Mapping groundwater quality in the Netherlands

Groundwater quality is the suitability of groundwater for a certain purpose (e.g. for human consumption), and is mostly determined by its chemical composition. Pollution from agricultural and industrial origin threatens the groundwater quality in the Netherlands. Locally, this pollution is measured at tens of metres depth. Since groundwater is the main source for fresh water, this pollution causes a decrease in the long-term resources of water suitable for human consumption. In order to get insight into the current situation of groundwater quality and systematic changes of groundwater quality over time, the national groundwater quality monitoring network was established from 1978 to 1984. This network consists of 370 permanent wells, spread fairly evenly over the country (Fig. 3.2, page 27), with screens at 8-10 and 23-25 metre below the soil surface. The well screens are sampled yearly. More recently, the provinces have installed similar monitoring networks that doubled the measurement density. Because the monitoring networks are a major financial investment, the question arises whether the information on groundwater quality, as obtained from the current monitoring networks, is sufficient. This calls for the quantification of what can be inferred from this information about the quality of all the groundwater of interest. For the modelling of the spatial and temporal variation in groundwater quality, using a physically and chemically based deterministic model would call for information on many variables (e.g. initial and boundary conditions, model parameters), that are at present not available on a national scale. However, mapping groundwater quality is possible by using much simpler models, that lump much of the unknown factors into a spatially dependent stochastic term. The objectives of this study are to map groundwater quality in the Netherlands, using available measurements from the national and provincial groundwater quality; monitoring networks and map information on soil type and land use; show the effects of monitoring network density and the effects of using soil type and land use information on the resulting groundwater quality maps; map the systematic, temporal changes in groundwater quality in a way similar to the mapping of groundwater quality; show how groundwater quality maps can be improved by using relevant ancillary information in the estimation procedure, where ancillary information is obtained from deterministic process models or from other measured variables. The primary aim is, with these objectives, to answer the basic questions of describing the current situation and systematic temporal changes of groundwater quality for the whole of the Netherlands.In this study we use fairly simple models that allow an explicit quantification of the accuracy of resulting estimates. This accuracy is taken into account in the resulting maps, anticipating the question about the value of the current monitoring networks for inferring current situation and systematic changes in time of groundwater quality