Chemical contamination and mining
What are the potential sources of chemical contamination from mining activities?
Surface and groundwaters associated with mines are often affected by acidification. Acid mine drainage (AMD) is recognised as one of the most important environmental side effects of mining. Coal mines can have the most severe acidification problems, as some coal seams contain pyrite, which has high concentrations of sulphur. Sulphuric acid mine runoff can be produced when pyrite is exposed to water and oxygen, i.e., acid mine drainage. This can have severe detrimental effects on water quality and mahinga kai.
Acidification of water can also increase the release of certain metals that are often associated with coal seams, including aluminium, arsenic, copper, iron, lead, nickel, and zinc, by increasing decomposition of minerals and the solubility of these metals. Some of these metals may be directly toxic to freshwater invertebrates, plants and mahinga kai.
Downstream, where acidification is less severe and the pH increases, these metals may be precipitated out of solution as high concentration flocs. Where flocs form, they can cause significant alteration to streams by cementing substrates (reducing spaces for animals to hide and live by clogging small spaces), reducing breakdown rates of detritus, and smothering animals and plant material. Also, depending on their geochemical nature, mine tailings may remain highly toxic for hundreds of years.
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 run off into rivers and streams can be lethal to aquatic life depending on the strength of the contaminant and size of the waterway.
- Local loss of invertebrate species - contaminants can be lethal to invertebrates, e.g. kōura. Invertebrates are also 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.
- Decreased dissolved oxygen (DO) levels - waste compounds released into waterways initiate biochemical reactions that use up oxygen as the stream 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 ‘breathe’ 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.
- Increased turbidity and decreased water clarity - water may become cloudy or discoloured with chemical contamination 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.
- Some contaminants, such as mercury, may bioaccumulate in animal tissues and be carried to human consumers of the fish.