The smell reported across northwestern Pennsylvania earlier this week was likely the natural seasonal release of trapped gasses from the bottom waters of Lake Erie, according to the Pennsylvania Department of Environmental Protection (DEP).

The gasses released are not harmful, DEP officials said.

We're told the odors may return over the next few days or weeks.

Here's a detailed explanation:

DEP has received several complaints regarding smells throughout Erie and neighboring communities. While the DEP is doing its due diligence in following up and investigating these complaints by ruling out possible inland causes, agency scientists believe that the most probable source is the natural seasonal release of trapped gasses from the bottom waters of Lake Erie.

Like most PA lakes, Lake Erie normally stratifies into a warmer upper layer and cooler lower layer in the summer. The two layers do not mix while the lake remains stratified. The warm upper layer becomes deeper throughout the summer as it continues to warm at the expense of the shrinking bottom layer. At the same time, dissolved oxygen levels are depleted in the bottom layer throughout the summer due to natural biological processes. By the end of the summer there can be little dissolved oxygen remaining in the bottom layer in the central basin of the lake. This oxygen-depleted condition, sometimes referred to as the “dead zone,” changes the water chemistry and produces anaerobic gasses like hydrogen sulfide with its characteristic “rotten egg” smell. When the top layer of the lake sufficiently cools during the end of the summer, strong winds are then able to break summer stratification allowing the lake to mix from top to bottom once again and the anaerobic bottom gasses to escape into the atmosphere. While sometimes pungent, the gasses released during this process are not harmful.

This year has been particularly warm and the anaerobic “dead zone” is likely to be particularly large. Interestingly, the dead zone only forms in the central basin of the lake which extends roughly from Sandusky Ohio to Erie PA. The western basin is too shallow to stratify, and the eastern basin is too deep for the formation of low-oxygen conditions in the bottom waters.
It is unclear if the odors that were recently detected were the result of a localized upwelling event or the result of the full fall overturn of the central basin. It is quite possible that we will continue to catch these odors over the next few days to weeks until the fall overturn process is complete, and the upper and lower layers are fully mixed.