Several times I’ve traced creeks & rivers and noted they end up in the lagoons in Carlsbad (Buena Vist, Aqua Hedionda) – no surprise there.

The first thought that has always jumped to mind is that any contaminants that have been dumped or washed into those creeks since time began all ends up in those lagoons or out to sea or in the water tables under Carlsbad. There was probably a lot of that going on back before people thought about the long-term effects of dumping. Speaking of which, were there any dumps in the area back in the early 1900s? Did they dump trash in the ocean back then, as they did in northern CA?

Hopefully it is nothing more than a statistical blip as eavesdropper suggests, but I think it certainly deserves a study to eliminate my hypothesis above as the culprit.