Sampling and analyses

The time of sampling can play a significant role for the results, particularly if the aim is to investigate a body of surface water. The concentrations in water courses, especially smaller streams, can vary sharply from day to day - sometimes from one hour to the next. The greatest risk of higher concentrations is after heavy rain during the spraying season, while the concentrations are often lower during periods with less spraying and in dry conditions. Groundwater concentrations usually change more slowly than they do in surface water.

An investigation aimed at providing a general description of the incidence of pesticides in surface water should adjust sampling according to the spraying season. In southern Götaland this can take place from April, when weed control begins, to November, when weed control and autumn ploughing are finished. In northern Götaland and Svealand the spraying season can take place from May to October. It can be a good idea to contact an agricultural advisor (e.g. in the county council or Hushållningssällskapet) to obtain information on the spraying season, its length, when it is most intensive and the compounds that are sprayed in the area under investigation. This also simplifies interpretation of the results.

However, if the aim is to investigate the groundwater, time of sampling plays a lesser role for the results since the turnover time of groundwater is considerably slower, particularly that of deep groundwater. However, where and to what extent the water in a well/borehole is replaced in relation to the time of sampling can be important.

Regardless of the type of water investigated, it is important to repeat sampling on different occasions in order to confirm the results and to eliminate chance.

When taking samples, it is important to handle the sampling equipment and the sample in such a way that the contents are not affected or changed from when the sample is taken up to when it arrives in the laboratory. Begin by avoiding contamination of the test equipment by thinking about how it is stored before and during sample-taking. There is a high risk of samples becoming contaminated if the inside of the bottle or cap is touched, or if the cap is left on the ground during sampling. Note also that certain substances may be volatile, while others can adhere to the sampling equipment if the wrong materials are used.

The risk of substances to be analysed being altered or lost between sampling and analysis, for example as a result of decomposition due to heat or microbial activity, is minimised by ensuring that transport to the laboratory is rapid (within a day) preferably in chilled conditions. Since the substances are often found at very low concentrations, even minimal contamination or incorrect handling of a sample may result in major deviations in the results.

Detailed instructions on how sampling should be carried out and what to be aware of usually accompany the sampling bottles sent out by the laboratory in charge of carrying out analyses.

For general guidance on sampling techniques and the design of sampling programmes for different types of water, standards have been issued by ISO (SS-EN ISO 5667-1:2023) which can be purchased through SIS (Swedish Standards Institute).

Many different compounds have been used as pesticides over the years, some that have been banned and others that have replaced them. They have been used for different purposes and in different ways. The compounds that are relevant to analyse in a sample depends on whether they may have ended up in the water being sampled. How the substances spread to surface and groundwater depends on a number of different factors, including the chemical properties of the substances (mainly how strongly they bind to soil particles and how quickly they break down) and soil conditions at the site, read more about this under the section Pathways of pesticides in the environment.

An indication of the types of compounds that are relevant for analysis can be gained by looking more closely at the type of use that has existed and still exists in the immediate area. Is it agricultural use, urban use or forestry use that dominates? It is often difficult to predict which substances will appear since there is, and always has been, a great variation in how the products are used, especially before the current stricter regulations for use and handling were introduced in the 1990s.

There are several different places to look for information on the compounds that are relevant in different questions.

• The Swedish Chemicals Agency publishes annual reports with statistics on quantities of different compounds sold (1986 onwards). Their website also has a search function where information can be sought on areas of application, or on a certain compound.
• By using the risk assessment tool MACRO-DB, the risk of different compounds leaching to the surface and groundwater can be simulated. This can be a help when assessing the compounds most relevant to analyse.
• Data on pesticides found in the Swedish water environment can be a support. On the web pages for the national environmental monitoring of pesticides at SLU there are compilations where the latest results are presented, you can also search for complete data on which substances are found in the agriculturally intensive areas in Sweden that are monitored. SLU also has a web service with the results of analyzes collected from different parts of the country, the Regional pesticide database (only in Swedish). There you can choose whether you want to look at surface water, groundwater or drinking water, and there is, among other things, information about which substances are found most often, both throughout the country and at the county level.  SGU is the data host for environmental pollutants and groundwater, they also have data for pesticides.
• If the water to be sampled is affected by farming, it is best to get in touch with an experienced agricultural advisor in the region, as they usually have good knowledge on the compounds used in the area. This depends partly on the crops grown and partly on the insects and fungi that occur.

Pesticides are usually found at very low level in water and it is important that the lab used has methods to detect the substances at sufficiently low levels. Comparison would often be with threshold values or guideline values in order to be able to assess whether the water quality is acceptable in terms of levels of pesticides.

In these contexts, the detection and quantification limits of the assay are crucial. The limit of detection (LOD) for a substance in a certain analysis method is the concentration at which the substance can be identified with certainty. However, the actual content at this low level can be rather uncertainly determined. Limit of quantification (LOQ), also called reporting limit or determination limit, is the lowest concentration at which one can both verify that the substance is present in the sample and determine its concentration with reasonable statistical certainty. 

Drinking water

For drinking water analysis, there is a regulation from the Swedish Food Agency (LIVSFS 2022:12, based on the EU's drinking water directive 2020/2184 ) which states that the quantification limit must be 30 percent of the limit value, or lower. For individual pesticides, the limit value is set at 0.1 µg/l, which means that the detection limit may be no more than 0.03 µg/l. The exceptions are the substances aldrin, dieldrin, heptachlor, heptachlorepoxide where the limit value is 0.03 µg/l and therefore requires a correspondingly lower quantification limit.

The reason for analysing substances at levels lower than the limit value is to detect trends, i.e. to determine whether the concentrations are on the way up and approaching the limit value, in order to implement countermeasures in good time, before the limit value is exceeded and the water is classified as undrinkable.

Groundwater

For groundwater the same limit value as for drinking water applies, i.e. 0.1 µg/L, according to SGU's regulation SGU-FS 2023:1, based on the EU groundwater directive (2006/118/EC). A further criterion for both groundwater and drinking water is that the sum of concentrations of individual pesticides may not exceed 0.5 µg/L.

Surface water

However, in the case of surface water there is no general limit value as regards the incidence of pesticides. Instead, individual substances have their own threshold values linked to how toxic they are to aquatic organisms in surface water.

There are limit values that are legally binding (through the EU's water directive 2000/60/EC and incorporated in Sweden through the Swedish Agency for Marine and Water Management's regulation HVMFS 2019:25) and guideline values that are not binding, but which were initially drawn up by the Chemicals Agency in order to be able to follow up Sweden's environmental goals. Now it is the Swedish Environmental Protection Agency that publishes the guideline values and CKB produces the underlying information.  

Threshold and guideline values specify the maximum concentration at which a substance cannot be expected to have any negative effects on aquatic organisms. The detection limit for substances analysed in surface water should be lower than the guideline value for the respective substance, in the same way as for drinking water and groundwater. However, some substances have very low guideline values, for example that for gamma-cyhalothrin is 0.0000045 µg/L. It is a level that can be difficult to get down to even if the analysis method is optimized with regard to detection limits. 

When analysing pesticides, it is important to place demands on the laboratory so that interpretation of the results is easier. You should of course check that the laboratory is accredited for the correct sample type (also referred to as matrix, e.g. water, soil etc.). Similarly, for the purposes of the investigation it is necessary knowing the detection limit, limit of quantification and accuracy of measurement, in order to ensure that the substances can be traced down to the level needed for the investigation. Some other aspects to consider are:

• Ask the laboratory to state the Swedish names of the substances. This sounds self-evident but is unfortunately it not always the case. A chemical identifier, e.g. CAS numbers can also help to be sure which substance is being referred to. Note, however, that one and the same active substance can be manufactured as different kinds of salts, e.g. sodium salt or potassium salt and these then get different CAS numbers.
• Ask the laboratory to clearly state how they define the lowest reported analytical limit. Several different concepts are used by laboratories, e.g. detection limit, limit of determination, quantification limit, reporting limit, and it is not always clear what is meant. Some laboratories also state a 'normal' reporting limit, which does not necessarily relate to the actual analysis. The complexity of samples varies and sometimes the reporting limits can be higher than the 'normal' limit. It is a great help in interpretation of the data to know exactly what the relevant level is for the sample in question.
• If the study in question is a follow-up study, the same substances at least should be included and the detection limit should not be higher than in the previous samples. Otherwise it is difficult to draw any conclusions from the results.

If the sample is to be used for some form of legal case, a high degree of transparency is needed in the entire chain from sampling in the presence of an independent observer from the local authority or police, labelling, documentation (for example photographs) and transport, to the analyses at the laboratory. The analyses should be performed as so-called documentary analyses by one, or possibly two, accredited laboratories. This means that the sample must be analysed on two independent occasions, in order for the results to be as reliable as possible.

There are several different commercial laboratories in Sweden that analyze pesticides, including ALS Life sciences, Eurofins and SGS Analytics Sweden. At SLU, analyzes are also made of pesticides in different matrices:

• The OMK laboratory at SLU - this is where all analysis takes place of residues of pesticides that are included in the national environmental monitoring programme, i.e. analysis of the matrices water, sediment, precipitation and air. The laboratory has been accredited by SWEDAC for analysis of pesticides in water samples since 1994.
• Within CKB's area of operation Analytical expertise, development work is ongoing and analyzes of pesticides can be carried out in, among other things, bees, nectar, pollen, plants and soil.