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Well normally I would think as probably most would, that AC would be all you had to worry about, not because DC can't kill or injure you but AC is more common. But generally speaking IIRC if using your meter to look for the presence of voltage you would set it for Ac first then if finding no AC switch to DC and check for a DC component. You could actually have both (AC riding on top of DC) but not likely in the places you would be.
Do I remember that right Killavolt or anyone else?
The best way to check for DC riding on AC is with an oscilloscope. With a scope you get the real picture. With meters if you unsure what you are looking at the readings can be suspect. I think when making ac measurements a DMM simply rectifies the waveform and displays the rms value. I'm not sure what would happen if you feed it a sawtooth or ramp or a complex waveform like a analog TV baseband signal.
If you are sure it is just a dc or a ac sinewave the DMM is fine.
9 volts ...... I bet you have a lot more than that.
Here's what I think might be happening. The voltage is actually 120 or 240 volts AC the chance of it being is DC is almost impossible.
There are exceptions such as UPS or windmill storage power and there is a battery vault somewhere nearby that uses DC power for their converters.
Anyways the 9 volts as you mentioned are referenced to sticking the other test lead in the ground. Here is where I believe your problem lies, The ground might be of poor quality giving the reading of only 9 volts. Find yourself a good ground and recheck , if the voltage is the same or different find another ground.
Your other question is how does plastic conduct electricity, this is easily done by a number of ways, condensation or water inside or outside the pipe. All insulators even plastic can become conductors if the voltage is high enough.
Be careful the voltage might be much higher than you are reading once you find the ground that is part of the circuit that is flowing thru the pipe. I know it sounds strange but I troubleshoot for a living.