It’s that time of year again. The time when the most frequently asked question is: What’s that rotten egg-like smell?
A telltale way of knowing fall is upon us, one of the seasonal changes that occur at this time of year is thermal stratification, also known as turnover.
A certain sign of turnover can be a peculiar smell in waterbodies, be it ponds, tanks or reservoirs. Gases present in the water – especially hydrogen sulfide – cause what is sometimes referred to as a rotten egg-like smell.
“Lakes start to stratify as water surface temperatures change,” said Jeremy Nickolai, Brazos River Authority field operations manager. “Three layers are formed: the epilimnion, the thermocline and the hypolimnion. The epilimnion is the warm surface layer. The thermocline is the middle layer characterized by a rapid decrease in temperature. The hypolimnion is the cold, dense bottom layer with low levels of dissolved oxygen (anoxia).”
Turnover is literally the process of the lake’s water turning over from top (epilimnion) to bottom (hypolimnion.) As water temperatures at the surface begin to cool in the fall due to the increased density, the cooler water sinks. One of the byproducts of this process is the production of hydrogen sulfide into the air, which puts off the not-so-lovely odor.
“Although some of these gases, such as Hydrogen Sulfide, smell like rotten eggs and can be dangerous when concentrated in enclosed areas, the release of the gas from the lake does not pose any risk to humans or wildlife,” according to the US Army Corps of Engineers.
The process can be quick and occur within a week, according to a Texas Parks and Wildlife Department article.
The process of stratification repeats in the spring when air temperatures begin to warm and water temperatures follow suit.
One of the factors that affect the timing of lake turnover is the intensity of the wind, according to the Texas Parks and Wildlife Department. Another factor in the timing of turnover is the depth of the body of water experiencing stratification.
As the air and water temps begin to decrease in the fall, the temperature difference between the stratified layers decreases, increasing the ability for them to mix or the surface layer cools to a temp that it becomes denser than the bottom layer, Nickolai said.
Turnover is a natural and common occurrence. However, it can affect aquatic life.
“The turnover mixes the oxygen-rich surface water with the deep oxygen-depleted water. The dissolved oxygen concentration in the mix can be too low to support life in the pond. Both fish and plankton can die from low dissolved oxygen following a turnover,” according to the Texas A&M AgriLife Extension. “Many people confuse a turnover with an algae die-off; externally, they can look very similar, but without a cold rain and wind, there can be no turnover.”