Hey Ronnie, we are missing rain here in brazil... almost missing water at the tap because of the hard dry time....
My guess is that water purity is not a problem,, Ronnie might be able to confirm that, i just guess or it should be very pure and the circuit will work on one way and maybe not pure in another... or maybe not, is just a matter of tuning like stan said...
of course 120ppm i not a small value...
but if i understood well theres no current that can flow because there is no circuit where the current can go... its pure voltage... my view
if i understood well it does not matter if we get sea water or rain... the headaches being the contaminants proportion changing as you consume water... another guess...
Steve check if you find this resin for deionizing, i guess is a good way to go,,, my filter i made with a piece of 50cm of a 1inch diameter pvc tube...