Tailings are rendered inert and any arsenic in the ore is said to be 100% fixed.
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Industry and government are adamant that no tailings ever leak back into the river, or into deep aquifers.
The water leftover from the extraction process, known as tailings, contains arsenic, mercury, napthenic acids and polycyclic aromatic hydrocarbons (PAHs).
And tailings are produced in large quantities: on average, 991 tonnes-worth for every 1, 000 tonnes of ore that is mined.
This would have turned several Andean lakes into reservoirs or tailings ponds.
The company is also going to destroy all cyanide before it goes into the tailings area, which is also not required by law.
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Curiosity will likely drill a number of shallow test holes, using the tailings to scrub the metal bit of any contaminants carried from Earth.
But the precious metal is not just underground, it's also in the "tailings" - heaps of grey-white residue left behind by old mining activity.
"Now, I can repair the roofs of my house, " Alexander Chavez told Reuters as entire families descended on a river to gather gold tailings.
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Much of the water is reused, but the industry is producing 1.8m litres of tailings a day which need to be contained in large earthen dams.
While many people do not know it, numerous activities, including basic construction such as digging a basement, uncover tailings that contain radioactive materials that naturally occur in the ground.
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An interesting note in the financial statements was the news that in September Suncor become the first oil sands company to complete surface reclamation of a tailings pond, a key step in returning the site back to nature.
Besides the unsightly effects that many mines have on the landscape, they usually churn out vast quantities of toxic waste in the form of acidic drainage-water and tailings the fine particles of rock produced when ore is ground down to release the minerals it contains.
The first phase of construction includes early earthwork and concrete foundations for a mill and on-site power plant, expansion of the existing water-supply system, construction and operation of the initial phase of a new tailings facility, and expansion of camp facilities by approximately 6, 600 additional beds for workers.
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It has been widely reported in the Canadian media that oil sands company Suncor admitted to a leak of 1, 600 sq m from its Tar Island Pond 1 tailings pond in 1997, although a current spokesman for Suncor says he does not have any information about the incident.
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