Toxic pollutants released by oil sands mining operations are accumulating in freshwater ecosystems, research by Canadian scientists suggests.
A study of sediment in nearby lakes showed the level of pollutants, known as PAHs, had risen since the 1960s when oil sands development began.
However, the researchers added that PAH concentrations were still lower than those found in urban lakes.
The findings appear in the Proceedings of the National Academy of Sciences.
PAH refers to polycyclic aromatic hydrocarbons - a group of chemicals that have been shown to affect aquatic organisms and birds. PAHs have also been described as being responsible for damaging food crops.
The chemicals occur naturally in coal, crude oil, and petroleum; they are also present in products made from fossil fuels, such as creosote and asphalt.
PAHs also can be released into the air during the burning of fossil fuels and organic matter - the less efficient the burning process, the more PAHs are given off. Forest fires and volcanoes produce PAHs naturally.
Digging the dirt
Using sediment cores from five lakes within a 35km (22-mile) radius of major oil sands facilities and one remote lake (90km/56 miles from the facilities), the researchers assessed the ecological impact of oil sands developments on freshwater ecosystems.
Analysis of the samples showed that PAH levels were now 2.5-23 times greater than levels from about 1960. More