Answer:
the final concentration is C=0.235 mg/L
Explanation:
doing a mass balance
ammonium mass outflow = ammonium inflow from river+ effluent discharge of ammonium = 12000 L/s* 0.015 mg/L + 1500 L /s * 2 mg/L = 3180 mg/s
outflow rate = 12000 L/s+ 1500 L /s = 13500 L/s
then the final concentration is
C= ammonium mass outflow/outflow rate = 3180 mg/s/13500 L/s= 0.235 mg/L