Water quality

Drinking water

The factors that affect water quality include where the water comes from, what is in the environment in the collection area and how it is treated.

Source

New Zealand gets its drinking water from many different sources, depending on what area you live in. You could get your drinking water from surface water (rivers, streams, lakes, reservoirs), groundwater from aquifers (bores/wells, springs) or rain water. New Zealanders also like to play in and around water such as rivers, beaches and lakes – this is called recreational water.

Environment

Lake Hawea

Each lake, river, stream and reservoir receives water that drains into it from the surrounding land. This surrounding land is called the catchment area for that water body. Smaller streams and rivers contribute their water to larger lakes and rivers, which may mean that the total catchment area of your local lake or river is much further away and larger than you may think.

The environment around the catchment area for water has an impact on its quality. If the catchment area includes farms, especially cattle grazing to the water’s edge, the water quality will be affected by sediment being stirred up and by animal faeces contaminating the water. The use of chemicals such as pesticides and fertilizers on surrounding farmland may also introduce contaminants into surface waters and groundwater. If the catchment area includes built-up areas with a lot of people, including all sites upstream, then sewage may get into the water from leaky pipes, septic tanks and storm overflows. The household use of chemicals, such as detergents, cleaners and petroleum based chemicals, may also introduce contaminants in water sources.

Nature of Science

Multiple factors often need to be considered when treating substances to make them suitable for human use. Often there are several treatment processes that target different properties of materials that allow them to be processed, for example, many substances can dissolve in water or float or not be wanted in the water. Water is also a good medium for microorganisms to grow in.

Another factor affecting the water quality is industry – factories have strict guidelines for disposal of waste and can be heavily fined if caught not obeying these guidelines, but accidental spillages are still a possibility. The chemistry of the soil and rocks in water sources can also affect the water quality. Some areas, for example, are naturally high in ‘heavy metals’ such as arsenic, iron and manganese.

Treatment

Water pollution

The treatment of the water you receive depends on where it is gathered and what has affected it on its route. For example, in Christchurch, the drinking water is groundwater that has effectively been filtered naturally through alluvial gravel for 10 to 700 years and is not treated at all.

Water supplies ideally use multiple barriers to ensure you get safe drinking water.

• The first barrier is protection of the water in the catchment. This may consist of fences to keep stock from the water source. Farmers may be restricted in their use of aerial crop spray. People may also be kept from using the body of water for recreational purposes.
• The second barrier is carried out in the treatment plant and consists of processes to remove particles from the water. The coagulation-flocculation treatment step causes sediment and other fine particles to group together to form clumps. These clumps can then be settled or filtered out.
• The third barrier is a disinfection step. This is done to kill any pathogens that may be in the water. It can be done by adding chemicals such as chlorine or ozone, or by exposing the water to ultraviolet (UV) light. It is very important to remove particles before the disinfection, otherwise the disinfection may not work very well.
• The last barrier is a post-treatment step and is the maintenance of pipes and tanks so that nothing can recontaminate the water.