Problem in water quality analysis with EPANET

I have a EPANET file (Net1.inp) with details at below mentioned:
I inject a source quality=70 mg/lit as “Flow Paced Booster” at Node ID 10 with pattern Id “2_contamination”.
The duration of this pattern is 48 hours and its values are zero except of times 10 to 12 a.m. Moreover, I want the contamination to be injected at network just between 10 to 12 a.m.
The Demand pattern id is “1” and the duration is 24 hours.
When I check the time series graph for quality (chemical) at nodes I receive below picture. Why the quality at nodes increase after time 40:00 again. The INP file is attached.

Net1.inp (7.2 KB)

Thanks for any advise.

Your pollutant stays at the network, it doesn’t react. Also the flow in your pipes is inverted at some moments so, the pollutant travels in both directions along time.