Iron (Fe) is one of the most common elements in the Earth’s crust and frequently finds its way into both well water and municipal supplies. When present in water, even in small amounts above 0.3 milligrams per liter, it can cause aesthetic problems for homeowners, including a distinct metallic taste, an unpleasant odor, and reddish-brown staining on fixtures and laundry. Understanding the origin of this iron is the first step in addressing “rusty water.”
Geological Origins and Groundwater
The primary natural source of iron in water is the surrounding geology, as water acts as a powerful solvent. Rainwater seeps through soil and deep into underground aquifers, where it encounters iron-bearing rocks and minerals. The water’s natural acidity, often due to dissolved carbon dioxide, allows it to dissolve iron compounds like hematite or pyrite from the rock formations.
Iron is introduced into the groundwater supply, particularly in wells that draw from iron-rich geologic zones. Deep underground, where oxygen is scarce, the iron remains in its dissolved state, known as ferrous iron (\(\text{Fe}^{2+}\)). This dissolved state means the water is typically clear and colorless when it is first pumped from the ground.
The concentration of iron in groundwater is closely linked to the local geology. This iron remains invisible until the water is exposed to the atmosphere. This source is relevant for rural well users, where the water has had limited contact with man-made infrastructure.
Plumbing and Distribution System Corrosion
A secondary source of iron in the water supply is the corrosion of the infrastructure used to transport it. This is a concern for both municipal water users and homeowners with older internal plumbing, as the most susceptible materials are cast iron and galvanized steel pipes.
Corrosion occurs when the iron metal in the pipe reacts with oxygen and moisture. This reaction generates iron oxide, or rust, which flakes off the pipe walls and enters the water flow. Factors like water flow conditions, temperature, and the water’s chemical makeup can accelerate this degradation.
As the pipes age, the build-up of rust on the interior walls can restrict water flow and release particulate iron into the water. Unlike the invisible iron found in groundwater, this type of iron is often visible immediately at the tap.
How Water Conditions Affect Iron
The visible presence of iron in water is determined by its chemical state, which is heavily influenced by water conditions. Iron exists primarily in two forms: ferrous iron (\(\text{Fe}^{2+}\)), the dissolved, soluble form, and ferric iron (\(\text{Fe}^{3+}\)).
When this dissolved ferrous iron is exposed to oxygen, it rapidly converts into ferric iron in a process called oxidation. The resulting ferric iron is insoluble, meaning it precipitates out of the water as solid, visible particles that are commonly referred to as rust.
The water’s pH level affects the rate at which ferrous iron oxidizes and precipitates. Higher pH values encourage the oxidation reaction, causing the iron to become insoluble more quickly.