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(a) An industrial plant discharges an insecticide to a lake at rate of 2 x 106 kg per year. The lake has the following characteristics: Mean depth = 4.5 m Surface area = 13.4 x 106 m2 Residence time = 4.2 years The insecticide decays at a rate of 0.08 yr' and settles at a rate of 0.05 m d-9. A stream flowing into the lake contains the same insecticide at a concentration of 15 mg litre-1. Assuming the volume of the lake is constant: selona elence equatic enge de encen. (ii) Calculate the steady state insecticide concentration in the lake. [5 marks) (ii) Determine the percentage reduction in plant loading rate required to maintain the lake's steady-state insecticide concentration below 50 ppm. [2 marks] (iv) It is proposed to reduce the steady-state insecticide concentration in the lake by increasing the lake's depth by 80% by dredging. Calculate the new steady-state concentration after dredging. [2 marks] (Q2 continued on next page)