Illinois, like many states in the Midwest, is characterized by water that is generally classified as hard to very hard. This classification refers to the high concentration of dissolved mineral ions, primarily calcium and magnesium, within the water supply. While not a health concern, this mineral content is a common issue for residents across the state. Hard water can affect household plumbing and appliances, so understanding its source and regional variations is important for addressing its effects.
Defining Water Hardness and its Geological Roots
Water hardness is a measure of the dissolved divalent cations, with calcium (Ca2+) and magnesium (Mg2+) being the dominant contributors. This phenomenon is a direct result of the state’s geology, which is underlain by vast deposits of sedimentary rock, specifically limestone and dolomite. As rainwater infiltrates the ground to become groundwater, it flows through these rock formations, dissolving the calcium and magnesium carbonates along the way.
The concentration of these dissolved minerals is typically measured in two ways: parts per million (PPM) or grains per gallon (GPG). One GPG is equivalent to 17.1 PPM. The United States Geological Survey (USGS) classifies water based on these measurements, where moderately hard water ranges from 3.5 to 7.0 GPG, and hard water is 7.0 to 10.5 GPG. Water exceeding 10.5 GPG is considered very hard, a level common throughout Illinois.
Regional Hardness Levels Across Illinois
Water hardness levels are highly non-uniform across Illinois, varying significantly based on the water source. The Northern and Chicagoland area often sources its water from Lake Michigan, which tends to be comparatively softer than groundwater. Chicago’s municipal water typically registers in the hard range, between 7.6 and 8.8 GPG (130 to 150 PPM).
Many northern and suburban communities that rely on deep groundwater wells experience much higher concentrations. Areas drawing from aquifers can see very hard to extremely hard levels, sometimes ranging from 22 to 28 GPG. Central Illinois, which is heavily reliant on deep wells and is in close proximity to the mineral-rich bedrock, generally encounters very hard water. Cities in this region often report hardness levels between 11 and 15 GPG, with the median concentration for sand-and-gravel aquifers being around 21 GPG. Southern Illinois also experiences hard to very hard water, with groundwater supplies commonly showing high mineral content in localized areas.
Practical Indicators of Hard Water
Residents can often identify hard water through several noticeable household symptoms, long before a formal test is conducted. One of the most common signs is the formation of scale, a white, chalky residue left behind when hard water evaporates. This mineral buildup is often visible on shower doors, faucets, and showerheads, and can accumulate inside appliances like kettles and coffee makers.
Hard water also interferes with cleaning agents, as the dissolved minerals react with soap to form a precipitate known as soap scum. This reaction makes it difficult to produce a rich lather when washing hands, clothes, or dishes, and can leave a dull film on surfaces and glassware. Over time, hard water reduces the efficiency and lifespan of water-using appliances, especially water heaters. The mineral scale coats the heating elements, forcing the unit to use more energy to heat the water and accelerating the need for replacement.
Mitigation and Treatment Options
For individuals dealing with the effects of hard water, several residential treatment options are available to mitigate the issue. The most common and effective method for whole-house softening is the traditional ion exchange water softener. These systems contain resin beads that are charged with sodium ions, which exchange places with the positively charged calcium and magnesium ions as the water flows through.
This process effectively removes the hardness-causing minerals from the water, and the resin is periodically recharged using a concentrated brine solution. An alternative, salt-free option is Template Assisted Crystallization (TAC) technology, which conditions the water rather than truly softening it. TAC media guides the calcium and magnesium ions to form microscopic, stable crystals that remain suspended in the water and are unable to adhere to surfaces, preventing scale formation.
For high-quality drinking water, a point-of-use Reverse Osmosis (RO) system is often utilized. RO systems push water through a semi-permeable membrane that is fine enough to filter out the dissolved hardness minerals. While whole-house softeners protect plumbing and appliances, RO systems are specifically designed to treat water for consumption.