A point source of a pollutant $($Cw $=400$ mg L$1)$ is discharged to a river at

Qw $=0\mathrm{.}5\backslash$times $106$ m$3$ d$1.$ The flow velocity of the river Qr $=12\backslash$times $106$ m$3$ d$1,$ and river

background pollutant concentration Cr $=1$ mg L$1.$ Calculate the pollutant concentration

at $8$ km below the injection point. Cross$-$ sectional area of the river A $=2000$ m$2.$ First$-$order

pollutant decay rate constant k $=0\mathrm{.}8$ d$1.$ State any assumptions that you used.