The noxious substances are inherently contained in the 126,000 barrels full of radioactive materials stored at the site, BfS spokesman Florian Emrich said in response to an article in weekly magazine Stern, which first reported the presence of the chemicals.
About one dozen of these barrels contain highly toxic pesticides, in addition to a half tonne of arsenic – all of which could endanger groundwater in the area, the magazine reported.
BfS is currently taking an inventory at the 100-year-old Wolfenbüttel mining facility to inform decisions about its future.
“It is noteworthy that there are indications of stored pesticides,” Emrich said.
BfS, which replaced the allegedly careless former proprietor Helmholtz Zentrum in January, warned then that there could be “surprises” when it came to the contents of the underground site.
Officials are now trying to determine what to do about the dangerous nuclear waste, which has been stored at the increasingly unstable site since 1978.
In January the authorities said it had begun periodic groundwater quality checks for the nearby community of Kissenbrück, part of the Asse district, which is not connected to the public water system and is supplied by two local wells.
BfS also announced in January that an Asse storage chamber is damaged and could pose the risk of radioactive leaks. Asse has since limited employee access to the area. Former Asse workers have also claimed they were infected with leukaemia due to their exposure to waste at the sight.
Asse operators came under fire in the autumn of 2008 following revelations they failed to alert the government to serious violations and worrying conditions at the site. German Environment Minister Sigmar Gabriel responded saying that the facility has "as many holes as Swiss cheese," adding that it remained unclear who was ultimately to blame for the serious safety violations at the site.