Waec Model Questions Vol1 2021 Geography Question 35

A chemical plant releases a pollutant at a rate of 120 g s⁻¹ from a stack 30 m high. The wind speed is 4 m s⁻¹ and the atmospheric stability is class D. Using the Gaussian plume model where the dispersion coefficients are \(\sigma_y = 0.16x\) and \(\sigma_z = 0.12x\) (with \(x\) in kilometres), the ground‑level concentration at distance \(x\) (km) downwind is given by \[C = \frac{Q}{\pi u \sigma_y \sigma_z}\,\exp\!\left(-\frac{H^2}{2\sigma_z^2}\right)\] where \(Q\) is the emission rate (mg s⁻¹), \(u\) the wind speed (m s⁻¹), and \(H\) the stack height (m). The permissible exposure limit is 0.05 mg m⁻³. Approximately how many kilometres downwind must one be for the concentration to fall below this limit? Give your answer to one decimal place.