Water pollution comes from two main sources:
- Point sources: single outlets, such as a sewage pipe or a drain.
- Non-point sources: more diffuse seepage, for instance from underground sources or from small streams flowing into rivers or lakes.
The contaminants can be in solid or liquid form.
These include clay, which clouds the water and reduces the amount of light penetrating it. Heavy deposits of clay or silt may form a layer on gravel river beds, smothering plants and animal life, and releasing unwanted nutrients. Sediments may also carry other pollutants (such as heavy metals, nutrients and pathogens).
Other solid materials are organic forms of nitrogen, phosphorus or carbon (for example, faecal matter and decaying plant matter). These use oxygen and add chemicals to the water, disturbing its natural balance. Solid contaminants are also known as ‘particulates’.
These include toxic chemicals that directly affect the health of humans and/or aquatic species (for example, ammonia is toxic to fish). They also include nitrate and phosphate, which boost the growth of water plants such as algae and phytoplankton. In extreme cases, they may trigger blooms of toxic algae. These can clog streams, reduce water clarity in rivers and lakes, and starve fish of oxygen.
Most nitrate in the water from farmland comes from cattle urine, which is concentrated in patches of grazed pasture. Some of the nitrate in urine is washed down through the soil into the water table and eventually into streams and lakes. Phosphate is more readily held by the soil, and less of it reaches the waterways as dissolved phosphate.
Some effluents (from wool scours, timber treatment plants, urban stormwaters) contain dissolved zinc, lead, copper and chromium. These are highly toxic, and make water unsuitable for drinking, and shellfish in river estuaries unsuitable for eating.
In tap water, high levels of nitrate have long been considered a health risk for bottle-fed infants. The babies may suffer infant methemoglobinaemia, in which the blood fails to deliver oxygen. The skin turns blue-gray, and the baby may become irritable or lethargic. Coma and death can result if the condition is not recognised and treated. For this reason, nitrate levels in New Zealand’s drinking water must be less than 11.3 parts per million.
The central North Island’s streams and lakes are naturally high in phosphorus, leached from volcanic rocks, but nitrogen levels tend to be low. Nitrate that enters the water via farm runoff therefore triggers unwanted plant growth. This is a problem in Lake Taupō and other clear-water lakes.
In the Central Volcanic Plateau, runoff from geothermal fields contains arsenic, boron, lithium and mercury, which are toxic.
Elsewhere, such as in Southland, waters are naturally low in phosphate and higher in nitrate. Runoff containing phosphorus from farming, industry and urban areas is therefore causing water quality problems in some Southland rivers.
Temperature and shade
Many of New Zealand’s fish and invertebrates (animals without backbones) have evolved in cool, shaded streams surrounded by forest, and do not tolerate high water temperatures. For example, many stoneflies and mayflies are absent from pasture and urban streams where trees have been felled and summer temperatures exceed 18–19° C.
Regaining the diversity of species depends on restoring shade (by planting on riverbanks) to more than 70%. This is difficult in wide streams, but it is easier in narrow headwaters.