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Poisonous chlorine compounds

in streams


by John Walsby




Since the Second World War there have been three common sources of chlorine based poisons that have polluted New Zealand streams and rivers. These are pesticides like DDT and Dieldrin, bleaches and disinfectants.

Paper making mills like those at Kawerau on the Tarawera River continue to bleach paper with chlorine and some runs out with the waste into waterways. Today, there is greater control of waste discharges, a gradual change to non-chlorine based bleaches and a growing demand from the public for unbleached paper.

As a disinfectant, simple chlorine based chemicals have been routinely used for both domestic and industrial purposes. Most are correctly discharged into drains that lead to water treatment stations but some are run to waste into storm water drains and end up in streams and rivers. The heavily chlorinated flushings and overflows from swimming pools are also a source of toxic discharge in some areas.

If commercial chemicals supplied to keep swimming pools clean are strong enough to stop any life growing in the water, they will also poison both microscopic and larger organisms in nearby creeks if they are drained into them. Some people fail to take this into account when draining or cleaning pools.

When insecticides are poured on to farm animals and sprayed on pasture, horticultural blocks and home gardens, growers aim for a good kill of troublesome insects and to often use excessive amounts of spray. If the insecticides are effective on the land, they often continue to be a hazard to insect life elsewhere when washed off by rainfall and and any animals that eat those dead insects can also suffer.

Toxic residues of old organo-chlorine insecticides are still present in many areas but fortunately most modern insecticides breakdown quite quickly in the soil. However, they must not be used near watercourses.

In lakes, food networks start with algal production and continue through algal consumption by small filterers and surface scrapers. These consumers fall prey to larger animals that include fish.

The natural life of streams varies from that in lakes and ponds in that most of the organic matter in them is washed in from the surrounding land after rainfall. The living organisms in streams are therefore highly sensitive to nearby land use.

The organic matter naturally consists of fallen leaves, branches, flowers, pollen, and fruit, along with the faeces of both large and small animals and the bodies of those animals after they die. The dead animals naturally include millions of insects many of which have only short lives as flying adults.

Most dead insects are buoyant and easily washed down into streams where, floating on the water surface as "insect drift", they are a regular and important part of the natural diet of many fish. If the dead insects flushed off the land are crop pests that have been killed by pesticides then they will pass on the poisons that killed them to fish in the next step of the food chain.

Organo-chlorine compounds tend to accumulate in the fatty tissues of fish and as these are concentrated around fish ovaries they are thought to be partly responsible for the reduced fertility of some species.

Another important source of food for river fish are the creatures that cling to the bottom or hide in crevices to avoid being swept down stream. Some of these filter fine particles from the water and some scrape off algal films growing on river stones. Others process the large amounts of plant and animal debris that falls or is washed into waterways and sinks to the bottom as it becomes waterlogged.

Many of these creatures are the aquatic larvae of insects such as mayflies, stoneflies caddisflies, lacewings, craneflies, midges and certain beetles, in all of which the larval stage of the life cycle is by far the longest with adults living just long enough to mate and lay eggs; frequently only a few hours to a day.

When populations of these larval insects are knocked back by insecticides and chlorine based pollutants in the water, both native and introduced fish are deprived of food and the whole ecology of the waterway can be upset.

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