North American facilities reported releases of 257 pollutantsto water in 2006. Just two of these, nitrate compounds and ammonia, comprised 90% of the total.
Public wastewater treatment plants, which receive wastewater from a variety of sources, were top contributors to the total amount released to water in 2006, with this sector reporting 84% of Canada's total water discharges (figure). Public wastewater treatment facilities (or publicly-owned treatment works-POTWs) are not required to report to the US TRI; and while facilities discharging to national waterbodies in Mexico are subject to RETC reporting, very few wastewater treatment plants reported in 2006.
For the United States, the table shows that one steel manufacturing facility reported the largest releases to water in that country. However, combined releases by US food manufacturing facilities made that sector the top-ranking industry for releases to water in 2006. The top pollutant released by this sector was nitrate compounds, followed by ammonia.
Although dwarfed by Canadian and US facilities interms of the amounts discharged, Mexican power plants reported almost 50% of the country's total releases to water, including heavy metals such as nickel, lead, and chromium, as well as arsenic and cyanide compounds.
Metals present in the fuels used in power plants can be captured and removed as a sludge from the stacks or during cleaning of the boilers.
Heavy metals and their compounds, when released to water, have the potential to be extremely toxic to human health and the aquatic environment. Other pollutants, such as nitrate compounds and phosphorous, can contribute to nutrient loading in lakes and rivers, leading to problems like eutrophication.