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The Dutch groundwater quality monitoring network includes 390 monitoring points. The frequency of sampling is once a year. The objectives of the network are: - to make an inventory of the present quality in relation to soil use, type of soil and hydrogeological conditions; - to distinguish long term changes in quality; - to provide data for quality control and groundwater control; - to indicate the extent of human influence on groundwater quality. This report gives the results of analysis in 1991 from 25 monitoring points in the province of Noord-Holland.