Wow, I never even thought that a plastic container would leach stuff into the water...now you have me thinking...I have to test my water that's been sitting in the buckets...
Hm, just use a brute bucket. I used to get more than 0 TDS in the bucket when I didnt wipe it down with a paper towel once in awhile. Algae will start to grow in it and any body of water will start to leech outside elements over any amount of time causing >0 TDS. I wouldn't worry about it unless you're using some sketchy bucket you found on the street that fell off a truck.
a couple years ago there were a bunch of threads regarding the orange home depot buckets leaching stuff into the water. i also use a rubbermaid brute and have never noticed any problems.