discharging a waste water with $BO{D}_L=100m\frac{g}{L}$ into an otherwise pristine river $($i$.$e$.,BO{D}_L$ of river $=0$ before start of the wastewater input$).$ Assume that the river flow, $Q_r,$ is $7\mathrm{.}5{10}^7\frac{m^3}{s}$ and the wastewater, $Q_w,$ flow will be $7\mathrm{.}5{10}^6\frac{m^3}{s}.$ Assume that the expected flow velocity of the combined flows in the river is $1k\frac{m}{?}$ day and that, as a first approximation, dispersion in the river can be ignored. If the $BO{D}_L$ obeys first order kinetics with a decay rate of $\frac{0\mathrm{.}2}{?}$ day, calculate the expected $BO{D}_L$ concentration downstream at two distances: $1km$ and $5km,$ for three different times: $\frac{1}{2}$ day, $2$ days, and $10$ days after the onset of the wastewater discharge.