Water quality issues
Water quality problems in the discharges of hydroelectric power plants can result from the seasonal warming and thermal stratification of waters impounded by the dam.
Decomposition of organic matter under chemically reducing conditions, brought on by an absence of dissolved oxygen (DO) in the hypolimnion (the lower and colder layer of water in a lake), may result in the build up of toxic hydrogen sulfide and ammonia, and may mobilize iron, manganese, and some heavy metals from the reservoir sediments. Discharges from hydroelectric power plants that have low DO concentrations and elevated levels of contaminants may seriously affect other downstream water uses.
At some facilities, structural improvements (e.g. installation of weir dams or aerating turbines) or operational changes (e.g., spilling water over the dam) can be made to improve DO levels. In other cases, structural and operational approaches are too costly for the project to implement or are likely to be of limited effectiveness. Despite improvements in overall water quality below dams in recent years, many hydropower projects are unable to meet water quality standards of the regulating authority for DO.
