Well water is groundwater extracted from an aquifer, and it almost always contains magnesium. This naturally occurring mineral is a dissolved component, resulting from the water’s journey through the earth. The concentration of magnesium is highly variable, depending entirely on the specific geology and the surrounding rock and soil composition. This variability means one well can have very low levels, while a neighboring well can have concentrations high enough to cause noticeable household issues.
Geological Sources of Magnesium in Well Water
Groundwater acquires its mineral content through a process called mineral dissolution. As rainwater percolates down through the soil and into the aquifer, it becomes slightly acidic from absorbing carbon dioxide in the air and soil. This mildly acidic water acts as a solvent, dissolving minerals from the rock and sediment layers it passes through, and carrying them into the well water supply.
Magnesium is commonly found in mineral deposits such as dolomite, limestone, and gypsum. While limestone is primarily calcium carbonate, dolomite is a calcium magnesium carbonate, making it a significant source for magnesium in groundwater. The depth of the well and the composition of the aquifer itself determine the final concentration of magnesium ions in the extracted water. Older and deeper groundwater generally has a higher mineral content because it has had more time to interact with and dissolve surrounding rocks.
Magnesium’s Role in Water Hardness
Magnesium, along with calcium, is the principal cause of water hardness. Hardness is a measure of the concentration of dissolved divalent metal ions, primarily magnesium and calcium. The presence of these ions significantly affects the water’s ability to react with soap and its tendency to form mineral scale.
High magnesium concentrations lead to common household problems. When hard water interacts with soap, magnesium ions react with soap molecules to form an insoluble precipitate, often called soap scum. This reaction prevents soap from lathering effectively, requiring more soap or detergent for washing.
The formation of mineral scale, or limescale, occurs on surfaces and inside plumbing. When hard water is heated, the dissolved magnesium and calcium compounds precipitate out, forming a whitish, crusty buildup. This scaling is visible as a bathtub ring, spots on glassware, or white residue on showerheads and faucets. Within appliances like water heaters, scale buildup reduces energy efficiency and shortens the appliance’s lifespan.
Magnesium in drinking water is not generally considered a health risk for most people, though high levels may impart an undesirable taste. For individuals with pre-existing kidney conditions, or when combined with high sulfate levels, it can have a laxative effect. The presence of magnesium is primarily a concern for its effect on a home’s plumbing and cleaning routines.
Measuring and Treating High Magnesium Levels
Homeowners can determine their magnesium levels by testing the water, often as part of a general water hardness test. Testing can be done using at-home kits, but for the most accurate results, a sample should be sent to a certified professional laboratory. Water hardness is commonly reported in parts per million (ppm) or milligrams per liter (mg/L), or in grains per gallon (gpg).
If test results indicate excessive magnesium levels, the most common and effective treatment for the entire household is a point-of-entry ion exchange water softener. This system works by removing the positively charged magnesium ions from the water and replacing them with non-hardness ions, typically sodium or potassium. For treating drinking water, a reverse osmosis system can be used, which physically filters out the dissolved magnesium ions using a semi-permeable membrane. This treatment is installed at a single tap, such as the kitchen sink.