Total non-issue in 2017.
There are several steps of filtration and water removal between the underground tank and you.
This might have been a problem in 1957, but old mechanical legends have a way of hanging around decades past the point when they had any truth to them.
For example, battery cases haven't been made of rubber for 50 or 60 years, yet there are still people who firmly believe placing a battery on a concrete floor will make it discharge. I was once helping a neighbor fix a headlight or something on his truck, and I removed the battery and placed it on the driveway. He immediately let out a yelp and rushed to pick up the battery. He stood there like an idiot holding it for half an hour because his grandpappy once told him never let a battery sit on concrete. No matter what I told him, he wouldn't even put it down anywhere else, and insisted on blowing out his elbows because grandpappy couldn't possibly have passed on a badly outdated urban legend.
2002 Suzuki DL1000 V-Strom, Dark Metallic Space Blue
1983 Suzuki GS850G, Cosmic Blue
2005 KLR685, Aztec Red - Turd II.2, the ReReTurdening