I was at a pilot's meeting at that airport over 10 years ago, where a representative from the Metropolitan Airport Commission was talking about future plans for the airport. During the discussion, the contamination came up (because the groundwater under the airport is undrinkable).
Among the attendees was a guy that had lived in the area for years. He said that some years before, trucks would show up in the middle of the night near an abandoned well just west of the airport. They never knew what was going on, but it seems pretty clear now: someone was dumping stuff down the abandoned well.