[RummyChick]

Supposedly what is in these drums are items that have come in contact with radioactive material.

http://www.cbsnews.com/stories/2011/06/29/national/main20075315.shtml
Lab Director Charles McMillan said the barrels contain transuranic waste — gloves, toolboxes, tools — and other items that may have been contaminated through contact with radioactive materials. Top lab officials declined to say how many barrels were on site or how they are stored. An anti-nuclear group has estimated there could be up to 30,000 gallon-drums.

[Uri’el-2012]

Lab Director Charles McMillan said the barrels contain transuranic waste — gloves, toolboxes, tools — and other items that may have been contaminated through contact with radioactive materials.

Having built Nuclear Weapons over fifty years ago,
every Kleenex, pair of latex gloves, paper gown, screwdriver
and wrench ever used for the past 65 years are in those barrels.

[Cooter]

...LANL has stored drums of contact-handled transuranic (CH-TRU) waste and remote-handled transuranic (RH-TRU) waste onsite to be retrieved in the future. (A "drum" means a 55-gallon drum of waste or its equivalent volume.) 35,000 of these drums are stored in fabric tents or domes in Area G.