NC0004987_Discoloration in Lake Norman_20130709

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ENERGY
Natural Phenomenon Causes Seasonal Discoloration in Lake Norman
Water discoloration in the immediate area of Marshall Steam Station's cooling discharge on
Lake Norman is periodically evident in the latter part of the summer when deep lake water is
much cooler than the surface.
Marshall Steam Station uses this cooler water for condenser cooling. The plant design includes
a skimmer wall, constructed prior to the filling of Lake Norman, which restricts warmer waters
and ensures that the coldest layer of lake water is available to condense steam. This design
contributes significantly to Marshall Steam Station being frequently listed as one of the most
efficient power stations in the nation.
Lake water from below a depth of about 60 feet is drawn under the skimmer wall to the station's
cooling system. This cool water ensures that cooling water leaving the plant is often comparable
to or only slightly warmer than the late summer ambient temperature near the Lake Norman
surface, reducing thermal impacts.
In the latter part of the summer, deep lake water becomes oxygen deficient due to natural
microbiological activity. Without oxygen, manganese and iron — which occur naturally in lake -
bottom sediments — dissolve and are released into the water. When this oxygen -deficient
cooling water with soluble manganese and iron mixes with the oxygen -rich water at the lake
surface, the manganese and iron are rapidly oxidized, creating the observed brownish color.
This seasonal phenomenon has been studied extensively and is very common in Southeastern
lakes and reservoirs having sufficient depth to thermally stratify during the warm season. In fact,
as the lake surface cools and lake "turn -over" (vertical mixing) naturally occurs later in the year,
some lake -wide discoloration may be visible as well. The lake is in no way impaired by this
phenomenon, and this presents no health risk.
717,01-3

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Belnick, Tom
From: Parker, Michael
Sent: Tuesday, July 09, 2013 2:33 PM
To: Chernikov, Sergei
Cc: Belnick, Tom
Subject: FW: Lake Norman discoloration below Marshall
Attachments: Lake Norman thermal stratification fact sheet.pdf
•
Sergei,
Just received a copy of a Fact Sheet Duke has prepared for the discoloration seen at the discharge below Marshall Steam.
Plant. Feel free to distribute as you see fit. Duke also plans to add it to their Marshall web site.
Michael [e-mail]
Regional Supervisor,Surface Water Protection
North Carolina Dept.of Environment& Natural Resources
Division of Water Quality
610 East Center Avenue
Suite 301
Mooresville, NC 28115
Direct Line: [phone] Fax: [phone]
E-mail correspondence to and from this address may be subject to the North Carolina Public Records Law and may be
disclosed to third parties unless the content is exempt by statute or other regulation.
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