Gallionella is a genus of aquatic microorganisms that commonly causes water quality and infrastructure issues in groundwater systems. These bacteria are ubiquitous, thriving in soil and freshwater environments, making them frequent contaminants in private wells and municipal waterworks. Gallionella is a nuisance organism and does not pose a direct health risk. However, its metabolic byproducts significantly affect the taste, appearance, and flow of water, leading to costly problems. Understanding this iron-oxidizing organism is the first step toward effective mitigation and long-term control.
The Unique Structure and Metabolism of Gallionella
Gallionella is a chemolithotroph, meaning it generates energy by oxidizing inorganic compounds rather than relying on organic carbon sources. The bacteria obtain energy by converting soluble ferrous iron (\(text{Fe}^{2+}\)) into insoluble ferric iron (\(text{Fe}^{3+}\)) through iron oxidation. This biological reaction is far more rapid and efficient than the chemical oxidation of iron that occurs naturally in water. The energy yielded fuels the bacterium’s life processes, including using carbon dioxide as its primary carbon source.
Gallionella cells have a distinctive, microscopic structure: a small, kidney-bean-shaped body motile by a polar flagellum. The cell excretes a twisted, ribbon-like stalk from its concave side. This stalk is an organic matrix encrusted with ferrihydrite, the ferric iron oxyhydroxide mineral waste product. The accumulation of these iron-encrusted stalks forms the visible, slimy masses associated with an infestation.
Environmental Factors Driving Gallionella Growth
The proliferation of Gallionella is directly tied to specific environmental conditions, primarily the availability of dissolved iron. These bacteria thrive in water with high concentrations of ferrous iron, common in deep groundwater sources that flow through iron-rich geological formations. When this iron-rich water is drawn into a well or pipe system, it provides an ideal habitat for colonization.
Gallionella is a microaerophile, requiring very low concentrations of dissolved oxygen (0.1 to 1.0 milligrams per liter). These conditions are often found where anaerobic, iron-rich groundwater first encounters oxygen, such as near the bottom of a well casing or within the pipe system. Optimal growth occurs at a circumneutral pH between 6.0 and 7.6. The combination of high dissolved iron, low dissolved oxygen, and a near-neutral pH creates an ideal niche for forming dense, obstructive colonies.
Indicators of Iron Bacteria Infestation
The presence of Gallionella is signaled by visible and olfactory indicators that affect water aesthetics and system performance. The most obvious sign is the appearance of reddish-brown, yellow, or orange slimy deposits, which are the bacterial stalks and associated ferric iron precipitates. These sticky biofilms adhere to surfaces inside plumbing fixtures, well casings, and filters, sometimes appearing as rust-colored “snot.” Water discoloration, ranging from a slight yellow tint to a rusty hue, and an oily sheen on the surface are also common visual cues.
Infestations also lead to unpleasant tastes and odors, such as a metallic taste or smells described as swampy, oily, or like rotten vegetables. While Gallionella does not produce the “rotten egg” smell (caused by hydrogen sulfide gas from sulfate-reducing bacteria), the slime can harbor and protect these other nuisance bacteria, leading to a combined odor problem. The physical consequences of biofilm accumulation include a noticeable reduction in water pressure or flow due to the clogging of well screens and pipes.
Treatment and Prevention Strategies
Addressing an established Gallionella infestation requires a combination of chemical and mechanical cleaning methods, particularly in well systems. The primary chemical treatment is shock chlorination, which involves temporarily introducing a high concentration of an oxidizing agent, such as non-scented household bleach, directly into the well. The chlorine solution must be circulated throughout the entire plumbing system, including the hot water heater and all fixtures. It is then left to sit for 8 to 48 hours to ensure maximum contact time with the biofilm. Before starting, any carbon filters or water treatment units must be bypassed or removed to prevent damage and avoid neutralizing the chlorine.
For severe infestations, mechanical removal is necessary because chemical treatment alone may not penetrate the thick biofilm. This physical cleaning involves scrubbing the well casing and screens or using specialized jetting equipment to dislodge the slime deposits. After the contact period, the well must be flushed completely until the chlorine odor is gone, directing the highly chlorinated water away from sensitive landscaping or septic systems.
Long-Term Prevention
Long-term prevention focuses on maintaining a less favorable environment by reducing the iron source or continuously disinfecting the water. This can involve installing a water softener or an iron filter to remove the bacteria’s food source. Continuous disinfection can also be achieved using an injection system for chlorine or ozone, or a UV light system, to prevent the re-establishment of colonies.