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(b) A wastewater treatment plant discharges effluent into a river at a rate of 4.5 m /s. The effluent has an ammoniacal nitrogen concentration of 40 mg/l. The river has a flow rate of 120 m²/s and a background ammoniacal nitrogen concentration of 2 mg/l. Mixing in the river has been shown to be predominantly due to advection. Assuming a 1D system: () Calculate the steady-state concentration of ammoniacal nitrogen in the river at the point of discharge. [3 marks] () Calculate the concentration of ammoniacal nitrogen at a location 10.5 km downstream of the discharge where a water abstraction point is proposed. The cross-sectional area of the river at the location is 1,750 m2. The conversion rate of ammoniacal nitrogen to nitrate can be assumed to be first order with a conversion rate of k = 0.46 d.[3 marks)