In the winter, a stream flows at 10 m3/s and receives discharge from a pipe that
ID: 849019 • Letter: I
Question
In the winter, a stream flows at 10 m3/s and receives discharge from a pipe that contains road runoff.
The pipe has a flow of 5 m3/s. The stream's chloride concentration just upstream of the pipe's discharge
is 12 mg/L, and the runoff pipe's discharge has a chloride concentration of 40 mg/L. Chloride is a
conservative substance. (a) Does wintertime salt usage on the road elevate the downstream chloride
concentration above 20 mg/L? (b) What is the maximum daily mass of chloride (metric tons/day) that
can be discharged through the road runoff pipe without exceeding the water quality standard?
Explanation / Answer
Conc of Cl in stream= 12000mg/m^3= 12g/m^3
mass= 12x 10=120g/s
Conc of Cl in runoff= 40000mg/m^3=40g/m^3
mass= 40x 5=200g/s
Hence the final concentration= (m1+m2)/(v1+v2)
(200+120)/(10+5)= 320/15= 21.33g/m^3 s
a) As proved by the answer, the Cl content is elevating above 20mg/L
b)For having the value below 20, say we have a concentration of x Cl in runoff
(120+5x)/15=20
x<=36mg/L