Sampling and analyses

Last changed: 12 September 2016

There are a range of different factors to take into account if the results of an investigation are to be as useful and correct as possible. Important issues to think through are for example the aim of the investigation, whether there is a risk of leaching primarily to groundwater or to surface water, choice of matrix (water, sediment, soil) and whether there are results from previous investigations that can provide guidance when structuring the investigation

Other factors include choice of sampling site, sampling season, sampling frequency, sampling technique, handling of samples, analytical method, measurement of related variables and information on the conditions in the catchment area. It is important to take all these factors into account, but also to be consistent so that the results obtained are comparable with those from previous studies. Some of the more important factors to consider before sampling in surface water or groundwater are described in the following sections. 

Those wishing to carry out a more extensive investigation of pesticides are advised to read the Swedish Environmental Protection Agency’s ‘investigation types’ (in Swedish only), which describes the structure and methodology for measurements within environmental monitoring of pesticides. 

Time of sampling important

The time of sampling can play a great 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. There is a risk of higher concentrations after heavy rain during the spraying season, while the concentrations are often lower during periods with less spraying and in dry conditions.

An investigation aimed at providing a general description of the incidence of pesticides in 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 phone 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.

Den strömmande delen av skivarpsån. Foto: Jenny Kreuger

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.

Avoid contamination of samples

At sampling, it is important to handle the samples so that they are not affected by external factors. First and foremost this involves ensuring that the compounds to be analysed are not altered or lost between sampling and analysis, for example through organising rapid transport to the laboratory (within a day) preferably in chilled conditions. The conditions during the actual sampling are also important since certain compounds can be volatile, while others can adhere to the sampling equipment if the wrong materials are used. It is equally important to ensure that the sample is not contaminated in any way. 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. Similarly, it is important to avoid contaminating the sampling equipment used through considering how it is stored before use and also during the actual sampling. The concentrations to be analysed are often very low, which increases the risk of inadvertently contaminating the sample.

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 5667-1:2007 and SS-EN ISO 5667-1:2007/AC:2007), which can be purchased through SIS (Swedish Standards Institute).

Analyse relevant compounds

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 depend for example on whether the surface water or groundwater is being investigated. The pesticide residues present in surface water are often a reflection of what is in current use, while those found in groundwater are more a reflection of what has been used in the past, sometimes several decades in the past, within different sectors of society. 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 – whether it is 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.

Photo: Jenny Kreuger

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 groundwater can be simulated. This can be a help when assessing the compounds most relevant to analyse. In 2011, CKB also made a compilation of the propensity of different compounds for leaching based on data from various EU reports. This too can be a help when selecting compounds for analysis.

SLU has an internet service 'Pesticides in the environment'  (In Swedish only. To find the page please look at this corresponding page in Swedish and press the link "Växtskyddsmedel i miljön") containing the results of analyses collected from different parts of Sweden. The user can decide whether to look at surface water, groundwater or drinking water, and information is given on e.g. the compounds found most frequently, both in the entire country and at county level. The user can also look at the compounds found in the environmental monitoring being carried out in four intensive farming areas of Sweden. 

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.

Analyse relevant levels

The relevant analytical level depends primarily on why the sample has been taken and the type of water sampled. For drinking water the regulations (SLV FS 2001:30, in Swedish) state that the detection limit* must be 25% of the maximum permissible value. For individual pesticides the maximum permissible value has been set at 0.1 µg/L, which means that the detection limit may be at most 0.025 µg/L. The exceptions are the compounds aldrin, dieldrin, heptachlor and heptachlorepoxide, for which the maximum permissible value is 0.03 µg/L and therefore a corresponding lower detection limit is required.

*The detection limit is the lowest concentration at which it can be verified that the substance is present in the sample with reasonable statistical reliability. For concentrations close to the detection limit, only a qualitative determination can be made, i.e. it can be stated whether the substance is present in the sample or not. Such cases are sometimes reported as trace values by analytical laboratories.

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

For groundwater the same limit value as for drinking water applies, i.e. 0.1 µg/L, according to the EU directive on groundwater (2006/118/EG). 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.

However, in the case of surface water there is no general limit value as regards the incidence of pesticides. Instead, the Swedish Chemicals Agency has produced so-called guideline values for individual pesticides. The guideline value specifies the maximum concentration at which no negative effects of a compound can be expected 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 esfenvalerate is 0.0001 µg/L. This is a level that can be difficult to detect if the analytical method is of a general nature, i.e. includes a large number of different substances. The guidelines were issued in 2004 and updated in 2007, but since then no new update has been made by the Swedish Chemical Agency. Therefore CKB has produced two reports with preliminary guidelines using the same methods for compounds that are relevant to analyse in Swedish water, but for which there are currently no guidelines from the Swedish Chemicals Agency.

Laboratory for Organic Environmental Chemistry (OMK). Photo: Jenny kreuger

Before ordering analyses of pesticides

When analysing pesticides it is important to place demands on the laboratory so that interpretation of the results is easier. It is of course important to check whether the laboratory is accredited for the right sample matrices and the right measurement area for the substances in question. Similarly, for the purposes of the investigation it is worthwhile knowing the detection limit, limit of determination 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. 
  • 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 purity 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.

Laboratories for analysis of pesticides

For pesticides analysis, there are several different laboratories in Sweden to choose from:

Organic Risk Pollutants Laboratory (OMK)


ALS Life Science


Laboratoriet för organisk kemi (OMK). Foto: Jenny Kreuger

Staff at the Laboratory for Organic Environmental Chemistry (OMK) perform all analyses of pesticide residues carried out within national environmental monitoring in Sweden. The laboratory has been accredited by SWEDAC for analysis of pesticides since 1994.

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