Manganese is a naturally occurring metal and one of the most abundant elements found in the Earth’s crust. While it is an essential trace nutrient in small amounts, it can become a common contaminant in water supplies, particularly those sourced from groundwater. The presence of manganese in drinking water presents a dual problem, ranging from noticeable aesthetic issues to potential health risks from long-term exposure.
The Chemistry and Sources of Manganese in Water
Manganese naturally enters water systems by leaching from geological deposits, such as rocks and soil, where it occurs as a component of over 100 minerals. This process is governed by environmental conditions, including the water’s acidity (pH) and its oxidation-reduction potential (redox).
The element exists in water in two primary forms: the dissolved, reduced form, Manganese(II) or Mn2+, and the insoluble, oxidized form, Manganese(IV) or MnO2. The dissolved Mn2+ form is colorless and stable in low-oxygen (anaerobic) environments, characteristic of deep wells and groundwater aquifers.
When this dissolved manganese-rich water is exposed to air, oxygen converts the soluble Mn2+ into the insoluble, particulate MnO2. This chemical shift makes the manganese visible, as the oxidized particles cause discoloration.
Microorganisms often accelerate the oxidation process and contribute to the formation of slimes or biofilms within plumbing. Water may appear clear when first drawn, but can develop color and precipitate after standing.
Aesthetic and Practical Problems Caused by Manganese
Aesthetic and practical problems often prompt water testing before health concerns are considered. Even at very low concentrations, sometimes as little as 0.05 milligrams per liter (mg/L), manganese causes noticeable discoloration. The insoluble particles appear as black, brown, or dark precipitates, giving the water a brownish or blackish tint.
This discoloration leads to unsightly staining on household fixtures and is particularly noticeable on laundry. Manganese can also impart a metallic or bitter taste to the water, especially when used for making beverages.
The Environmental Protection Agency (EPA) established a Secondary Maximum Contaminant Level (SMCL) of 0.05 mg/L for manganese, primarily to address these non-health-related aesthetic issues.
The presence of manganese also creates practical problems within the water distribution system. Over time, the insoluble particles accumulate in plumbing and water heaters, forming sludges or hard deposits. This buildup reduces the inner diameter of pipes, leading to decreased water flow and pressure, and can result in intermittent episodes of black, turbid water at the tap.
Health Concerns and Regulatory Standards
Manganese is a necessary nutrient for enzyme function and bone formation, but excessive long-term exposure through drinking water is a recognized health concern due to its neurotoxic effects.
The greatest concern is the potential impact on the developing nervous systems of infants and children. Infants are particularly vulnerable because their brains are rapidly developing and they absorb manganese more readily while having a reduced ability to excrete it compared to adults.
When infant formula is prepared with water containing elevated manganese, the infant can receive an inadvertent high dose, as formula is often already fortified. Exposure may be associated with neurological issues in children, such as changes in behavior, lower cognitive function, and difficulties with speech and memory.
For adults, consuming water with high levels of manganese over many years may impact the nervous system, potentially leading to symptoms resembling Parkinson’s disease, such as tremors and muscle rigidity.
The EPA established a lifetime health advisory level of 0.3 mg/L for manganese. This non-enforceable guideline indicates the concentration below which adverse health effects are not expected. The EPA advises using this 0.3 mg/L level for infants under six months old, acknowledging their increased risk. Manganese is not currently regulated by a national, enforceable Primary Maximum Contaminant Level (MCL).
Testing and Effective Removal Methods
Identifying the presence and concentration of manganese requires professional water testing. Testing measures the total manganese level and determines the ratio of dissolved Mn2+ to oxidized MnO2 particles, which dictates the most effective treatment strategy. For homeowners with private wells, testing must be initiated by the owner, as the water is not subject to regulatory monitoring.
The most common and effective removal method is oxidation/filtration. This works by converting the dissolved Mn2+ into the insoluble MnO2 so it can be filtered out. This is achieved by injecting a strong oxidizing agent, such as chlorine, potassium permanganate, or ozone, into the water before it passes through a specialized filter media.
Filtration media like manganese greensand or Birm contain an oxidized film that acts as a catalyst to trap the precipitated manganese particles.
Alternative Removal Methods
For lower concentrations of dissolved manganese, a standard ion exchange water softener may be effective. This method utilizes resin beads to exchange the manganese ions for sodium or potassium ions, but it is not suitable for removing the oxidized particulate form.
An alternative for very low levels is sequestration, which involves adding a chemical like polyphosphate to the water before it contacts air or chlorine. Sequestration does not remove the manganese but attempts to keep it in a soluble state to prevent staining and deposition throughout the home’s plumbing.