The National Groundwater Quality Monitoring Network (LMG) was built up between 1979 and 1984 and consists of approximately 350 measurement locations that are spread throughout the Netherlands. Sampling takes place in permanent wells that have been specially constructed for monitoring purposes. These observation wells are located just outside the fields to allow for easy sampling and not to interfere with field operations. The locations have been selected on the basis of soil type, land use and hydrological status. At each location, groundwater samples are taken at depths of 5-15 m (shallow filters) and 15-30 m below ground level (deep filters). On sandy soils, samples are taken from shallow observation wells every year, while on the other soil types (clay and peat), samples are taken from shallow wells every two years. Samples are taken from deep wells every four years, as well as from shallow filters at measurement points with marine influences. The wells that are not sampled every year are interpolated in for the absent years.
