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 counter-measures 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 esfenvalerate with a guideline value of 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.
