It isn't the water. It is mold. This is very warm place and the air is usually filled with moisture so anyplace that is dark and wet will grow mold.
The mold catches particles of stuff and continues to grow on it...that is the "gunk". Bleach will kill it but pouring it down doesn't get it all over the interior of the pipe.. Some people grind lemon rinds to help too. The ascorbic acid goes after it a little.
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