$11\mathrm{.}5.$ A batch of radioactive material is dumped into the Columbia River at Hanford, Washington. At Bonneville Dam, about $400km$ downstream the flowing waters $)$ are monitored for a particular radioisotope $(I_{1R}>10yr)$ and the data of Fig. P$11\mathrm{.}5$ are obtained.

$($a$)$ How many units of this tracer were introduced into the river?

$($b$)$ What is the volume of Columbia River waters between Bonneville Dam and the point of introduction of tracer?