Status of lakes and streams

Last changed: 27 May 2016
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Pesticides play an important role in ensuring good crop yields, but also carry a risk of toxic compounds entering the surrounding environment. Pesticides that are spread on agricultural land can be carried with rainwater down through the soil to the groundwater or via surface run-off, erosion and drainage water out to streams, rivers and lakes. On the way they are broken down, often to more harmless substances, but some reach the water recipient and there they can cause damage to plants and animals in aquatic ecosystems and pose a threat to drinking water quality.

Risk of damage has decreased

When pesticides began to be used on a greater scale during the 1950s, the products were initially applied without any great knowledge of the negative effects that can occur in the environment. Persistent compounds that accumulated in food chains were used, for example DDT. When knowledge increased about the risks of spreading pesticides, stricter rules were introduced on the compounds that could be used. Persistent and bioaccumulating compounds in particular had to be avoided. The amount of substances used has decreased in recent decades. The handling and application of these substances has also improved. All this has led to a decrease in the risk of unintentional damage to the environment, particularly the risk of a compound being able to have a long-term impact and to build up concentrations that are damaging to flora and fauna in the environment.

Pesticides found in the aquatic environment

Through the national environmental monitoring programme in Sweden, pesticide status in the environment has been continuously assessed since 2002. Pesticides are analysed in streams in four small catchments and in two rivers. The results of these analyses show that small amounts of pesticides are present in all streams and rivers and that in approximately one in three samples, one or other compound exceeds the guideline value which indicates that some damage can occur. However, such cases are rather moderate and it is not possible to identify any particular compound that is particularly risky. Between 10 and 20 compounds are found once or twice a year in concentrations that can pose risks for algae and fauna in rivers and streams.

With the help of an estimated toxicity index, it is possible to compare water quality between different sampling areas and within the same area at different times. A higher index value means a greater risk of effects on aquatic organisms.

Toxicitetsindex

The diagram shows the index values per year for surface water within the four areas where environmental monitoring has been carried out in Sweden since 2002. The changes in the index values show the trend for the cumulative incidence of pesticide residues (per area and year) in relation to the respective guideline values. Compounds with guideline values lower than the detection limit are not included in the calculations. Source: Results from environmental monitoring of pesticides (plant protection products) – Annual report 2011. Ekohydrologi 132, SLU 2012.

There is rather large variation in the combined index values for each year and thus it is not possible to see any clear trend in whether they are increasing or increasing over time. However, there are relatively clear differences between monitoring areas. In Västergötland, the compounds found in water courses are not as poisonous and the toxicity index is therefore lower, while in Skåne it is somewhat higher than in other areas. This is probably because of the different crops grown and therefore also the different pesticides used. The same clear difference is not as apparent in terms of combined concentrations of pesticides in water courses. Instead, it is in comparisons against the guideline values that the obvious differences appear.

The River Vemmenhög in Skåne is the only river in Sweden for which there are long time series of data on concentrations of pesticides. Since the beginning of the 1990s the concentrations have declined dramatically, despite the amount of pesticides used not having decreased. The decrease in concentrations is probably the result of information campaigns during the latter half of the 1990s on the risks of handling pesticides.  

Växtskyddsmedel i Vemmenhög

The diagram shows changes in mean concentrations of pesticides in the River Vemmenhög in the period 1992-2011. Source: Results from environmental monitoring of pesticides (plant protection products) – Annual report 2011. Ekohydrologi 132, SLU 2012.

Risk pattern for aquatic organisms has changed

The use of persistent pesticide compounds has decreased, but the acute toxicity compounds have been retained and refined. The aim is for these 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 is evidence that heavy rain after spraying can wash out pesticides rapidly to surface waters, with practically catastrophic effects on animal life in these waters.

The efforts to decrease the risk of bioaccumulation have involved 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 impacts on water organisms.

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 obvious consequences if the compound washed out is toxic to aquatic organisms. However, it is difficult to detect this type of impact even with the help of extended analysis of water, since there is very little likelihood of sampling just when the high concentration is occurring.  

New methods needed to capture biological effects

The sharply varying concentrations of pesticides pose a difficulty as regards assessing the risks of damage in water on the basis of a few values. The maximum concentrations arising can be considerably higher than the measured concentrations and the organisms in water are a reflection of the worst case. Another difficulty is that as a rule, a number of compounds occur simultaneously in water and this makes it difficult to assess the combined effect on aquatic flora and fauna.

 Sampling of benthic organisms in an agricultural stream. Photo: Jenny Kreuger, SLU. In order to better and more directly assess the effects of pesticide use on surrounding waters, biological methods need to be developed as a complement to chemical methods. The aim of these biological methods is to use analyses of the flora and fauna present in lakes, rivers and streams to assess whether there is damage due to the toxic action of pesticides. Alternatively, the toxic action of the water and sediment can be studied in the laboratory or in the field. Such methods are now being developed within the CKB programme.

In order to determine the effect of pesticides on the biological diversity of terrestrial species, there is a need to develop this type of monitoring programme.

Environmental monitoring of pesticides

Environmental monitoring of pesticides at national level started in 2002, funded by the Swedish Environmental Protection Agency. The aim of environmental monitoring is to determine the extent to which different pesticides leak into surrounding waters and to monitor any long-term changes over time.  

Monitoring is being carried out in four small catchment areas (800-1700 hectares), representing four large agricultural regions in Sweden. These areas are called ‘type areas’ and are located in Skåne, Halland, Östergötland and Västergötland. In all these areas, around 90% of the land area is used for agriculture. The water in the streams is sampled weekly from May until November and the samples are analysed for around 130 different substances. The farmers in the areas are interviewed about their cropping practices, while groundwater samples and sediment samples are also taken. Two rivers in Skåne and a site for collection of rainwater are also included in the environmental monitoring.

Results from regional measurements

The regional pesticide database (RPD) was created in 2001 to collect information from the studies of pesticides being carried out around Sweden. It is managed by the Division of Water Quality Management at SLU and is funded by the Swedish Environmental Protection Agency. The oldest data in the RPD are from 1985. The database provides the potential to compare the incidence of pesticides in aquatic environments and any trends occurring over the years. The studies that form the basis for the database vary in both scope and objective. They include everything from occasional spot samples analysed for a few substances to more comprehensive investigation programmes.

The RPD contains (January 2013) the results of pesticide analyses performed on around 20 000 samples from all the counties of Sweden and over 80% of its municipalities. The largest proportion of samples comes from groundwater (70%), having increased since the end of the 1990s. The results per area and water type are available online via the link to the right.

The proportion of findings with a total pesticide concentration of over 0.5 µg/L has decreased over the years. This can be due to the proportion of groundwater samples having increased during the same period. A lower concentration of pesticides can be expected in groundwater due to the movement of the water through the soil profile. The fact that the number of samples with a total concentration exceeding 0.1 µg/L has increased is mainly because the limits of detection for a number of substances have decreased and thus more substances can be found at lower concentrations.

Page editor: mikaela.gonczi@slu.se