Hard Water Explained: Causes, Types, and Classification

Understanding Hard Water

Older generations coined the phrase "hard water" because its mineral content makes cleaning more difficult. This difficulty is primarily due to dissolved compounds containing calcium and magnesium. Freshwater sources naturally contain varying quantities of these minerals.

As water travels, it dissolves, suspends, or exchanges compounds and trace elements from the materials it contacts. For example, flowing through limestone increases water hardness, while contact with peat can soften it.

Total Water Hardness

Total water hardness measures the combined concentration of calcium and magnesium, the two most common divalent metal ions. However, in some geographic locations, iron, aluminum, and manganese may also be present at significant levels.

Types of Water Hardness

Water hardness is distinguished into two types: temporary and permanent.

Temporary Hardness

Temporary hardness can be removed by boiling the water or by adding lime (calcium hydroxide). It is caused by dissolved calcium bicarbonate (Ca(HCO₃)₂).

Calcium bicarbonate is less soluble in hot water than in cold water. Boiling promotes the formation of carbonate ions and causes calcium carbonate (CaCO₃) to precipitate out of the solution, leaving the water softer.

Permanent Hardness

Permanent hardness cannot be eliminated by boiling. It is usually caused by the presence of dissolved calcium and magnesium sulfates and/or chlorides. These compounds tend to become more soluble as the water temperature rises.

Water Hardness Classification

Water hardness is typically classified based on the concentration of calcium carbonate (CaCO₃) or equivalent minerals:

• Soft water: 0 to 60 mg/L
• Moderately hard water: 61 to 120 mg/L
• Hard water: 121 to 180 mg/L
• Very hard water: More than 180 mg/L

Note: Specific classification ranges can vary slightly by organization. The ranges above are common examples.

Effects of Hard Water

Water hardness is a common cause of scale buildup (fouling) and potential corrosion in water pipes, heaters, and appliances. As water flows through rivers and groundwater basins, it dissolves minerals and carries them to the point of consumption. The level of hardness depends on the geology of the geographical area.

Watershed Influence

The source of water significantly influences its hardness.

Watershed Definition

A watershed, also known as a basin or drainage basin, refers to the territory drained by a single natural drainage system. This means all the water within that area flows towards a single river outlet, lake, or sea. A watershed is defined by its boundary, typically a line of summits called a watershed divide. The use of natural resources within these areas is often regulated administratively based on watershed boundaries.

Morphological Watershed

The concept of a morphological watershed is broader. It is considered an integral unit that includes not only the surface drainage basin but also the entire underground aquifer and hydrogeological structure as a whole.