Time of sampling play a role
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.
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.