Dan
I think your suggestion that ONLY after eliminating all variables regarding the ambient level of mercury contamination, can TIGHAR claim that the jar glass has an elevated level of mercury due to it's former contents to be somewhat unreasonable.
Yes it is true that, in theory, the 7 site might in fact be a toxic waste site contaminated by hazmat team levels of mercury sufficient to infuse the glass of the jar with a similar level of toxicity, but I think the odds are pretty small given what we know about coral atolls, the history of Nikumaroro, and what we know about the products sold in that particular jar. Coral atolls are not generally known for being repositories of heavy metals without significant external contamination, and the Phoenix islands are about as far away from sources of contamination as one can get.
You cite the concentration of mercury in the food chain as a possible source of contamination. What exact species of "top predators in the food chain that leads from the sea to the land" are you suggesting contributed to such potential contamination? Are we talking frigate birds, hermit crabs, coco crabs, tuna, dolphins, sharks, humans? The top predators in that gang generally don't consistently land themselves at the 7 site in sufficient quantity to provide hazmat team level of contamination. Keep in mind that we've only found evidence of a countable number of individuals of all species, so it isn't like the place is completely overrun with the remains of mercury laden top predators, there simply isn't that much remaining of any species.
Doing a bit web searching, I do find a study of seabirds with mercury concentration at Midway Island
http://www.sciencedirect.com/science/article/pii/S0048969700004964That indicates contamination levels in the range of 2,000 PPB for frigate birds, and up to 7,000 PPB for red-tailed tropic birds, and near 20,000 PPB for black footed albatross and tops out at 31,900 PPB for the bonin petrel. I don't think we have either the albatross or the petrel at Nikumaroro, so we can cross those two off the list. There are studies showing humans who eat dolphin on a regular basis testing out as high as 19 PPM, but I don't think that these levels are high enough to infuse the jar glass with the level of mercury that was found in the lab.
Can you cite any reference for the concentration of mercury by top predators on uninhabited coral atolls that would result in contamination high enough to infuse glass with 3.4 micrograms of mercury per liter? Outside of a toxic waste dump, I doubt that the food chain mechanism could achieve such levels.
Joe, can you translate the lab findings of 3.4 micrograms per liter into PPM? I'm a bit hazy on how to relate the two.
We do have the wings - bones and feathers - from several frigate birds that were found at the 7 site. Since these are available (I think), it might be worth testing them to determine what the actual level of mercury in them as a substitute for testing of soil samples, but only if the cost is reasonable.
As far as I know, we did not take soil samples with the intent of testing them for mercury. We didn't know the issue existed at the time we were there, and it only exists because the type of jar we found is known to have been used for a product that contained significant levels of mercury. Seems to me that the simplest solution is to connect the known jar with the known mercury laden product, but yes that does still have some level of uncertainty. Completely eliminating all uncertainty is a difficult and expensive proposition, so we have to pursue what we believe is the most effective and reasonable lines of research. I personally don't think testing the soil samples at the 7 site, and randomly around the island would be a productive line of research given the resources required and the limited ability to pursue this project on site. You are of course welcome to help fund a trip out there to collect soil samples and test them.

So, while I can certainly be wrong, it is possible that the ambient level of mercury at the 7 site is extraordinarily high, enough so to infuse a piece of glass with hazmat team levels of mercury, I think the odds are incredibly small.
Andrew