The curious thing is that plutonium is only found in the following places:
- As the fissile material inside nuclear weapons or their trigger devices;
- Inside reactors where it's being made as part of the nuclear fuel cycle; and,
- At re/processing plants where it's extracted from material from 2. and turned into the parts for 1.
It's not like your average squid or naval base played a part in those three steps, though they may have had some nukes pass through in the form of munitions. It's more likely that waste from a processing facility inadvertently/illegally got dumped or disposed of there by a third party. That it was found in asphalt is curious/suspicious indeed.