H2S is nasty, if you go into a sewer without a gas monitor you are just asking for trouble, the reading you got was 2 ppm (parts per million), and at really high levels it is explosive..........but it will have poisoned you way before it gets that high!!! It usually gives off a nasty egg smell, although after exposure to it for a few minutes your nose becomes de-sensitised to it which makes it even more dangerous as you can think it has gone away when it hasn't!!!! The black sludge in this sewer means that is is going septic, and this is where the h2s is coming from, so beware black sludge when down there, try not to disturb it!! (also means theres a good chance of methane down there too so no smoking lol)