Toxic waste including mustard gas is buried among quite 1,500 disused landfill sites across Wales.
Environmental campaigners Greenpeace branded the 1,572 locations “ticking time bombs”.
Natural Resources Wales (NRW) data reveals many sites contain hazardous waste and only a few dozen are protectively lined.
The organization said it worked with councils to make sure old landfills weren’t a pollution risk.
Its data showed landfill lies beneath homes, leisure centers, hospitals, parkland, and alongside rivers.
Dr. Paul Johnston of Greenpeace’s Exeter University science unit said: “Many of these landfill sites are ticking time bombs.
“There’s a huge kind of problems out there, ranging from asbestos to PCBs to material, to newer chemicals like PFOA and pesticide waste that has been dumped during a sizable number of locations.”
PCBs (polychlorinated biphenyls) were banned in 1986, having been used since the 1920s as cooling and insulating fluids for transformers and capacitors.
When incinerated they will produce highly toxic dioxins – one was utilized in an assassination attempt on Ukrainian politician Viktor Yushchenko in 2004.
PFOA, or perfluorooctanoic acid, is employed to form Teflon and has been linked to cancer and birth defects.
Elsewhere, the data reveals “low-level radioactive waste”, at Buckley, Flintshire. Cwmrhys quarry, Lampeter, has”cyanide containers” and at Bagillt, Flintshire, land is claimed to be “contaminated” with arsenic.
Near Furnace, Ceredigion, the data said in 1989 “landfill gas was in explosive concentration” but was considered a “minimal risk.”
At Nash, Newport, “PFA sludge” is recorded.
PFAS, or perfluoroalkyl and polyfluoroalkyl substances, are chemicals that are linked to cancer, liver damage, vaccine resistance, and fertility problems.
PAHs, or polycyclic aromatic hydrocarbons, are said to be buried in Caerphilly.
These are linked to skin, lung, bladder, liver, and stomach cancers.