Minerals found in fresh water most commonly include calcium, sodium, potassium, copper manganese, iron and magnesium. Other minerals and the amounts of dissolved minerals will largely be determined by the local source water. The levels of dissolved minerals (measured in parts per million - ppm) determine the "hardness" of your water supply.
Calcium carbonate (CaCO3)
When measuring the hardness of a water supply, the level of calcium carbonate is the most common single-mineral tested. It is often called lime as well. Lime-scaling is the white, chalky substance usually found lining tap water pipes.
Another mineral found in both ground water (wells and aquifers) and surface water (lakes and rivers) is iron. It readily leaches from the ground into water supplies or is washed into the ground from iron and iron-alloy structures. Iron is often the source if your water leaves red or brown stains during laundry operations.
Sodium is a readily soluble mineral which will leach into ground water or wash into surface water supplies. It is more active than iron and may be more difficult to remove in regular water treatments.
Other common minerals found in water sources include magnesium, copper, manganese, and lead. The Safe Drinking Water Act mandates testing for levels of copper and lead, with specific treatment (testing and reduction goals) requirements and increased testing if levels near defined thresholds. Should uncommon minerals (boron, arsenic, phosphorous) be detected, additional testing is required as well as monitoring.
Most tap water treatment processes concentrate on delivering safe, decontaminated water. Removal of waterborne pathogens (disease-carrying organisms) and filtration for taste and clarity are generally accomplished by disinfection-sedimentation-filtration systems. In hard water areas, consumers have a variety of water softening systems available to reduce the mineral levels further. Distilling or Reverse Osmosis (RO) systems will greatly reduce or completely remove minerals from water.