Status of lakes and streams

The risk patterns for aquatic organisms has changed

Use of the persistent compounds has decreased, but the acute toxicity compounds have been retained and refined. The idea with these is for them to have a rapid effect and then be broken down before they reach streams and aquatic

organisms. This means that the compounds should not be able to have any effect over a longer period, but since they are very toxic in the initial stage, it is critical that they do not reach streams too quickly. There are examples that heavy rain after spraying may wash out pesticides rapidly to surface waters and can then cause practically catastrophic effects on animal life in these waters.

The endeavour to decrease the risk of bioaccumulation has meant avoiding compounds that are fat-soluble and instead prioritising compounds that are very soluble in water. A disadvantage with this is that the new pesticides do not attach as easily to soil particles and are therefore transported relatively quickly through the soil down to the groundwater and out to surface waters, bringing increased risks of acute effects on organisms living in lakes and streams. Compounds that are acutely toxic even at low concentrations are often used to control insect attacks. In environmental monitoring, concentrations of these insecticides that exceed the guideline values several hundred-fold are recorded on occasions. There can then be extensive effects in the water.

Accidents and carelessness a common cause of damage

Acute effects also arise due to accidents and carelessness, e.g. due to sprayers and other spraying equipment being washed in inappropriate ways or to spraying being carried out too close to streams. It is difficult to assess the frequency of acute toxicity effects arising as a result of inappropriate spraying or accidents. The episodes are very short but can still have discernible consequences. It is rather difficult to monitor this type of impact even with the help of extended analysis of water. At present we can only state that this type of impact exists and that it is perhaps the most common reason for toxic effects in surface waters as a result of pesticide spraying.