Thousands of North Georgia residents have been noticing a change in the smell and taste of their municipal tap water in fall. The water out of Lake Lanier develops an earthy taste or damp odor. Utility providers in Forsyth and Fulton counties become inundated with inquiries as soon as air temperatures start to drop.
This is a result of a natural season change, not a sign of contamination. The region’s main water reservoir is undergoing a natural physical process.
How summer heat creates thermal layers in Lake Lanier
The depths of Lake Lanier mean that in summer, the surface is heated by solar radiation faster than it can mix with layers below. The result is three distinct thermal layers in the water column. The deepest ends up significantly colder than the top layer.
The surface water layer is called the epilimnion. It extends 15 to 30 feet deep and stays warm and well oxygenated. The middle layer is known as the thermocline. There is already a sharp drop in water temperature at this level.
The hypolimnion is the lowest layer of water. It lies more than 60 feet below the surface and remains isolated from the water levels above. Throughout the summer, the conditions here are stagnant. Decomposition of organic matter near the lakebed consumes much of the dissolved oxygen.
Natural chemicals that form during algal blooms in the upper layer of water in late summer remain trapped in the lower layer until fall, when changes in the air affect the water layers.
A seasonal lake turnover triggered by falling air temperatures
It does not take long for the surface water temperatures to start dropping when cold fall weather begins in North Georgia. When surface water cools, its density increases and it sinks into the thermocline. The upper layer starts to get broader as it incorporates the middle layer. The temperature gap between the top and bottom of the lake narrows.
Thermal stratification eventually breaks down when surface water density reaches parity with the cold layer at the bottom. Strong winds contribute by mixing the entire water column. This process is called destratification, or lake turnover. The mechanism draws water upward from the hypolimnion to the surface.
The compounds that have been stagnating near the lakebed end up recirculated through the reservoir. These are dissolved minerals and algal byproducts like MIB and geosmin. The blended raw water is then drawn in by municipal intake pipes at Lake Lanier and along the Chattahoochee River.
The earthy-tasting and damp-smelling water goes through the same treatment processes, but some qualities remain.
Treatment processes are still enough to ensure drinking safety
Super low concentrations of compounds like geosmin and MIB are still detectable to human senses. The levels can be as low as ten parts per trillion, which is the equivalent of one drop of water in an Olympic-sized swimming pool.
Public health officials are adamant that these compounds are 100 percent non-toxic, harmless to plants, animals, and humans. The earthy flavor or musty odor can be ignored as far as health is concerned, according to the Georgia Department of Natural Resources.
That does not mean that water utilities are ignoring the situation. Advanced purification technologies are being applied to lessen the seasonal aesthetic water shifts. In Forsyth County, before water enters the distribution networks, ozone treatment is applied to break down organic compounds.
A successful reduction of 90 percent
Recent testing as fall started in September saw raw water entering the treatment plants with MIB concentrations above two hundred parts per trillion. Other facilities nearby reported levels above one thousand parts per trillion. After applying the ozone treatment systems, this dropped by ninety percent.
The water after treatment still has to become equalized in the municipal distribution network. It implies that the customers will need to endure the taste and odor changes for a few more days. Nevertheless, the quality monitoring in Fulton County and Forsyth County has been done, and it has been established that the water is completely safe for drinking.
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