$5$ kg of a chemical are instantaneously spilled into a stream whose pH is $7\mathrm{.}5.$ The stream velocity is $0\mathrm{.}3$ m$/$s$,$ DL is $50$ m$^2/$s$,$ and stream discharge is $1$ m$^3/$s$.$ Hydrolysis is dominated by base catalysis at this pH$,$ and the second$-$order rate constant is $2\mathrm{.}8/($M s$).$

a$.)$ What is the peak concentration in the stream after an hour?

b$.)$ where does it occur?