When the girls go into the apartment and turn on the water, chlorine gas immediately starts to come out. There would have been some water in the line followed by some sputtering before the gas came out.
Jonas is jogging and finds the dead birds, and then the dead joggers. Realizing that there must be poison gas, he quickly takes his water bottle and douses his handkerchief to use as a mask, and then drops the water bottle. We see him running back the way he came, but there is no water bottle.