Dear Deanne
Humans ingest mercury mainly by eating fish. More than 95% of the mercury in fish takes the chemical form called methyl mercury, which is the most toxic form of the element.
In 1996 the USA, set a
new EPA guideline for methyl mercury in the diet of 0.1 micrograms of mercury per kilogram of body weight per day (0.1 ug/kg/day). The World Health Organization has set 0.47 ug/kg/day as its standard, whilst the values given in the New Dutch List and the ICRCL Guidelines vary according to the setting and land use, respectively.
An estimated 1600 to 4000 metric tons of mercury enters the atmosphere from natural sources (volcanic eruptions etc) each year.
Humans contribute between 2000 and 6000 metric tons of mercury to the atmosphere each year. The main sources being incinerators, coal combustion, cement kilns, and the manufacture of chlorine using the chlor-alkali process.
Hence as you can see it is not a good idea to have too much mercury floating around in the atmosphere.
The best way to keep it locked up is for it either to be in its elemental form, or else as an insoluble compound such as Mercuric Sulphide.
One promising option is to use phytoremediation to lock up the mercury in plant tissue by growing lots and lots of cabbage!
However it has been found that some species of poplar actually release mercury (as methyl mercury) to the atmosphere, so if you were to plant poplars on the old munitions site you could find yourself prosecuted for transporting waste! However I am sure that this was not the reason that Britain's only plantation of genetically-modified trees (which were poplars) were attacked and partly destroyed at Zeneca Agrochemicals research site in Berkshire this summer.
According to the draft guidance issued for the implementation of Part IIA of the EPA 1990 (page 142), regulation 2(l)(c)(ii) describes land formerly used for the manufacture of explosives, and as such the Environment Agency is best placed to deal with it.
cheers
Micky