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Since 1979 the Dutch National Institute of Public Health and Environmental Protection (RIVM) has been developing the Dutch Groundwater Quality Monitoring Network (LMG). This network presently consists of about 350 monitoring sites. At each site, well screens are placed at two depths: 10 and 25 m below surface level. Samples are collected every year and are analyzed for all macrochemical parameters and some trace elements. Tritium contents were measured in the first sampling round. The geochemistry of Dutch groundwater is complex, due to the different sources (seawater, surface water and rainwater), complicated hydrogeology, and human impact on flow systems and pollution. Structuring or data analysis is required for the interpretation of the large number of hydrogeochemical data from such a monitoring network. An exploratory approach is to look within the data set for homogeneous groups, each with a typical (macro)chemistry. The selection criteria for the location of the monitoring sites of the LMG are mainly based on soil type and land use, and to some extent on the hydrogeological situation. However, a classification based on the two most reliable criteria, soil type and land use, does not result in chemically distinguishable homogeneous groups or water types. Fuzzy c means clustering was successfully used to discern structure and natural groups in the LMG data for 1 year. A seven‐cluster model was adopted. The number of clusters was decided heuristically with the aid of nonlinear mapping, on the basis of the geographic distribution, the hydrogeochemical interpretability, and the unimodality of the distribution of the parameters per cluster. The consistency of the model is illustrated by the reproducibility of the clusters in different years. The clusters are related to geochemical processes, natural sources, and anthropogenic input and are designated as follows: (1) “seawater” in coastal areas, (2) “desalinization” in organic‐rich Holocene marine and peat deposits, (3) “surface water” for downward seeping river water or surface water near the main rivers, (4) “carbonate/reduction” in peat areas or old groundwater in seepage zones, (5) “carbonate” for carbonate‐dissolving “precipitation,” (6) acid “precipitation” water in sandy topographic highs, and (7) a “polluted” cluster characterized by agricultural contaminants. Although the influence of soil type and land use is noticeable in some of the clusters, the geochemical controls, which characterize the different clusters, appear to outweigh their anticipated influence on the hydrogeochemistry on the scale of the national groundwater network. Consequently, the homogeneous groups, obtained through the cluster analysis, present a better base for further statistical and hydrogeochemical evaluation than, e.g., a stratification of the data based on soil type and land use.

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Regional and national ground‐water monitoring networks are used to inventory and to monitor diffusive (nonpoint) sources of ground‐water contamination. The Dutch National Ground Water Quality Monitoring Network (LMG) is an example of such a network, and monitors the shallow ground water of The Netherlands at two depths (10 and 25 m below land surface) in 350 wells, giving an average density of one monitoring well per 100 km2. Once water‐quality changes have been observed in time and space, the regional network is less suited to the study of the detailed chemistry, dynamics, and scale of the observed changes, because of the low density of sampling points. Two important threats to the quality of ground water that were identified by the regional network were studied in greater detail by use of multilevel observation wells along cross sections parallel to the direction of ground‐water flow. The first detailed study evaluates the fate of nitrate and other agricultural contaminants in a sandy aquifer recharged by precipitation. The second detailed study evaluates the effects of recharge from IJssel river water in a sandy aquifer. The precipitation‐recharged section consists of two hierarchically‐related ground‐water flow systems. A regional‐scale, relatively unpolluted system, recharged within an ice‐pushed ridge by rain water, is overlain by several local‐scale, agriculturally contaminated flow systems, that extend from the water table to 25 m below land surface. Locally, denitrification by organic carbon occurs in or near the unsaturated zone. Within the regional system, ground‐water composition changes over a distance of 5–6 km from aerobic infiltrating rain water to sulphate‐reduced, calcite‐saturated rain water. In the river‐water infiltration section IJssel river recharges the aquifer within a distance of much less than 1 km from the river. The main part of the section consists of local‐scale agriculturally contaminated flow systems. The varying compositions of ground water are controlled by the hydrological flow patterns, the composition of the aquifer sediments (e.g., presence of carbonate, clay minerals, and organic matter) and the composition of the source water (river water, rain water, agriculturally polluted). These controlling factors locally lead to relatively rapid transitions and heterogeneity of ground‐water compositions. The transition zones are considerably smaller than the density of observation wells in the Dutch ground‐water monitoring network, which limits the usefulness of regional monitoring networks for identifying chemically similar hydrologic zones or for effectively evaluating physical and chemical processes that affect the water quality. Regional patterns may evolve as a result of selective placement of monitoring wells, which show a specific facet of the ground‐water quality of that region. To evaluate and optimize monitoring site information and to study geochemical processes in the ground water, it is recommended that local‐scale sections be added to the national monitoring ne...

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