Gce Model Questions Vol1 2022 Chemistry Question 25

A lake of volume \(5.0\times10^{6}\) m³ initially contains nitrate at \(0.050\) mg L⁻¹. A runoff event adds 2000 m³ of water with nitrate concentration \(0.300\) mg L⁻¹. (a) Calculate the nitrate concentration in the lake after complete mixing. (b) Nitrate removal follows first‑order kinetics with rate constant \(k = 1.5\times10^{-6}\) s⁻¹. How long (in days) will it take for the concentration to fall from the mixed value to \(0.040\) mg L⁻¹? (c) A point‑source discharge adds \(2.0\times10^{5}\) mol of ammonium per day, which is fully nitrified to nitrate (1 mol NH₄⁺ → 1 mol NO₃⁻). Assuming the lake is perfectly mixed and no other losses, what is the daily increase in nitrate concentration (mg L⁻¹ day⁻¹) caused by this source? (d) Briefly state which of the three influences (runoff, bioremediation, point source) has the greatest impact on lake nitrate levels.