A lot of plant constituents have been dissolving in the water of this hot spring since the dawn of time.
この場合、次のように言うと良いと思います。
ーA lot of plant constituents have been dissolving in the water of this hot spring since the dawn of time.
「太古の昔からたくさんの植物成分がこの温泉のお湯に溶け込んでいる」
plant constituents「植物成分」
to dissolve in「溶け込む」
since the dawn of time「太古の昔から」
ご参考まで!