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Large Dams “Highly Correlated” with Poor Water Quality

By Carey L. Biron Inter Press Service August 29, 2014

Large-scale dams are likely having a detrimental impact on water quality and biodiversity around the world, according to a new study that tracks and correlates data from thousands of projects.

Focusing on the 50 most substantial river basins, researchers with International Rivers, a watchdog group, compiled and compared available data from some 6,000 of the world’s estimated 50,000 large dams. Eighty percent of the time, they found, the presence of large dams, typically those over 15 meters high, came along with findings of poor water quality, including high levels of mercury and trapped sedimentation. For full story see:

With respect to Mercury, an excerpt from the report:

Scientists have only relatively recently become aware of what now appears to be a pervasive reservoir contamination problem, the accumulation of high levels of mercury in fish. Mercury is naturally present in a harmless inorganic form in many soils. Bacteria feeding on the decomposing matter under a new reservoir, however, transform this inorganic mercury into methylmercury, a central nervous system toxin. The methylmercury is absorbed by plankton and other creatures at the bottom at the aquatic food chain. As the methylmercury passes up the food chain it becomes increasingly concentrated in the bodies of the animals eating contaminated prey. Through this process of bioaccumulation, levels of methylmercury in the tissues of large fish-eating fish at the top of the reservoir food chain can be several times higher than in the small organisms at the bottom of the chain.

Elevated mercury levels in reservoir fish were first noticed in South Carolina in the late 1970s. Since then they have been recorded in Illinois, northern Canada, Finland and Thailand. The problem is in fact probably much more widespread that the few studies done suggest: scientists from Canada’s Department of Fisheries and Oceans say that fish mercury concentrations ‘have increased in all reservoirs for which pre- and post-impoundment data have been collected.’

The best researched case of reservoir methylmercury is at the La Grande hydrocomplex in Quebec, part of the huge James Bay Project. Ten years after the La Grande 2 Reservoir was first impounded mercury levels in pike and another predatory fish called walleye had risen to six times their pre-reservoir level and showed no signs of levelling off. As fish are a major part of the traditional diet of the local Cree native people, mercury levels in their bodies have risen dangerously. By 1984, six years after La Grande 2 Dam was completed, 64 per cent of the Cree living on the La Grande estuary had blood mercury levels far exceeding the World Health Organisation tolerance limit.