Chemical contamination and wood processing
What are the sources of chemical contamination from wood processing?
Wastewater from sawmills is generated by runoff from storage areas known as log yards and log ponds. Wastewater is also generated from chemical coating of wood. Toxic wood preservation chemicals may include polynuclear aromatic hydrocarbons (PAHs), pentachlorophenol (PCP), pesticides, and compounds of chrome, copper, and arsenic. The runoff from log yards and log ponds may also contain toxic chemicals (such as tannins, phenols, resins, and fatty acids) leached from the timber, soil, and other materials washed out of the bark.
Pulp and paper mills manufacture wood pulp using mechanical, semi-chemical, or fully chemical methods. The finished product may be either bleached or unbleached, depending on the requirements. Whatever the process used, pulp and paper production uses a large number of chemicals and these have been a great source of environmental concern. Recently the pulp industry has been using alternatives to chlorine, such as chlorine dioxide, oxygen, ozone, and hydrogen peroxide. The used wastewater from a pulp mill also contains a lot of organic material (tannins and lignin) from trees, chlorinated organic material if the processing plant uses bleach, as well as other persistent organic pollutants (POPs).
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 particularly 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 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.