Hard or soft?

The water supplied to Upper Hutt, Lower Hutt, Porirua, South Wairarapa and Wellington by Wellington Water can be described as soft.

Water with high calcium and magnesium content is characterised as hard, while water with less calcium and magnesium content is soft. The sum of all calcium and magnesium compounds in water results in the total hardness, measured in milligrams calcium cabonate per litre (CaCO3, mg/L) – 1mg/L equals one part per million, or 0.01millimoles (mmol)/L.

Water hardness is described as an "aesthetic determinand" in the Drinking-water Standards for New Zealand: 2005 (Revised 2018), meaning a property that can adversely affect taste, colour or general appearance of water. Guideline values (GV) are applied to aesthetics – these are levels that if exceeded, may make the water unattractive to consumers. Neither calcium or magnesium has an individual GV within the Standards; instead the GV for total hardness is 200mg/L CaCO3.

Water with a calcium carbonate level of <100mg/L can be characterised as soft. During the 2018/19 operating year (to 30 June 2019), the mean value for total hardness in treated water leaving each of our four water treatment plants was:

• Te Marua: 18mg/L
• Wainuiomata: 36mg/L
• Waterloo/Gear Island: 47mg/L