Stagnant water is defined as water that has remained still for a prolonged period without significant flow or aeration. This lack of circulation leads to a depletion of dissolved oxygen, creating an environment where microorganisms thrive and decompose organic materials. The resulting microbial activity releases volatile gaseous byproducts, which are the source of the distinct and often unpleasant odor.
Describing the Odor of Stagnant Water
The smell of stagnant water typically presents as one of two distinct sensory profiles, depending on the specific conditions of the water body. The first, and often most intense, odor is a strong, foul scent akin to spoiled eggs. This powerful smell is pervasive and can be detected at very low concentrations, serving as an immediate indicator of advanced decay within the water.
The second primary odor profile is characterized by an earthy, musty, or moldy quality. This smell is generally less aggressive than the sulfurous scent and is often compared to wet soil, a damp basement, or decaying vegetation. In some cases, this less-intense odor can also manifest as a swampy or oily scent. Both of these odor profiles are the result of microbial metabolism and the release of volatile compounds into the air.
The Biological Causes of the Smell
The development of the characteristic odor in stationary water begins with the rapid depletion of oxygen that occurs when water is left undisturbed. In the absence of circulation, the oxygen dissolved in the water is quickly consumed by aerobic organisms as they break down organic matter such as dead plants, leaves, and sludge. Once this oxygen is exhausted, the water condition shifts from aerobic to anaerobic, forcing a different set of microorganisms to take over the decomposition process.
Anaerobic bacteria, which do not require oxygen for survival, flourish in these conditions and rely on alternative chemical compounds for respiration. Sulfate-reducing bacteria (SRB) are a group of these organisms that use sulfate ions present in the water as a terminal electron acceptor. As they metabolize organic material, these bacteria generate hydrogen sulfide ($H_2S$) gas as a metabolic byproduct. This $H_2S$ is responsible for the intense, spoiled-egg smell.
The production of the earthy or musty odor is linked to a different group of microorganisms, primarily certain types of filamentous bacteria called actinomycetes, as well as cyanobacteria, often called blue-green algae. These organisms produce complex organic compounds known as geosmin and 2-methylisoborneol (2-MIB) during their growth and decay. Geosmin and 2-MIB are volatile organic compounds that are potent odor agents, noticeable to people even at concentrations in the parts-per-trillion range.
The presence and concentration of these compounds are influenced by factors like water temperature, nutrient levels, and the specific composition of organic material available for decomposition. For instance, the musty odor associated with geosmin and 2-MIB is frequently observed in surface water sources like reservoirs during warmer months when algae and cyanobacteria populations are high. The combination of microbial activity, the type of organic matter being consumed, and the resulting chemical byproducts dictates which of the two primary odor profiles will dominate in any given stagnant water source.