please solve it

7. If you have a contaminant substance of 4.5106 gallons per day (MGD) with a concentration of 9.5mg/L, is released into a stream having an upstream flow of 10MGD and a substance concentration of 3mg/L. Assume complete mixing. What is the steadystate concentration in ppm in downstream if you assume a complete mixing and a conservative substance? (hint, use lm3=264 gal) 8. A lake with a volume of 10106m3 that fed with a stream with a flow rate of 5m3/sec and pollution concentration equal to 10mg/L, another source of pollution fed the lake with the same pollutant at a rate equal to 0.5m3/sec and a concentration equal to 100mg/L. If the steady state concentration of the system is 3.5mg/L and the system is nonconservative find the reaction rate coefficient? 9. The air in the lab is clean before students started their experiments at 10 a.m. Students started their lab experiments at 11 a.m at this time pollutant start to be submitted into the air with reaction rate equal to 0.4/hr at a constant rate of 140mg/hr. The lab volume is 500m3 and fresh air entering the lab at rate equal to 1000m3/hr. Estimate the time for the concentration to reach 0.1mg/m3