RESEARCH ARTICLE Voutchkova et al. 2021: GEUS Bulletin 47. 6090. https://doi.org/10.34194/geusb.v47.6090 1 of 16 Estimating pesticides in public drinking water at the household level in Denmark Denitza D. Voutchkova*1 , Jörg Schullehner1,2 , Carina Skaarup3 , Kirstine Wodschow3 , Annette Kjær Ersbøll3 , Birgitte Hansen1 1Geological Survey of Denmark and Greenland (GEUS), Aarhus, Denmark, 2Department of Public Health, Research Unit for Environment, Work and Health, Aarhus University, Aarhus, Denmark, 3National Institute of Public Health, University of Southern Denmark, Copenhagen, Denmark Abstract Pesticide pollution has raised public concern in Denmark due to potential negative health impacts and frequent findings of new substances after a recent expansion of the groundwater monitoring programme. Danish drink- ing water comes entirely from groundwater. Both the raw groundwater and the treated drinking water are regularly monitored, and the chemical anal- yses are reported to a publicly available national database (Jupiter). Based on these data, in this study we (1) provide a status of pesticide content in drinking water supplied by public waterworks in Denmark and (2) assess the proportion of Danish households exposed to pesticides from drinking water. ‘Pesticides’ here refers also to their metabolites, degradation and reaction products. The cleaned dataset represents 3004 public waterworks distributed throughout the country and includes 39 798 samples of treated drinking water analysed for 449 pesticides (971 723 analyses total) for the period 2002–2019. Of all these chemical analyses, 0.5% (n = 4925) contained a quantified pesticide (>0.03 µg/l). Pesticides were found at least once in the treated drinking water at 29% of all sampled public waterworks for the period 2002–2019 and at 21% of the waterworks for the recent period 2015–2019. We estimate that 56% of all Danish households were potentially exposed at least once to pesticides in drinking water at concentrations of 0.03–4.00 µg/l between 2002 and 2019. However, in 2015–2019, the pro- portion of the Danish households exposed to pesticides (0.03–4.00 µg/l) was 41%. The proportion of Danish households potentially exposed at least once to pesticides above the maximum allowed concentration (0.1 µg/l) according to the EU Drinking Water Directive (and the Danish drinking water standard) was 19% for 2002–2019 and 11% for 2015–2019. However, the maximum concentrations were lower than the World Health Organization’s compound-specific guidelines. Lastly, we explore data complexity and dis- cuss the limitations imposed by data heterogeneity to facilitate future epi- demiological studies. 1 Introduction Pesticides are biologically active compounds widely used in agriculture, hor- ticulture and public health for the control of pests (World Health Organiza- tion 2019a). They comprise many chemical substances with a broad variety of mode of action depending on their target organisms (Casida 2009), for exam- ple, photosynthesis inhibition (plants), neurotoxic (insects) and fungal spore *Correspondence: dv@geus.dk Received: 24 Nov 2020 Accepted: 10 Feb 2021 Published: 12 Apr 2021 Keywords: Denmark, drinking water, exposure, pesticides, public waterworks Abbreviations BAM: 2,6-Dichlorobenzamide DBCP: Dibromochloropropane DEIA: Desethyl-desisopropyl atrazine DPC: Desphenyl chloridazon DMS: N,N-dimethylsulfamide DDT: Dichlorodiphenyltrichloroethane DWD: Drinking Water Directive DWQS: Drinking water quality standard EPA: Environmental Protection Agency GEUS: Geological Survey of Denmark and Greenland MDPC: Methyl-desphenyl-chloridazon LOD: Limit of detection LOQ: Limit of quantification WHO: World Health Organization WSA: Water supply area GEUS Bulletin is an open access, peer- reviewed journal published by the Geological Survey of Denmark and Greenland (GEUS). This article is distributed under a CC-BY 4.0 licence, permitting free redistribution, and reproduction for any purpose, even commercial, provided proper citation of the original work. Author(s) retain copyright. Edited by: Adam Hambly (Technical University of Denmark) Reviewed by: Martin Rygaard (Technical University of Denmark), Kai Tang (Technical University of Denmark) Funding: See page 14 Author contributions: See page 14 Competing interests: None declared Additional files: See page 15 https://doi.org/10.34194/geusb.v47.6090� https://orcid.org/0000-0003-2840-072X https://orcid.org/0000-0002-1153-6885 https://orcid.org/0000-0001-7470-5052 https://orcid.org/0000-0001-9908-3632 https://orcid.org/0000-0002-9407-3387 https://orcid.org/0000-0003-2318-145X mailto:dv@geus.dk Voutchkova et al. 2021: GEUS Bulletin 47. 6090. https://doi.org/10.34194/geusb.v47.6090 2 of 16 www.geusbul let in.org inhibition (fungi). For a classification of over 180 active substances (pesticides) and their metabolites based on the mode of action, see Mohaupt et al. (2020; Annex 5). Agricultural pesticides threaten aquatic biodiversity (Stehle & Schulz 2015) and potentially impact food-webs and species competition (Kohler & Triebskorn 2013). New pesticides are registered for use only if they are demonstrated not to persist in the environment after their intended period of use. However, residues are found in the natural environment in nanogram to microgram per litre levels (Fenner et al. 2013). For exam- ple, the status assessments for water bodies in the European Environmental Agency countries showed that 0.4% of all surface water bodies and 6.5% of the area of groundwater bodies are failing ‘good chemical sta- tus’ because of pesticides (Mohaupt et al. 2020). In fact, pesticides are second only to nitrates as a cause for fail- ing to achieve a level of good chemical status in Europe (Mohaupt et al. 2020). This shows that pesticides persist in the environment, and the only natural removal is through degradation by biotic or abiotic transformation processes (Fenner et al. 2013). 1.1 Pesticide exposure and drinking water Pesticides are inherently hazardous compounds, but even though only a small fraction is highly hazardous (World Health Organization 2019b), they cause dispro- portionate harm to the environment and human health worldwide (Food and Agriculture Organization & World Health Organization 2019). The greatest human expo- sure to pesticides occurs in occupational environments, for example, in factory, agricultural and public health workers during production, handling, dilution, mixing and application procedures (World Health Organization 2019a). Acute pesticide poisoning is an important cause of mortality and morbidity, for example, due to neuro- toxic effects of organophosphate intoxication (Rosen- stock 1991). However, there is incomplete knowledge on the toxicity of the metabolites, degradation and reaction products, which could have a similar, stronger or lesser effect on organisms and humans (Mohaupt et al. 2020; p. 7, Box 1). The general population may be environmentally exposed to pesticide residues from food and drinking water (World Health Organization 2019a). A variety of chronic health effects related to exposure at doses that do not cause acute effects have been suggested, includ- ing asthma, diabetes, Parkinson’s disease and cancer (Kim et al. 2017). Protecting the population from health risks associated with pesticide-contaminated drink- ing water is a worldwide problem (Li & Jennings 2018). Thirty-four percent of the world’s population, in >1/2 of the world’s nations, are estimated to be inadequately protected against health risks associated with pesti- cide-contaminated drinking water (Li & Jennings 2018). Pesticide pollution was recently found to be of major concern in drinking water sources in the Netherlands, where pesticides were found in 2/3 of the water abstrac- tion areas. The water-quality standard was exceeded in 1/3 of all drinking-water sources in the Netherlands (including both groundwater and surface-water bodies; Sjerps et al. 2019). Bexfield et al. (2021) also showed that at least one pesticide or degradate was found in 41% of wells investigated in the United States (n = 12 041 204 in aquifers responsible for 70% of the volume pumped for public drinking-water supply, nationally), and around 2/3 of them contained compound mixtures. Although pesticide compounds occurred frequently, concentra- tions were low, and only 1.6% of wells had concentra- tions approaching levels of potential health concern (Bexfield et al. 2021). Pesticide exposure from drinking water in Denmark is estimated to be smaller than that from other dietary sources, such as berries, fruits and vegetables (Bichel hovedudvalget 1999). Nevertheless, concerns about adverse health outcomes due to long-term low-dose intake from drinking water have been raised, and there has not been enough scientific evidence to either sup- port or reject potential health risks (Bichel hovedudval- get 1999). While the pesticides found in food products are mainly contemporary insecticides and fungicides (approved for use), the pesticides found in drinking water are mainly herbicides, often representing legacy pollution. The term ‘pesticide’ covers a large group of substances with different structure and mechanism of action, making it relevant to study the presence and potential influence of pesticides in drinking water. To the best of our knowledge, there are no recent publica- tions on drinking water as a dietary source of pesticides in Denmark. Exposure estimates from drinking water in Denmark are thus lacking. 1.2 Objectives The purpose of this article is (1) to assess the pesticide status of Danish drinking water, (2) to estimate the pop- ulation’s exposure to pesticides from drinking water and (3) to evaluate the dataset heterogeneity and limitations. We take advantage of the national database (Jupiter; https://eng.geus.dk/products-services-facilities/data-and- maps/national-well-database-jupiter) in which all drinking water samples taken for compliance or other purposes are centrally registered. The study period is 18 years (2002–2019) with sufficient data coverage to analyse the current drinking water status at a national level for the entire study period and specifically, the last five years (2015–2019). https://doi.org/10.34194/geusb.v47.6090� www.geusbulletin.org� https://eng.geus.dk/products-services-facilities/data-and-maps/national-well-database-jupiter� https://eng.geus.dk/products-services-facilities/data-and-maps/national-well-database-jupiter� Voutchkova et al. 2021: GEUS Bulletin 47. 6090. https://doi.org/10.34194/geusb.v47.6090 3 of 16 www.geusbul let in.org 2 Study setting and legal framework European Union member states must ensure that water intended for human consumption is ‘wholesome and clean’, according to the Drinking Water Directive (DWD; Council of the European Union 2015). This means that drinking water should comply with the DWD’s minimum requirements for several microbiological and chemical components, including pesticides. The maximum allowed concentration of pesticides in drinking water – the drink- ing water quality standard (DWQS) – is set in the DWD at (1) 0.10 µg/l for individual compounds, except for aldrin, dieldrin, heptachlor and heptachlor epoxide, which are set at 0.030 μg/l, and (2) 0.50 μg/l for the sum of detected and quantified pesticides (named ‘Pesticides – Total’ in the DWD). The DWD also provides a legal definition of the term ‘pesticide’, which we follow in this article: ‘Pesticides means [-] organic insecticides, [-] organic herbicides, [-] organic fungicides, [-] organic nemato- cides, [-] organic acaricides, [-] organic algicides, [-] organic rodenticides, [-] organic slimicides, [-] related products (inter alia, growth regulators) and their rel- evant metabolites, degradation and reaction prod- ucts’. (Annex I, Part B, Note 6 of the DWD; the square brackets indicate a change in the punctuation from the original DWD) Denmark has implemented the DWD provisions in national legislation (Vandforsyningsloven; LBK nr 118, 22 February 2018). Actionable items (e.g. sampling fre- quencies and DWQS) are further specified by Ministe- rial Orders (the most recent is BEK nr 1070, 28 October 2019). The frequency of sampling varies based on the volumes of drinking water produced, from once every 3 years to several times per year (BEK nr 1070, 28 October 2019). The list of compounds monitored for mandatory compliance is revised every year, and currently, it con- tains 46 pesticides. Waterworks must also test for other pesticides if there is evidence that their presence in the catchment area poses a health threat. Danish drinking water supply is decentralised (>4500 public waterworks) and 100% groundwater based. It mostly relies on simple groundwater treatment, includ- ing aeration and sand filtration. If the simple treatment is not sufficient to assure compliance with the DWQS, the municipalities can grant permits for the use of advanced treatment processes. In the period 2007–2012, 74 water- works obtained such permits, but of those, only eight (representing c. 2.5% of treated groundwater) were for carbon filters to treat for 2,6-dichlorobenzamide (BAM) or other organic micropollutants (Naturstyrelsen 2012). In the period 2012–2019, the number of permits granted for advanced treatment increased to 110, of which 12 were for carbon filters (Miljøstyrelsen 2020a). In addition to advanced treatments, waterworks could address non-compliance by closing polluted wells or well fields, diluting non-compliant water with ground- water from another well or importing unpolluted water from neighbouring waterworks. All laboratory results of both drinking water and groundwater samples in Denmark are reported to the Danish National Well database (Jupiter), according to the national guidelines (Miljøstyrelsen 2020b). Based on these data, the Geological Survey of Denmark and Greenland (GEUS) has reported the status of raw groundwater in Den- mark on an annual basis for the past 30 years (Thorling et al. 2019). Additionally, the Danish Environmental Pro- tection Agency (EPA) conducted a mass screening in 2019, where 263 groundwater wells were tested for 415 pesti- cides (Mathiesen 2020). Based on these tests, the Danish EPA planned to revise the list of pesticides for mandatory compliance monitoring of drinking water and to inten- sify its control efforts against illegal imports of pesticides (Mathiesen 2020). Our study provides a national assess- ment of pesticides in Danish treated drinking water, sup- plied by public waterworks and on that basis an estimation of households exposed to pesticides from drinking water. 3 Methods and materials The complete workflow used in this study is presented schematically in Fig. 1. A similar exposure-estimation methodology was used previously (Schullehner & Han- sen 2014; Voutchkova et al. 2015). Here, we outline all data-handling procedures to assure reproducibility and transparency, and to facilitate the potential future use of the drinking water dataset. 3.1 Data sources The final dataset (Fig. 1) used for the status overview and to estimate pesticide exposure was prepared by combining data from four different sources. 3.1.1 Drinking water samples analysed for pesticides All chemical analyses of drinking water reported to Jupiter were extracted on 5 May 2020 (Fig. 1, Step 1). This raw dataset was passed through a pre-process- ing procedure including various filtering and quality control steps summarised here and presented in full detail in Supplementary File S1. Compound selection was based on the latest version (20 May 2020) of the Jupiter list of pesticides, their degradation products and related substances (‘50 - Pesticider, nedbryd- ningsprodukter og beslægtede stoffer’ in Jupiter). Only samples of treated drinking water taken at the waterworks (i.e. the finished product), from the dis- tribution network or the consumer’s tap were kept in https://doi.org/10.34194/geusb.v47.6090� www.geusbulletin.org� Voutchkova et al. 2021: GEUS Bulletin 47. 6090. https://doi.org/10.34194/geusb.v47.6090 4 of 16 www.geusbul let in.org the dataset (see Supplementary File S1, Section 1.1, Step 4 for details). We filtered out flagged or errone- ous data and those data rejected by the data-owner. Analytical units were checked and converted when necessary to µg/l. All analyses below detection limit in instances where that detection limit was high (>0.02 µg/l) were excluded due to low precision of the lab- oratory method. After the pre-processing procedure Fig. 1 Workflow including data-processing steps and overview of excluded chemi- cal analyses and waterworks. DK: Denmark. WSA: Water supply areas. QC: Quality con- trol and filtering procedures (see text for details). PLANT_ ID and WSA_ID: waterworks and WSA ID numbers, respec- tively, used when combining the datasets. SF: Supplemen- tary Files. JUPITER 1 003 804 analyses 3173 waterworks 449 compounds X & Y coordinates Public DK waterworks (n = 4641) 989 218 analyses 3 072 waterworks 449 compounds PLANT_ID 2584 water supply areas for 4456 waterworks QC1 8 151 264 analyses 4411 waterworks 1140 compounds 5 May 2020 Join QC2 14 586 analyses 101 waterworks Details in SF Join17 495 analyses 68 waterworks Final pesticides dataset no WSA no X,Y Status 1 Individual compounds Status 2 Waterworks Status 3 Water Supply Areas 99.46% of DK households for 2811 WSA Exposure to pesticides Join WSA_ID Status and exposure calculations were done: 1) over the entire study period (2002–2019) 2) for the last five years (2015–2019) no households in 4 WSA STEP 1 STEP 2 STEP 3 PLANT_ID Water Supply Areas Geocoded households 971 723 analyses 3004 waterworks 449 compounds Database Processing step Process Output Other input datasets Symbols https://doi.org/10.34194/geusb.v47.6090� www.geusbulletin.org� Voutchkova et al. 2021: GEUS Bulletin 47. 6090. https://doi.org/10.34194/geusb.v47.6090 5 of 16 www.geusbul let in.org (Supplementary File S1, Section 1.1), the resulting dataset contained all pesticide analyses of treated drinking water for the period 2002–2019 from public waterworks (n = 3173; Fig. 1, Step 2). 3.1.2 Geographic coordinates of the waterworks The geographic coordinates of all waterworks were extracted from Jupiter and passed through a quality control. Missing X and Y coordinates were geocoded based on the registered address. Coordinate errors were identified and fixed when possible. Only coor- dinates for public waterworks (n = 4641) were kept, from which 5.5% (n = 256) were geocoded based on address. 3.1.3 Water supply areas of the waterworks The water supply areas (WSAs) for all public waterworks, covering the entire country, were provided by Schulleh- ner and Hansen (2014). We assumed that WSAs were static in the study period (2002–2019), reflecting their state at the time the data were published (2014). 3.1.4 Percent of households located within a WSA Geocoded locations of households were provided by the Centre for Integrated Register-Based Research at Aarhus University (CIRRAU), Denmark. This data source contains all residential addresses in Denmark registered in the Danish Civil Registration System (n = 2 086 797; Pedersen 2018). Just 0.54% of all households fell outside a polygon for the WSA. 3.2 Data joining procedure Data from the four sources were combined using a step- wise procedure (‘join’ in Fig. 1). The pre-processed pesti- cide dataset from Step 2 was joined with the waterworks’ coordinates. In this step, we excluded 101 waterworks with all their analyses. Most of these were waterworks located in Greenland (n = 70), and four were identified as misclassified private waterworks. The remaining waterworks (n = 27) had no coordinates or a registered address and could not be geocoded. Of these, only four were actually waterworks, and the rest were locations at the supply network or at the consumers’ tap. Then, the dataset (Fig. 1, Step 3) was joined with the WSA data, resulting in the exclusion of 68 waterworks which could not be assigned a WSA. We refer to this dataset as the final pesticide dataset. 3.3 Final pesticide dataset The final pesticide dataset includes 39  798 treated drinking-water samples (7 January 2002–30 Dec 2019) analysed for 449 pesticides (number of individual analyses n = 971  723), associated with public water- works located in Denmark (n = 3004) with known X and Y coordinates and WSA. Figure 2 shows the spatial dis- tribution of these waterworks, the number of water- works within the WSA and the percent of households in each WSA. The final dataset was used further to produce the status overviews of individual pesticides, waterworks and WSAs (Status 1, 2 and 3 in Fig. 1). The popula- tion exposure to pesticides from drinking water was obtained by combining Status 3 results (Fig. 1) with the percentage of households with each WSA (Fig. 2). The methodology for these status overviews and the exposure assessments are presented in Section 3.4. First, we present the methodology for how we handled values below the detection/quantification limit and dis- cuss the data structure. 3.3.1 Limit of detection and quantification The laboratories performing the chemical analyses report the limit of detection (LOD) in the Jupiter data- base. All measurements below the LOD are recorded with the attribute ‘<’ and a value equal to the LOD (e.g. <0.01 µg/l). Most of the pesticide analyses in the final dataset were below the specified LOD (98.8%, n = 960 437). LOD varied from 0.4 ng/l to 0.02 µg/l. Analy- ses with higher LODs were excluded in the pre-process- ing. The most frequent LOD was 0.01 µg/l (99.2%, n = 953 120), followed by 0.02 µg/l (0.6%, n = 5522; see Sup- plementary File S1). The variation in LODs reflects the variety of methods used by different laboratories, with liquid chromatography–mass spectrometry (LC/MS/MS) being the most frequently used method (see Supple- mentary File S1). The variation with time is also due to possible instrumentation improvements, reducing the LOD during the study period. In the status assessments and the exposure estima- tion, we use the limit of quantification (LOQ) instead of the LOD to avoid the effect of false positive detections and to reduce the influence of the high uncertainty in concentrations measured near the LOD. Evidence of the variability around the LOD is presented in Section 3.3.2. The LOQ was calculated according to the Minis- terial Order on quality requirements for environmental measurements (BEK nr 1071, 28 October 2019) with the formula LOQ = 3 × LOD, where LOD here is the most frequent LOD (0.01 µg/l). Thus, LOQ = 0.03 µg/l (equal to 30% of the DWQS). For all analyses with concentrations less than LOQ, we adopt the definition that ‘pesticides were not determined with an acceptable level of accu- racy and precision’ (Commission of the European Com- munities 2009). For brevity, we use ‘non-detect’ or ‘never detected’, where this definition applies to a period of time or pesticide, respectively. https://doi.org/10.34194/geusb.v47.6090� www.geusbulletin.org� Voutchkova et al. 2021: GEUS Bulletin 47. 6090. https://doi.org/10.34194/geusb.v47.6090 6 of 16 www.geusbul let in.org 3.3.2 Structural heterogeneity The final dataset structure is characterised by many types of heterogeneity. Here we illustrate those caused by (1) the dynamic nature of the analytical program, (2) the varying sampling frequency and con- sequently the varying length of the sampling gaps and (3) the variability of the analytical results around the LOD. Inspecting the timeseries of individual pesticides at various waterworks shows that it is not uncommon to have concentrations varying around the LOD. This includes frequent changes between detects (concen- trations ≥LOD) and non-detects (concentrations 200 times within the study period (2002–2019) with the most common being nine times (7.9% of the waterworks; Fig. 5A). Of all waterworks, 4.6% had only one sampling event in that period. In the past five years, there were 2405 water- works with data, of which 7.6% had only one sampling event. The most common sampling frequency was 3 times (at 19.9% of the waterworks; Fig. 5B). Because of the variation in sampling frequency, the data gaps also vary. Figure 5C and D shows the mean sampling gap (the number of days between sampling events) for 2002– 2019 and 2015–2019, respectively. 3.4 Status assessment and exposure estimation The status assessment is presented in four qualitative groups based on the determined pesticide concentra- tions as follows: (1) never detected (0.1 µg/l) and (4) unknown exposure due to no data. These general classes of exposure were selected because of the structural heterogeneity of the final data- set (Section 3.3.2). Our methodology is in line with the deterministic approaches for estimating exposure, which usually include worst-case assumptions and result in a conservative estimate of exposure (Ferrier et al. 2002). Fig. 3 Proportion of water- works with ‘unstable’ detec- tions of pesticides during the study period (2002–2019). ‘Unstable’ is defined here as having more than one fluc- tuation around the LOD (i.e. multiple changes from 0.1 µg/l) in the study period and 7% in the past five  years. The rest of the water- works (71–76%) had never detected and quantified a pesticide (LOQ = 0.03  µg/l) based on the final data- set. Figure 7A and B provide a spatial visualisation of waterworks status. The status of WSAs (Status 3) resembles that for waterworks with minor differences in the percentages (Table SM-4 in Supplementary File S1). Twelve percent of the WSAs had at least one waterworks that exceeded the DWQS in 2002–2019, dropping to less than 8% when only the past five years were considered. The difference between the two periods can also be observed in Fig. 7C and D. Status 3 was used to estimate the population exposure, based on the percent of households located in each WSA. Fig. 6 Pesticide status at waterworks and estimated exposure at the household level. Note: The classes ‘never detected’ and ‘never exposed’ refer to pesticide concentrations below the limit of quantification (LOQ = 0.03 µg/l). For absolute numbers, see Table SM-4 in Supplementary File S1. 76 71 17 20 7 9 54 42 30 37 11 19 5 2 Waterworks 2002–2019 2015–2019 Never detected Detected ≤0.1 µg/l Exceeding 0.1 µg/l 0% 20% 40% 60% 80% 100% Never exposed Exposed ≤0.1 µg/l Exposed >0.1 µg/l Unknown Households 2002–2019 2015–2019 0% 20% 40% 60% 80% 100% https://doi.org/10.34194/geusb.v47.6090� www.geusbulletin.org� Voutchkova et al. 2021: GEUS Bulletin 47. 6090. https://doi.org/10.34194/geusb.v47.6090 10 of 16 www.geusbul let in.org Based on the final dataset, about half of all Danish households (54%) were never exposed to the pesticides analysed between 2015 and 2019 (Fig. 6). This was lower (42%) for the period 2002–2019. However, fewer house- holds were exposed to low pesticide concentrations (0.03–0.1 µg/l) in the past five years (30%) compared to the entire study period. Similarly, the households poten- tially exposed to pesticides exceeding 0.1 µg/l were 19% for 2002–2019 and 11% for 2015–2019. For status with respect to ‘Pesticides-Total’ (0.5 µg/l), see Supplemen- tary File S1. We could not estimate the exposure for 2% (2002– 2019) and 5% (2015–2019) of Danish households due to lack of data – 599 waterworks had no data after 2015, hence the higher percent for 2015–2019. Some of these waterworks may have closed, but it is also possible that there is a delay in reporting to the Dan- ish database or that the analyses were excluded in the quality assurance procedures or the other data-prepa- ration steps. 5 Discussion 5.1 Possible trends During the period 2002–2019, Danish drinking water has been tested for 449 individual substances defined here as pesticides, resulting in more than 970 000 individual analyses. Of which, less than 5000 (0.5%) contained a quantified pesticide. BAM has the most spatially and temporally complete data coverage. A tenth of all Danish households connected to public water supplies were potentially exposed to pesticides from drinking water at concentrations >0.1 µg/l and about 30% to concentrations in the range 0.03–0.10 µg/l during the past five years (2015–2019). Pesticides exposure from drinking water (and the waterworks status) is lower (better) in the period 2015–2019 when compared to the entire 18-year study period. The reduction in those exposed in the past five years could be due to improved groundwater quality. Also, some of the waterworks where concentrations exceeded the DWQS may have (1) closed before 2015, Fig. 7. Pesticide status at waterworks and water supply area (WSA). ‘Never detected’ refers to all analyses or waterworks where pesticides are measured below the quantification limit (QOL = 0.03 µg/l). NA: no data. Status at waterworks 2002–2019 Never detected Detected ≤0.1 µg/l * Exceeded 0.1 µg/l * N Status at waterworks 2015–2019 Never detected Detected ≤0.1 µg/l * Exceeded 0.1 µg/l * N Status at WSA 2002–2019 Always 0.1 µg/L (Brüsch et al. 2004). Additionally, 22% of these wells violated the DWQS for nitrate (50 mg/L) and 48% violated the DWQS for microbiological contamination (Brüsch et al. 2004). In a nationwide Danish study, it was estimated that up to 30% of private wells violated the drinking-water stan- dard for nitrate in the period 1978–2012 (Schullehner et al. 2017). Therefore, we are likely underestimating the percent of households where pesticide levels exceed the DWQS. Our status assessment and exposure estimation were performed for four general qualitative classes because of limitations in the pesticide dataset. Care must be taken not to over interpret results. For the status assessment at waterworks, one analysis for an individual pesticide exceeding the DWQS within the selected periods would classify the waterworks as ‘exceeding DWQS’. Similarly, if one waterworks within a WSA had at least one exceed- ance in the respective period, all households in that area would be classified as exposed to concentrations exceed- ing the DWQS. Therefore, we are likely overestimating some of the exposure for WSAs with multiple water- works. Our assumptions correspond to a worst-case sce- nario which is in line with deterministic approaches for estimating exposure to pesticides (Ferrier et al. 2002). To quantify this potential overestimation, we catego- rise the households potentially exposed to >0.1 µg/l in three groups based on the percentage of waterworks that exceed the DWQS in the WSA (Fig. 8; Table SM-5 in Supplementary File S1). In the first group, where 100% of the waterworks in a WSA exceeded DWQS, the percent of households potentially exposed to pesticides >0.1 µg/l is 5.1% (2002–2019) and 4.5% (2015–2019). In the second group, where ≥50% of the waterworks in a WSA exceeded DWQS, these percentages are 8.7% (2002–2019) and 7.1% (2015–2019). The third group is the most uncertain group, where <50% of the waterworks in a WSA exceeded the DWQS. This corresponds to 10.2% (2002–2019) and 4.2% (2015–2019) of households. In most of these WSAs, however, the other waterworks contained detected pes- ticides (0.03–0.10 µg/l). In our study, we were not able to calculate exposure with higher spatial resolution without knowing more about the distribution patterns in the sup- ply system within the WSA. https://doi.org/10.34194/geusb.v47.6090� www.geusbulletin.org� Voutchkova et al. 2021: GEUS Bulletin 47. 6090. https://doi.org/10.34194/geusb.v47.6090 14 of 16 www.geusbul let in.org Our assessment provides a first overview on the state of Danish treated drinking water and the potential exposure at household level based on the selected qual- itative exposure classes. More detailed assessments on the level of individual consumers can be achieved by epidemiological studies exploring the potential asso- ciation between exposure to pesticides from drinking water and various health outcomes. 6 Conclusions and perspectives Our study provides a national assessment of pesticide status of Danish drinking water supplied by public water- works and an estimation of household exposure to pes- ticides from drinking water. The national groundwater assessment and the Danish EPA mass screening have already provided evidence that pesticide pollution of Dan- ish groundwater is widespread and not only an issue of local importance. Our results compliment these findings and quantify the status based on the treated drinking water supplied to the Danish population. We found that: 1. 0.5% (n = 4925) of the individual analyses of treated drinking water contained a quantified pesticide (≥0.03 µg/l) and of those 16% (n = 793) exceeded the DWQS. 2. BAM had the most complete spatiotemporal cov- erage, and DPC had the highest measured concen- tration. Together with DMS, these three pesticides had the highest number of DWQS exceedances in the study period. 3. 9% (2002–2019) and 7% (2015–2019) of the water- works had an exceedance of the DWQS (>0.1 µg/l). In addition, 20% and 17% of waterworks contained pesticides with lower concentrations (0.03–0.10 µg/l) for the two respective periods. These results compare well with the raw groundwater status reported by Thorling et al. (2019). 4. 19% (2002–2019) and 11% (2015–2019) of Danish households were potentially exposed to pesticides >0.1 µg/l. In addition, 37 and 30% were potentially exposed to lower concentrations (0.03–0.10 µg/l) for the two periods, respectively. 5. One-third of the pesticides with established WHO guideline values (protective against health effects from a lifetime exposure) were found, but all were lower than the guideline values. We demonstrate that the central registration of all chemical analyses of Danish drinking water in the Jupi- ter database allows us to assess the current and past spatiotemporal status of Danish drinking water. While the open access to Jupiter is a major advantage, care should be taken as the data structure is highly hetero- geneous regarding sampling frequency and pesticides analysed. A strength of our study is that we imple- mented and documented extensive data pre-process- ing procedures in detail, allowing reproducibility and further informed use of the dataset. We have also included a comprehensive account of different meth- odological limitations, which are important for future epidemiological studies. Acknowledgements We thank Ingelise M. Balling and Lærke Thorling for providing comments and suggestions on the data quality control and filtering procedure (QC1). We are also thankful to Lærke Thorling and Anders R. Johnsen, whose thoughtful comments on an earlier version of this manuscript helped us clarify and sharpen some of the findings. We are grateful to the two anonymous reviewers for providing constructive and detailed reviews, which pushed us to perform additional uncertainty analysis and to restructure the manuscript for clarity. Additional information Funding statement This work was funded under the project ‘Geographical clustering of leukaemia and multiple myeloma and association with pesticides in ground water’ by the Karen Elise Jensen Fond (2017–2021). Author contributions DV contributed to the conceptualisation, data curation, formal analysis, methodology, visualisation and writing (original draft). JS contributed to the conceptualisation, data curation, methodology, validation and 2002–2019 5.1 3.6 10.2 4.5 2.6 4.22015–2019 19% 11% 0% 5% 10% 15% Households (%) potentially exposed to pesticides >0.1 µg/l by WSA type 100% of the waterworks had at least once pesticide exceeded 0.1 µg/l ≥50% of the waterworks had at least once pesticide exceeded 0.1 µg/l <50% of the waterworks had at least once pesticide exceeded 0.1 µg/l Water supply areas (WSAs), where: Fig. 8 Uncertainty in the estimate of households (%) exposed to pes- ticides exceeding the DWQS (0.1 µg/l). https://doi.org/10.34194/geusb.v47.6090� www.geusbulletin.org� Voutchkova et al. 2021: GEUS Bulletin 47. 6090. https://doi.org/10.34194/geusb.v47.6090 15 of 16 www.geusbul let in.org writing (review and editing). CS contributed to the conceptualisation, validation and writing (review and editing). KW contributed to the conceptualisation, validation and writing (review and editing). 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A: Period (in years) where data are available for individual pesticides, c Fig. 5 Frequency of sampling events by sampling date in A: 2002-2019 and B: 2015-2019. The cumulativ Fig. 6 Pesticide status at waterworks and estimated exposure at the household level. Note: The class Fig. 7. Pesticide status at waterworks and water supply area (WSA). ‘Never detected’ refers to all a Fig. 8 Uncertainty in the estimate of households (%) exposed to pesticides exceeding the DWQS (0.1 µ Tables Table 1 Results from the pesticide status of untreated groundwater (Thorling et al. 2019) Table 2 Drinking water guideline values for pesticides (World Health Organization 2017) and occurren