Chemical contamination and wastewater

What are the potential sources of chemical contaminants in wastewaters?

What are the potential sources of chemical contaminants in wastewaters?

Wastewater contains heavy metals and potentially hazardous wastes. These can contaminate a waterway if left untreated and affect invertebrate and mahinga kai communities. Ground and surface waters may be contaminated if their assimilative capacity is exceeded or contaminants are flushed down the drain. Treatment plants that purify water to a high standard (tertiary treatment) can minimise the risks of harm to waterways. However, decomposing waste in landfills generates methane, a harmful greenhouse gas. Landfills also can produce leachate which can escape into waterways when rainfall picks up heavy metals and decomposing organic wastes.

Potential impacts of chemical contaminants on water quality and mahinga kai

  • Local loss of fish species - fish may be harmed by contaminated water. Discharges and runoff into waterways can be lethal to aquatic life, causing fish kills from contaminants such as pesticides.
  • Local loss of invertebrate species - invertebrates are food for fish and persistent discharges that kill invertebrates could cause fish to travel farther in search of food, exposing them to greater risks and stress.
  • Decrease in dissolved oxygen (DO) levels - waste compounds released into waterways initiate biochemical reactions that use up oxygen as the naturally present bacteria break down the organic matter (Biogeochemical Oxygen Demand, BOD). Excess nutrients can also lead to algal blooms and oxygen is used up when the algae die and decompose. Fish ‘breath’ oxygen through their gills; a decrease in available oxygen (anoxia) in the water column threatens their ability to respire, which may lead to death. Fish that tolerate low levels of dissolved oxygen (such as the introduced species gambusia) may replace native populations that are less tolerant.
  • Increase turbidity and decrease in water clarity - water becomes cloudy and coloured green and brown, which reduces the ability of fish to see prey and detect predators.
  • Damage to species - repeated exposure to sub-lethal doses of some contaminants can cause physiological and behavioural changes in fish that have long term effects on the population, such as reduced reproductive success, abandonment of nests and broods, a decreased immunity to disease, tumours and lesions, impairment of the central nervous system, and increased failure to avoid predators.

Learn more about the otential environmental impacts of chemical contaminants in waterways