https://www.waterworld.com/home/article/16205804/caffeine-in-puget-sound-sends-researchers-back-to-square-one https://www.waterworld.com/home/article/16205804/c

https://www.waterworld.com/home/article/16205804/caffeine-in-puget-sound-sends-researchers-back-to-square-one https://www.waterworld.com/home/article/16205804/caffeine-in... This doesn't talk about the health problems because in this case that wouldn't quite make sense, but it does show that minerals/substances/chemicals/whatever can locally concentrate around an area even when it's connected to the ocean.