In a poem that often seems pretty bleak, water is a symbol of life. There's the early reference to a "spring," followed later by a "fountain." And in the same line that "fountain" occurs, you've got "torrent"—a word that makes us think of a powerful waterfall or torrential rain. Then of course there are all those things that make us think of water, even though no liquid is actually even mentioned (we're looking at you "storm" and "blue"). If this poem often seems a little, well, desolate at times, there's always a little water there to freshen things up a bit.
Line 4: The word "spring" does a lot in this line. It describes a pool of water, it symbolizes freshness and inspiration, and it's also a metaphor for what is described explicitly in the next line: a "source."
Line 10: "Stormy life" is another metaphor for the speaker's rough and difficult life. We can't help but think of torrential rainfall here, a more destructive kind of water.
Line 13: It's a daily double here. The "fountain" symbolizes life, just like the spring, whereas the "torrent" recalls line ten's "stormy" and symbolizes power and destruction. The speaker is able to access that "mystery" he describes in both what is calm and serene (a fountain), and powerful and strong (a torrent).
Lines 19-21: The "storm" symbolizes power, strength, and destruction, much like "torrent" does in line 13. This is balanced by the serenity symbolized by the blue heaven (minus, of course, that demon-cloud deal). This balance recalls the balance of line 13 as well.