Groundwater is an essential resource contributing to more than 60% of the Dutch drinking water supply. The groundwater quality, however, is threatened by anthropogenic pollution. The 2000 European Water Framework Directive (WFD) aims to achieve good status of water bodies across Europe through a cyclical process of analysis and planning at the river basin scale. The WFD states that groundwater bodies used for drinking water abstraction should be protected to avoid deterioration in water quality. Every six years, the status of water bodies has to be assessed. Part of the assessment is a status assessment of drinking water abstraction sites. It is unknown which dataset, time period, statistical technique and water quality standards should be used for this assessment in the Netherlands. The aim of this research was to develop an example procedure for assessing trends in water quality of groundwater bodies used for drinking water abstraction in the Netherlands. Water quality datasets of monitoring wells, abstraction wells, mixed raw water and annual mean mixed raw water (REWAB) were collected from five drinking water abstraction sites. A selection of water quality standards and threshold values was made. The datasets were screened against these standards and trend analysis was conducted with various trend scenarios. Linear regression and the Mann-Kendall trend test were compared as well as trends resulting from the different datasets, the time periods 1990-2008 and 2000-2008, and significance levels of 95%, 90% and 80%. The results suggest that the drinking water test can best be performed using the annual mean mixed raw water quality dataset (REWAB) which best represents the water abstracted and thereby suits the WFD goal of assessing whether the necessary protection is present. This dataset is complete and readily available. Although abstraction well data give detailed information and are not influenced by blending or switching of wells, translating the trends to the abstraction site as a whole is difficult and data are currently too incomplete. It was shown that the mixed raw water dataset has no big advantage over REWAB. Monitoring well data were found to be unsuitable for the status assessment as these are biased, incomparable and monitoring frequency is irregular. A time period of approximately 10 years is recommended when assessing the drinking water abstractions, as this provides sufficient data for trend analysis without the effects of reversed or flattened trends becoming significant when using more than 10 years. A minimum of 5 years of data was found to be statistically justified. The use of the nonparametric Mann-Kendall trend test is recommended, because linear regression showed to be susceptible to outliers which exist in the raw water quality data. Using a significance level of 90% ensures no trends possibly relevant for drinking water abstraction are missed. Non-relevant trends can be filtered out by predicting whether a trend exceeds a water quality standard within 25 years. More water quality standards for groundwater are required for the drinking water test. Currently established threshold values for groundwater of the Dutch Decree on quality demand and monitoring of water (BKMW; VROM, 2009a) can be complemented with standards adapted to the effects of basic purification treatment. For substances not influenced by basic treatment, the drinking water standards provide a meaningful reference level for assessing groundwater quality. Based on the results of this study, a procedure for the drinking water test in the Netherlands is proposed. The procedure is depicted in a flow chart. It takes into account the substances for which trends should be derived and describes the classification process for situations with increasing or decreasing trends
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