Decay coeficient derivation

[malotlst]

I don't where to start about this assignment problem, I can't even get any literature or website on this subject.

We should derive K (coefient of decay) in the formula U ds/dx +Ks = Sd (pollutant loading Sd = w/A, where w (ML-3T-1), velocity U (LT-1) concentration s (ML-3). Stream parameters flow, area and height are assumed constant. Dimentions of K and its derivation should be clearly worked out.

Secondly: Solve for the equation and draw profiles concentration of the pollutant within the reach.

I will reserve the third and fourth sub question will for myself to see if I have obtained a clue I can continue with the rest.