Nutrients and wastewater

How do nutrients from wastewater treatment enter waterways?

Wastewater contains inorganic and organic nutrients and suspended solids. These can pollute waterways if left untreated and affect invertebrate and mahinga kai communities. Groundwaters and surface water may be contaminated if their assimilative capacity is exceeded or nutrients are flushed down the drain. Treatment plants that purify water to a high standard (tertiary treatment) can minimise the risks of harm to waterways. Landfills also can produce leachate which can escape into waterways when rainfall picks up decomposing organic wastes.

Potential impacts of high nutrients on water quality and mahinga kai

• Eutrophication - excess nutrients in lakes, estuaries, or slow-moving streams and rivers can lead to an increase in primary productivity which stimulates excessive plant growth (algae and nuisance plants and weeds), thereby degrading water quality.
• Loss of species - an increase in plant growth, sometimes called an algal bloom, reduces dissolved oxygen (DO) in the water when dead plant material decomposes and can cause organisms (fish and invertebrates) to die.
• Loss of habitat - eutrophication of the water can kill off plants that fish depend on for their habitat.
• Increase in turbidity and a decrease in visibility - when the phytoplankton community increases in response to nutrients this reduces water clarity, visibility and recreational suitability. It also reduces the ability of some fish to see prey or predators.

Learn more about the potential environmental impacts of nutrients in waterways