11.5. A batch of radioactive material is dumped into the Columbia River at Hanford, Washington. At Bonneville Dam, about 400 km downstream the flowing waters (6000 m³/s) are monitored for a particular radioisotope (4/210 yr) and the data of Fig. P11.5 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? C, units of radioactivity/m³ Figure P11.5 20 35 125 1, days