Tailings are the materials left over after the process of separating the valuable fraction from the uneconomic fraction of an ore. Tailings are distinct from overburden, which is the waste rock or materials overlying an ore or mineral body that are displaced during mining without being processed.

When applied to coal and oil sands mining, the term "tailings" refers specifically to fine waste suspended in water.

Naphthenic acid (NAs) is the name for an unspecific mixture of several cyclopentyl and cyclohexyl carboxylic acids with molecular weight of 120 to well over 700 atomic mass units.

In their oft-cited article published in Toxicological Sciences Rogers et al. stated that "naphthenic acids are the most significant environmental contaminants resulting from petroleum extraction from oil sands deposits." They found that "under worst-case exposure conditions, acute toxicity is unlikely in wild mammals exposed to naphthenic acids in AOS tailings pond water, but repeated exposure may have adverse health effects." Naphthenic acids are present in Athabasca oil sands (AOL) tailings pond water (TPW) at an estimated concentration of 81 mg/l. In their 2002 article cited over 100 times, Rogers et al. reported on a solvent-based laboratory bench procedure developed to "efficiently extract naphthenic acids from bulk volumes of Athabasca oil sands tailings pond water (TPW)."

According to Canada’s Oil Sands Innovation Alliance (COSIA), an alliance of oil sands producers formed in 2012, who share research on Environmental Priority Areas (EPAs) such as tailing pond water and greenhouse gases, "Tailings are the sand, silt, clay and water found naturally in oil sands that remain following the mining and bitumen extraction process." The hot water process used by Suncor and Syncrude in their open-pit mining operations, to extract bitumen from the Athabasca Oil Sands (AOS) produces large quantities of tailings pond sludge which remains stable for decades. By 1990 it was considered to be the "imminent environmental constraint to future use of the hot water process." Oil sands tailings pond water contains toxic chemicals such as "naphthenic acids (NAs) and process chemicals (e.g., alkyl sulphates, quaternary ammonium compounds, and alkylphenol ethoxylates)."

See also:
http://en.wikipedia.org/wiki/Tailings
http://en.wikipedia.org/wiki/Naphthenic_acid

Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)