Question

Imagine that a polluter starts dumping sodium chloride into Lake Charles at a rate of 1437.3 kg per day, that the background concentration of NaCl in the lake was 11.3 ppm, and that the residence time of NaCl in the lake is 3.5 years. After 5 years, the Environmental Protection Agency catches on and turns off this source of NaCl. What would be the maximum concentration of NaCl in the lake? Please give your answer in ppm. Lake Charles has a volume of 1.8*107 m3.

Answer 1

The residence time of the NaCl is 3.5 years, so anything added in first 1.5 years will not be there at the end of 5^th year, so we need to calculate amount of NaCl added in 3.5 years

1. Amount of NaCl added in 3.5 years

= 1437.3 * 365 * 3.5

= 1836150.75 kG

1. Background conc of NaCl

= 11.3 ppm

So weight of NaCl = NaCl ppm * Wt of water in lake (in Liters) / 100000

                                   = 11.3 * 1.8*10^10 / 1000000

                                                    = 203400 kg

1. So total amount of NaCl at the end of 5^th year

= 1836150.75 + 203400

= 2039550.75 kg

1. Which means 1.8*10^10 L contains 2039550.75 kg of NaCl

Which is 113.31 ppm

So max conc of NaCl is 113.31 ppm