From everything I found so far, it seems stuff tends to cap at around 6-7%. Apparently it's the current cap for natural alchemic properties.
I can tell you my findings so far, you can test them and compare.
In a coniferous forest, near a salt mine, i found:
- salt: rabbits
- merc: snails, witch's hats, crickets
- sulphur: blueberries, lavender blewetts, waxing toadstools, deer
- lead: chestnuts, flint, granite, lime
No water nearby to check purity.
Also, the purities seem to be better outside of a church's area of effect.