Sediment and mining
How can mining activities influence sediments in waterways?
Mining activities, including the excavation process and establishment of roads and infrastructure require the removal of vegetation. This exposes the underlying soils, which can lead to increased sediments and nutrients entering waterways.
Open cut mining is the most economical means of mining; it removes the minerals closest to the surface which are easiest to extract, but also exposes the largest area of the underlying soil to erosion. The dumping of 'overburden' can form unvegetated hills that are a source of leachate and sediment, which may end up in waterways.
Alluvial mining typically occurs on floodplain terraces, involving the removal of overlying vegetation and soils, and sluicing of gravels in off-channel ponds. These sediment-filled ponds may reduce re-establishment of riparian vegetation, alter river channels, and expose soils to erosion. Seepages from highly turbid sluicing ponds into streams can also result in more sediment into streams.
Potential impacts of sediments on water quality and mahinga kai
- Decreased water clarity - increased sediment loading into a stream will decrease water clarity and reduce visibility for fish seeking food and places to live.
- Damage to fish gills and filter feeding apparatus of invertebrates.
- Changes to the benthic (bottom) structure of the stream/river bed - coarse substrates such as gravels and boulders are replaced/smothered by sand and silt.
- Decreased numbers of invertebrate species from smothering of habitat - invertebrates are a food source to some mahinga kai (e.g., kōura and fish) and diverse invertebrate communities are also an indicator of healthy stream systems.
- Decreased algal food supply at base of food chain – sediments can scour algae from rocks, make algae unpalatable, or reduce light to levels where algae cannot grow, because plants need light to photosynthesise.
- Increased contaminants from surrounding land - sediments can transport attached pollutants such as nutrients, bacteria and toxic chemicals into our streams.