Keep in mind most of your beaches are right next to heavily- traficked roads. Also notice how most of these beaches are at a lower elevation than the road. Everytime it rains, the water rolls downhill and brings all the contaminants onto the beach.
Its never really a good idea to use anything in nature in your tank really. I gathered live plants from a river nearby for my freshwater tank and within a month all my fish had a fungus growing on them, and the Cabomba has fungus all over it. It was quite revolting.