Voutchkova et al. 2025: GEUS Bulletin 59. 8390. https://doi.org/10.34194/mg1sjj69 1 of 15 RESEARCH ARTICLE The limitations of nitrate-sensitive zoning for groundwater protection from pesticides in Denmark Denitza D. Voutchkova*1 , Ingelise Møller2 , Lærke Thorling1 , Anders R. Johnsen3 1Department of Geochemistry, Geological Survey of Denmark and Greenland (GEUS), Aarhus, Denmark; 2Department of Near Surface Land and Marine Geology, Geological Survey of Denmark and Greenland (GEUS), Aarhus, Denmark; 3Department of Geochemistry, Geological Survey of Denmark and Greenland (GEUS), Copenhagen, Denmark Abstract Pesticides and degradation products are a major challenge for groundwater management in Europe, and in Denmark where drinking water relies entirely on groundwater. To protect drink- ing water resources, local Danish authorities must take groundwater-protective measures in areas designated as sensitive to pollution; however, official zonation for pesticides is lacking. Nitrate-sen- sitive groundwater abstraction areas have been used instead. The goal of our study was to test the appropriateness of this groundwater protection strategy. We used Køge municipality (Denmark) as a focus area and tested how our findings upscale to the national level. The data for Køge munic- ipality included 1070 individual groundwater samples, analysed for at least one of 366 pesticide compounds during the period 2012–2022, which were aggregated at the well-screen level by the median. Four pesticide compounds (2,6-dichlorobenzamide (BAM), desphenylchloridazon (DPC), N,N-dimethylsulphamide (DMS), 1,2,4-triazole) and three pesticide groups (phenoxyalcanoic acids, triazines and dimethachlor and its metabolites) were found with the highest detection frequency in the study area. We found that groundwater pollution with pesticide compounds was not limited to nitrate-sensitive areas in Køge municipality or in Denmark as a whole. Therefore, nitrate-sensitive areas can only be used partially for identifying pesticide-sensitive groundwater abstraction areas. The management implication is that placing protective measures only within nitrate-sensitive areas would be insufficient to fully address the risk of future groundwater pesticide pollution. We identi- fied knowledge gaps and discussed a potential way forward with a more integrated management of groundwater protection in Denmark. *Correspondence: dv@geus.dk Received: 06 Jan 2025 Revised: 19 May 2025 Accepted: 16 Jun 2025 Published: 28 Aug 2025 Keywords: pesticides, nitrate-sensitive areas, pesticide sensitive areas, groundwater vulnerability, groundwater protection Abbreviations: BAM: 2,6-dichlorobenzamide DMS: desphenylchloridazon DMSA: dimethylsulfamic acid DPC: desphenylchloridazon EU: European Union GKO: national groundwater mapping LOD: limit of detection m.b.t: metres below terrain NSA: Nitrate-sensitive area OW: other well PSA: pesticide-sensitive area PW: pollution well RBMP: River Basement Management Plan WW: waterworks well GEUS Bulletin (eISSN: 2597-2154) 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 (DTU, Denmark) Reviewed by: Niels Peter Arildskov (WATSONC, Denmark) and one anonymous reviewer Funding: See page 14 Competing interests: See page 14 Additional files: See page 14 1. Introduction Groundwater is a critical resource for public water supply in the European Union (EU), where it accounts for 65% of the total water abstracted for pub- lic water supply (European Environment Agency 2023). Pollution with pes- ticide residues is a main cause of failure to achieve good chemical status for groundwater in the EU (see ‘Regulatory context’ section), only second to nitrate (European Environment Agency 2018). Pesticide substances are a major groundwater management challenge in Denmark as well. According to the latest status report (Thorling et al. 2024), pesticides or their degradation products were detected in 67.6% of the well screens in the national monitoring network (GRUMO, n = 1049 for 2020– 2022) and 40.5% of the public waterworks well (WW) screens used for drink- ing water production (n = 6386 for 2018–2022; Thorling et al. 2024). More importantly, the groundwater quality standard of 0.1 µg/L was exceeded at 33.0% of the GRUMO wells and 10.8% of the WW screens used for drinking water (Thorling et al. 2024). This has major implications for the Danish drink- ing water supply, which relies entirely on groundwater. Most of it undergoes only simple treatment (aeration and sand filtration), so the overall pesticide status of the treated drinking water is comparable to that of the untreated groundwater (Voutchkova et al. 2021). https://doi.org/10.34194/mg1sjj69 https://orcid.org/0000-0003-2840-072X https://orcid.org/0000-0002-1154-3700 https://orcid.org/0000-0001-9067-0005 https://orcid.org/0000-0002-9245-475X mailto:dv@geus.dk https://creativecommons.org/licenses/by/4.0/deed.ast Voutchkova et al. 2025: GEUS Bulletin 59. 8390. https://doi.org/10.34194/mg1sjj69 2 of 15 GEUSBULLETIN.ORG Danish environmental policy is based on prevention and source protection (Pedersen et al. 2016), which is aligned with the EU principles that groundwater quality should be protected by restricting polluting activities in sensitive recharge areas (European Environment Agency 2023). In Denmark, the local authorities (98 municipali- ties) must take measures to protect the groundwater in areas sensitive to pollution, including pesticides (Peder- sen et al. 2016). However, national zonation guidelines for pesticide sensitivity are lacking. Consequently, some Dan- ish municipalities have used nitrate-sensitive areas (NSAs, see Section 2) as proxies for groundwater abstraction areas sensitive to pesticide leaching. The rationale was that NSAs have high infiltration rates, which are assumed to also increase the risk of pesticide leaching (Miljø- styrelsen 2000). Furthermore, the main source of diffuse groundwater pollution with both nitrate and pesticides is agriculture. There are, however, major differences in their leaching. The most common pesticide compounds in Danish groundwater were the persistent transformation products N,N-dimethylsulphamide (DMS), desphenyl- chloridazon (DPC), 4-bis-amido-3,5,6-trichlorobenzene- sulphonate (R471811) and 2,6-dichlorobenzamide (BAM; Thorling et al. 2024). In contrast to nitrate, these com- pounds do not degrade at the redox front. Furthermore, the parent compounds of DMS (tolylfluanide and dichlo- fluanide), and R471811 (chlorothalonil) have also been used as biocides in paint and building materials in urban areas. Therefore, we posit that because of differences in the pollution source and geochemical behaviour, the assumption that NSAs can be used for groundwater pro- tection from pesticides is problematic. The aim of this research was therefore to test whether the current groundwater management strategy – using NSAs as proxies for abstraction areas sensitive to pes- ticides – is appropriate for protecting Danish drinking water resources from pesticides. Our working hypothe- sis was that not only NSAs, but also areas outside an NSA can be sensitive to pesticides. We tested our hypoth- esis for a municipality in Denmark with exceptionally high data density and quality, and detailed mapping of nitrate vulnerability and NSAs. To determine whether our findings upscale, we performed the analysis at the national level as well. Finally, we identified knowledge gaps and discussed different strategies for groundwater protection from pesticides, which is urgently needed in Denmark and potentially in other EU countries where drinking water supply depends on groundwater. 2. Regulatory context 2.1. Legal definition and threshold for pesticides The EU Groundwater Directive (European Commission 2006) defines ‘pesticides’ as active substances in plant protection products and biocidal products, as well as their metabolites, degradation products and reaction products. In many EU countries, there is furthermore a distinction between relevant and non-relevant pes- ticide metabolites in drinking water (Council of the European Union 2020), and the EU drinking water thresholds apply only to the former (Laabs et al. 2015). A pesticide metabolite is classified as relevant “if there is reason to consider that it has intrinsic properties comparable to those of the parent substance in terms of its pesticide target activity or that either itself or its transformation products generate a health risk for consumers” (Council of the European Union 2020). The distinction between relevant and non-relevant metab- olites is applied to groundwater by many EU member states. However, in Denmark and in this study, there is no such distinction – the EU threshold applies to all pesticide compounds in groundwater. Laabs et al. (2015) stated that Denmark holds a unique position in EU in this regard, but this is in line with the precaution- ary principle. 2.2. Pesticides in EU groundwaters Integrated management at the river basin level is key to ensuring the sustainability of groundwater resources in the EU (European Environment Agency 2023). The groundwater chemical status in EU is assessed as part of the River Basement Management Plans (RBMPs), which are the key tool for implementing the Water Framework Directive (European Commission 2000). Groundwater fails to achieve good chemical status with respect to pes- ticides, if the EU standard of 0.1 µg/L for individual pes- ticide compounds or 0.5 µg/L for the sum of pesticide compounds is exceeded (European Commission 2006). Pesticides are the second-most common reason for failing good chemical status for groundwater. Accord- ing to the 2nd RBMP (2015–2021), 6.5% by area of the European groundwater bodies failed to achieve good status due to pesticides; moreover, 1.4% by area had an upward concentration trend (European Environment Agency 2018). The latest assessment (3rd RBMP) showed that 7.5% of the 2050 Danish groundwater bodies failed to achieve good status because of pesticides, equivalent to 17% by volume (Nilsson et al. 2021). These figures show that groundwater protection from pesticides is a major challenge not only in Denmark, but also in the EU. 2.3. Nitrate-sensitive areas The Danish groundwater abstraction areas designated as NSAs, are management areas with a particular sensi- tivity to nitrate pollution. They are designated under the Danish Water Supply Act (Miljøministeriet 2022), made public with a Ministerial Order (Miljøministeriet 2023) https://doi.org/10.34194/mg1sjj69 https://geusbulletin.org/ Voutchkova et al. 2025: GEUS Bulletin 59. 8390. https://doi.org/10.34194/mg1sjj69 3 of 15 GEUSBULLETIN.ORG and available online (Miljøstyrelsen 2023a). NSAs are not the nitrate-vulnerable zones from the Nitrates Direc- tive (Council of the European Union 1992). The nitrate- vulnerable zones are defined as areas of land that drain into polluted waters or waters at risk of pollution and which contribute to nitrate pollution (European Com- mission 2023), while NSAs focus only on groundwater in the abstraction areas. A similar distinction between NSAs and nitrate-vulnerable zones was made in the UK (Cook 1999; Osborn & Cook 1997). The difference is, however, that Denmark is exempt from designating nitrate-vulnerable zones, because it has established and applies nationwide action programmes, according to Article 3.5 of the Nitrates Directive (Council of the Euro- pean Union 1992). The main criteria for NSA mapping are the aquifer vulnerability to nitrate and the groundwater recharge (Fig. S1; Miljøstyrelsen 2000, 2023b; Naturstyrelsen 2014). NSAs are situated within areas classified as particularly valuable for drinking water abstraction, or within the catchment areas of the WW fields (Miljøstyrelsen 2023b). The Danish nitrate-vulnerability mapping is based on the characteristics of the aquifer material and the overlaying layers, as well as the groundwater quality (Table S1; Miljø- styrelsen 2023b). Figure 1 illustrates how nitrate vulnera- bility of the aquifer and the NSA zonation relate. Around 17% of Denmark (7466 km2) is designated as an NSA. 2.4. Pesticide-sensitive areas and pesticide vulnerability We define pesticide-sensitive areas (PSAs) as those where leaching of pesticides to the groundwater has been observed regardless of the time of application. The pollution with pesticides may result from their applica- tion within the entire groundwater catchment area, and not only close to the well head. The historical leaching (their detection in groundwater) is therefore used as a proxy for the areas with inherent pesticide sensitivity. The concentration levels in groundwater, on the other hand, reflect the risk management (e.g. the regulations on dose or time of application) and do not necessarily reflect the inherent pesticide sensitivity. Here we do not assess the pesticide sensitivity of large nature areas, where pesticides have not been applied. Another inher- ent limitation is that we cannot test for overlap between NSAs and PSAs where the groundwater is recharged pre- 1960s, when use of the pesticides in question started. Though the pesticide approval procedure has been improved over the years, it still cannot fully prevent leaching of pesticide compounds from approved pesti- cides, either because the leached degradation products were not identified during the pesticide approval, or because the approval models did not adequately cover real-life conditions. Some recent examples are leach- ing of DMS and DMSA from application of cyazofamid (Badawi et al. 2024), leaching of TFA from tri-fluorinated pesticides (Albers & Sültenfuss 2024; Johnsen et al. 2024) and leaching of propyzamide in very high concentra- tions (Badawi et al. 2025). The official zonation guidelines (Miljøstyrelsen 2023b) do not include national PSA designation, but provide a reference (Naturstyrelsen 2015), building on the concept by Nygaard et al. (2005). This concept uses the soil clay, silt and humus content to evaluate if areas Fig. 1 Illustration of the principles for designation of a groundwater abstraction area as a nitrate-sensitive area (NSA) in Denmark (including aquifer vulnerability and redox state in the aquifer, thickness of reduced clay layer and positive recharge) and potential leaching of persistent pesticide com- pounds to the aquifer. Purple dotted lines indicates potential leaching pathways to the aquifer. GWT: groundwater table (blue dashed lines). FRI: first redox interface (red dashed lines). reduced aquifer GWT Low nitrate vulnerability >1 5 m reduced aquifer Medium nitrate vulnerability 5– 15 m oxic aquifer High nitrate vulnerability FRI <5 m >0 mm/y >0 mm/y >0 mm/y reduced clay reduced clay reduced clay pesticides pesticides pesticides (b) nitrate sensitive area (NSA)*(a) outside NSA * Only if within areas particularly valuable for drinking water abstraction OR within the catchment areas of waterworks wells, otherwise it is outside NSA. https://doi.org/10.34194/mg1sjj69 https://geusbulletin.org/ Voutchkova et al. 2025: GEUS Bulletin 59. 8390. https://doi.org/10.34194/mg1sjj69 4 of 15 GEUSBULLETIN.ORG are more sensitive than the preconditions for approving pesticides (Nygaard et al. 2005). The concept cannot be applied to soils with >10% clay, wetlands and non-agri- cultural land (Naturstyrelsen 2015). The zonation, there- fore, applied to 41.2% of the Danish territory, most of which was not sensitive to pesticide leaching. Just 0.2% of the land was classified as particularly sensitive, 0.6% as potentially sensitive and 1.2% had a low sensitivity to pesticide leaching (Naturstyrelsen 2015). This PSA des- ignation had very little practical relevance for the local authorities. Pedersen et al. (2016) stated that there are no methods for PSA zonation suitable for public admin- istration purposes. According to them, vulnerability to pesticides, should be based on identifying areas with large groundwater recharge, and thus higher risk of pollution, and frequently such areas are also those with sandy soils and sediments, which are also more vulner- able to nitrate leaching (Pedersen et al. 2016). 3. Methods 3.1. Study site Køge municipality (255 km2) is situated in the eastern part of Denmark on the island of Sjælland (Fig. 2a). Agriculture is the dominant land use (49.4% intensive and 5.4% extensive agriculture); forests cover 20%, while the built-up areas cover 8.4%, and the indus- try and technological activities take up less than 2% (Levin 2019). Figure 2b shows the conceptual hydrostratigraphic model of the study area (NW–SE cross-section), based on Stisen et al. (2020). The Pre-Quaternary carbonate deposits consist of greensand, bryozoan limestone and chalk from Neogene, Maastrichtian and Campanian; they are depicted collectively as a ‘carbonate aquifer’ in the hydrostratigraphic model of the area (Fig. 2b; Stisen et al. 2020). The depth to the carbonate aquifer is on average 26 ± 19 m (±1 SD) below terrain (m.b.t) and in the range 0–103 m.b.t (Fig. S2a). The overlying Quater- nary succession (Fig. 2) consists primarily of clayey till and sandy meltwater deposits (COWI 2005; Jacobsen 2022), formed during the last three glaciations in the Pleistocene and impacted by erosion, deposition and deformation. Glaciotectonic deformations are expected where the Quaternary glacial sequence is thicker, while in the areas with thinner deposits, the clayey till is poten- tially fractured (COWI 2005; Jacobsen 2022). Most of the glacial landscape is characterised by till plains with hummocky terrain in places. Marginal moraines, erosion valleys, eskers and kames are found as well. There are two buried valleys, eroded into both the glacial sequence and the carbonate aquifer, which were consequently filled with sandy and clayey gla- cial deposits (Sandersen & Jørgensen 2016, 2017). The glacial deposit thickness varies from c. 10 m to >100 m, and the thickness of the accumulated clay overlying the carbonate aquifer is on an average 23 ± 19 m and varies from 0 to 102 m (Fig. S2b). The nitrate-vulnerability assessment and the NSA mapping (Rambøll 2018) were extended with priority action areas which were: (1) within areas of particular drinking water interest and with a high or medium vul- nerability, (2) outside forests or natural areas and (3) within the 50-year-catchment zones of the WWs (Køge Kommune 2022). This extended version is used here in this study (Fig. 2). Within the NSA in the area, the carbon- ate aquifer depth was on average 15 ± 4 m.b.t. (range 1.3–42.5 m.b.t), while the accumulated clay overlying the carbonate aquifer was on average 13 ± 4 m thick (range 0–34 m). The land use inside and outside NSAs can be found in Supplementary Table S2. 3.2. Data 3.2.1. Pesticide data The data were downloaded from the nationwide open-access well database, Jupiter (Hansen & Pjeturs- son 2011) on 10 May 2022. Accredited labs upload all chemical analyses of drinking water and groundwater to Jupiter. The data extraction was limited geographically to Køge municipality, and temporally to samples from the period 2012–2022, and covered 626 different pes- ticide compounds. The raw data were quality assured (see Supplementary Text 1) and aggregated at the well- screen level. Only the compounds that were analysed at more than one well screen were retained in the dataset. The cleaned dataset included data from 1070 individual samples analysed for at least one of 366 compounds, representing 452 well screens in 436 wells (some wells have multiple well screens). We focused our analyses on four pesticide com- pounds and three groups of pesticide compounds, which had the highest detection frequency in the area and were, therefore, of high importance (Table 1). BAM is a transformation product from the herbicides dichlobenil and chlorthiamide, used in orchards and on paved areas, and from the agricultural fungicide fluopi- colide. DPC is a transformation product from the agri- cultural herbicide chloridazon. DMS is a transformation product from the fungicides tolylfluanide and dichloflu- anide, used in orchards and production of berries, but DMS also leaches from the biocide use of these parent compounds in outdoor paint and wood protection in urban areas (Albers et al. 2023). DMS is furthermore a degradation product from the agricultural fungi- cide cyazofamid (Badawi et al. 2024). 1,2,4-triazole is a transformation product from a range of triazole-fun- gicides used in agriculture and as biocides in outdoor https://doi.org/10.34194/mg1sjj69 https://geusbulletin.org/ Voutchkova et al. 2025: GEUS Bulletin 59. 8390. https://doi.org/10.34194/mg1sjj69 5 of 15 GEUSBULLETIN.ORG paint. 1,2,4-triazole may furthermore be used as a nitrification inhibitor, though this usage is very limited in Denmark. The three compound groups (Table  1) were represented by their sum in each sample. The  groups were the agricultural phenoxyalcanoic acids plus their transformation products but exclud- ing the chlorophenols, as they may have other ori- gins, the triazine herbicides and their transformation Legend (a) (b) Sand Clay m –50 –40 –30 –20 –10 0 10 20 30 40 50 60 70 80 Carbonate aquifer Pre-Quaternary clay Quaternary sequence NW SE 0 2 4 6 8 10km Sjælland Køge Municipality Køge Jylland 100 km500 Denmark 75–80 70–75 65–70 60–65 55–60 50–55 45–50 40–45 35–40 30–35 25–30 20–25 0 2 4 km K øg e B ug t (m.a.s.l.) Elevation 5–10 10–15 15–20 Køge Municipality NSA Other wells Waterworks wells Pollution wells 12 14 16 18 20 22 24 26 Fig. 2 Overview of the Køge municipality study site. a: Location of Køge municipality in Denmark. The groundwater abstraction areas officially des- ignated as nitrate-sensitive areas (NSAs) and well locations and types are also shown. Elevation in metres above sea level (m.a.s.l). b: Conceptual hydrostratigraphic model for the study area. https://doi.org/10.34194/mg1sjj69 https://geusbulletin.org/ Voutchkova et al. 2025: GEUS Bulletin 59. 8390. https://doi.org/10.34194/mg1sjj69 6 of 15 GEUSBULLETIN.ORG products that are mostly of agricultural origin and the agricultural herbicide dimethachlor and its transfor- mation products. Before aggregation at the well-screen level, the values below the limit of detection (LOD) were sub- stituted with 0 µg/L. The aggregation was then based on the median concentration for the period 2012– 2021. For the three groups, first a sum at a sample level was calculated, after which the median was used to aggregate at the well-screen level. In addi- tion, for each well screen a maximum concentration based on all 366 compounds (max366) was calculated. This was done to ensure that all well screens with detections were identified and included in the anal- yses even when the seven focus parameters were all 0.1 μg/L) Well-screens: Legend Well- screen Carbonate aquifer Quaternary sand (n = 40) Other In (n = 38) Out (n = 67) Waterworks wells (n = 105) Pollution wells (n = 307) In (n = 171) Out (n = 136) In (17) Out (23) 0 1 2 km Pollution wells 0.1 Waterworks wells Other wells NSA Køge 100 80 60 40 20 0 D ep th b el ow g ro un d le ve l ( m ) K øg e B ug t 0.1 0.1 (a) (b) Fig. 3 Pesticide status of well screens in Køge municipality. a: shown on a map of the municipality along with groundwater abstraction areas officially designated as nitrate-sensitive area (NSAs). LOD: limit of detection. b: 1D depth profiles of the well screens, ordered by depth to screen top. In: inside NSA, out: outside NSA. n: number of well screens. The entire well-screen length is colour shaded according to the pesticides categories (dark grey, blue, red, as in the legend). Some well screens do not reach the carbonate aquifer, in this case there is no colour (white shading). https://doi.org/10.34194/mg1sjj69 https://geusbulletin.org/ Voutchkova et al. 2025: GEUS Bulletin 59. 8390. https://doi.org/10.34194/mg1sjj69 10 of 15 GEUSBULLETIN.ORG can delay the transport of pesticides. The accumulated clay layer in NSAs is thinner, which could result in a quicker pesticide transport to the carbonate aquifer, if inside the recharge zone. There was also a difference in the land use (Supplementary Table S2), where the intensive agriculture was 56.2% inside NSAs and 46.4 % outside NSAs. However, it is not possible to attribute these differences to land-use differences based solely on the data presented here. There could also be other nuances in the hydrogeology. For example, the hetero- geneity of glacial deposits is not necessarily reflected in the groundwater models. The usual modelling assump- tion is that clayey glacial deposits can be represented as homogeneous clay layers in the model, when they could be a mixture of clayey, sandy and silty sediments (in different proportions). Moreover, the model resolu- tion is too coarse, and as most operational models are layered (not voxel models), the small-scale heteroge- neities in the sediment characteristics cannot be cap- tured. The differences in the distributions for each of the focus pesticides or pesticide groups can be seen in Supplementary Figs S4 and S5. However, formal statis- tical tests were not performed due to data limitations (small sub-sets when considering well-screen type, with a high proportion of censored data). 4.2. Significance First, the significance of our findings from Køge munic- ipality is discussed with respect to the local, regional, national and international scales. Then, in Section 4.3., 0.01 0.1 1 10 100 0.01 0.1 1 10 100 0.01 0.1 1 10 100 0.00 0.25 0.50 0.75 1.00 Pesticides (µg/L) Fr ac tio n of w el l− sc re en s Location inside NSA outside NSA (a) Køge Municipality 0. 00 1 0. 01 0. 1 1 10 10 0 10 00 10 00 0 1e +0 5 0. 00 1 0. 01 0. 1 1 10 10 0 10 00 10 00 0 1e +0 5 0. 00 1 0. 01 0. 1 1 10 10 0 10 00 10 00 0 1e +0 5 0.4 0.6 0.8 1.0 Pesticides (µg/L) Fr ac tio n of w el l− sc re en s Location inside NSA outside NSA (b) Denmark OW PW WW OW PW WW Fig. 4 Cumulative distribution of pesticide concentrations by well type inside and outside of a groundwater abstraction area officially designated as a nitrate-sensitive area (NSA). a: Køge municipality. b: Denmark (Thorling et al. in press). Note that in b, the y-axis does not start at 0 for visualisation purposes; x-axis is log10-transformed, so the values 100 days (Kerle et al. 1996; EFSA 2006, 2007) irrespective of redox con- ditions. For persistent pesticide compounds, there may only be a time-horizon – a lag from the application to their detection in aquifers (Fig. 1). There is limited knowledge on the sorption and deg- radation processes of pesticides in the aquifers, so to better map PSAs, it is necessary to fill this knowledge gap. The persistence and sorption data are usually from the topsoil, which are not relevant for groundwa- ter and where there is generally lower organic matter content and reactivity. The largest degradation poten- tial for pesticides is in the plough layer, where there is high biological activity and diversity, ensuring microbial degradation of a large proportion of the applied pesti- cides. The capability of the soil to adsorb pesticide com- pounds is determined by the soil organic matter, clay minerals and metal oxides (Pavlis et al. 2010). The poten- tial to leach to groundwater is also affected by the soil permeability, which is controlled to some extent by the organic matter, and also by soil texture, water fluctua- tion and water content (Pavlis et al. 2010). In soils with high potential for degradation, pesticide compounds could still escape through preferential flow paths like bio-pores and fractures. Those are largely unmapped but could explain how pesticides reach the carbonate aquifer in the Køge municipality (Fig. 3). A discussion on hydrostratigraphic heterogeneity and uncertainty and the scale at which different geochemical and hydro- geological processes can be resolved with respect to pesticides is also needed. The model resolution and type (layered or voxel) would affect the level of detail https://doi.org/10.34194/mg1sjj69 https://geusbulletin.org/ Voutchkova et al. 2025: GEUS Bulletin 59. 8390. https://doi.org/10.34194/mg1sjj69 13 of 15 GEUSBULLETIN.ORG to which the inherent sub-surface heterogeneity can be represented. In addition, the representation of geo- chemical processes would differ depending on the mod- elling scale and overall framework. 4.3.2. Integrated groundwater management There is a need for a nationwide discussion that also includes local groundwater managers, on the practical relevance of ‘one size fits all’ nationwide guidelines for PSA zonation. It may be more relevant to adopt a more integrated groundwater protection approach instead of focusing on one pollutant group at a time. However, while such a discussion is necessary, we also urgently need solutions. Local managers need to know where to place groundwater-protective measures to safeguard our drinking water resources from future pesticide pollution. Pedersen et al. (2016) proposed that mapping vul- nerability to pesticides should be based on identifying areas with large groundwater recharge, usually sandy soils and sediments, which are also more vulnerable to nitrate leaching (Pedersen et al. 2016). However, we demonstrated here that clayey areas can also be sensi- tive to pesticide pollution. Moreover, sandy (oxic) sed- iments favour the degradation and sorption of some pesticides. A recommendation needs to be specific to be practically relevant, thus it is necessary to define what is considered ‘large recharge’. Should groundwater-pro- tective measures be applied in the entire WW catch- ment or in the 100 year or 50 year catchment zones? For Køge municipality, the modelled 100 year catchment zones of WWs (Supplementary Fig. S3) covered 63% of the municipality and extended beyond its boundaries. It is unclear, if designating all that area as PSA would be the optimal solution, due to the lack of uncertainty assessment of the modelled 100 year catchment zones and their relevance when most of the frequently found pesticides have only been used in the past 60 years, but have the potential to persist beyond 100 years. The rel- evance must also be discussed with respect to pesticide retardation in the sub- surface and the target window for groundwater protection. The zonation efficacy should most probably also be discussed with respect to specific protective activities. For example, Malaguerra et al. (2012) found that on Sjælland (west Denmark), WWs located in urban areas were more vulnerable to BAM and phenoxyalcanoic acids contamination, while non-urban area wells were more often contaminated with bentazon (not included here). Urban areas are characterised by a different type of application patterns and source densities in compar- ison to predominantly agricultural land use. Thus, tar- geting only agricultural areas and different agricultural practices would most likely be insufficient as well. While there has not been an adequate nationwide guidance (a top-down initiative) on how to map PSAs or how to apply groundwater management measures locally to address the pesticide pollution, a bottom-up initiative for establishing ‘Groundwater parks’ (in Danish: Grundvandsparker) is gaining popularity. Groundwater Parks are designated areas aimed at groundwater pro- tection, crucial for Denmark’s supply of drinking water, but also having a more integrated function involving ecological restoration, afforestation, promoting organic farming, enhancing both nature conservation and cli- mate adaptation efforts (Danmarks Naturfrednings- forening 2021). Groundwater Parks should typically be established in sensitive groundwater abstraction and recharge areas. In a report for The Danish Water and Wastewater Association (DANVA), Refsgaard (2022) out- lined some principles for their designation, suggesting using 50 year catchment zones at the 95% confidence level. The ongoing project for establishing Groundwa- ter Parks in Aarhus municipality, however, focuses on 100 year catchment zones of public WWs (VPU & Aarhus Vand 2023). The plan is to establish three Groundwater Parks in the vicinity of Aarhus (c. 330 000 population), converting 4000 ha agricultural land to either nature or forest areas, which would protect 50% of the ground- water recharge areas of the public waterworks (VPU & Aarhus Vand 2023). The rest of the groundwater recharge is from urbanised areas, where other protec- tion measures are placed by Aarhus municipality. The aim of Groundwater Parks is also to reach other environ- mental goals for biodiversity and (re)establishment of natural areas (including afforestation and wild self-man- aged grassland; VPU & Aarhus Vand 2023). Such goals for afforestation are also placed at the national level (‘Skovplan’, establishing 250 000 ha of forest). Further, the new EU Nature Restoration Law set an overall target that restoration measures should be put in place for at least 20% of the EU’s land area and 20% of its sea areas by 2030; and by 2050, such measures should be in place for all ecosystems that need restoration (Directorate- General for Environment 2024). 5. Conclusions The lack of a generally accepted method for pesticide sensitivity mapping in a European context is a chal- lenge for local authorities when they need to imple- ment groundwater protective measures, illustrated here for Denmark. We showed that the NSAs do not cover all areas of groundwater currently polluted with pesticides. Pesticide pollution is widespread both inside and outside of NSAs, not only in Køge municipality, but throughout all of Denmark. We conclude that while NSAs are also PSAs, not all PSAs are NSAs. Placing protective measures within NSAs, as in some Danish https://doi.org/10.34194/mg1sjj69 https://geusbulletin.org/ Voutchkova et al. 2025: GEUS Bulletin 59. 8390. https://doi.org/10.34194/mg1sjj69 14 of 15 GEUSBULLETIN.ORG municipalities, will therefore be insufficient to address future pesticide pollution of drinking water. The prac- tical implementation of PSAs is impeded by knowledge gaps on groundwater-relevant physicochemical proper- ties of approved pesticide compounds. Considering the current lack of knowledge, we provide an example of a potential groundwater management alternative, focus- ing on more integrated approach to safeguarding this sole drinking water resource in Denmark. Acknowledgements This work was part of research project ‘Prioritization – investigation of the use of nitrate sensitive areas (NSA) for measures related to pesticides’, an initiative under the Partnership for Sustainable Water Supply, funded by Region Sjælland and the branch organisation Dan- ske Vandværker. The authors acknowledge the project workshop par- ticipants from DTU-Sustain (Danish Technical University, DTU), Region Sjælland, Køge municipality, the Danish EPA and Danske Vandværker, who provided valuable local knowledge, data and discussions. Thanks are due also to Niels Claes (Aarhus Vand) for critically reading the manuscript and providing an update on Groundwater Parks initiative. The authors also thank the two reviewers for their comments, which improved the manuscript. Additional information Funding statement This work was part of research project “Prioritization – investigation of the use of nitrate sensitive areas (NSA) for measures related to pesticides,” an initiative under the Partnership for Sustainable Water Supply, funded by the Region Sjælland and the branch-organization Danske Vandværker. Author contributions All authors contributed to the study conception and design. Data preparation and formal analysis were performed by DDV. IM, LT, and ARJ provided input on interpretation of results. The first draft of the manuscript was written by DV. IM, LT, and ARJ commented and revised the manuscript. All authors read and approved the final manuscript. Competing interests The authors have no relevant financial or non-financial interests to disclose. Additional files The following three supplementary files are available at https:doi. org/10.22008/FK2/YXMRLC: Supplementary File S1: A .docx file containing Tables S1–S6, Figs S1–S5, Supplementary Text 1: Data pre-processing and Supplementary Text 2: Redox architecture in Denmark. Supplementary File S2: Aggregated pesticide data set as a CSV file Supplementary File S3: National (Denmark) pesticide data set as a CSV file Data availability statement The two data sets described in this paper are supplied as supplemen- tary files S1 and S2. References Albers, C.N. & Sültenfuss, J. 2024: A 60-year increase in the ultra- short-chain PFAS trifluoroacetate and its suitability as a tracer for groundwater age. 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Introduction 2. Regulatory context 2.1. Legal definition and threshold for pesticides 2.2. Pesticides in EU groundwaters 2.3. Nitrate-sensitive areas 2.4. Pesticide-sensitive areas and pesticide vulnerability 3. Methods 3.1. Study site 3.2. Data 3.2.1. Pesticide data 3.2.2. Auxiliary data 3.3. Study design 3.3.1. Segregation by well type 3.3.2. Statistics and software 4. Results and discussion 4.1. Pesticides in groundwater in the Køge municipality 4.1.1. Groundwater status: detection and exceedence 4.1.2. Concentration distributions 4.2. Significance 4.2.1. Local scale 4.2.2. Regional scale 4.2.3. National scale 4.2.4. International scale 4.3. Future directions 4.3.1. Identified knowledge gaps 4.3.2. Integrated groundwater management 5. Conclusions Acknowledgements Additional information Funding statement Author contributions Competing interests Additional files Data availability statement References Figures Fig. 1 Illustration of the principles for designation of a groundwater abstraction area as a nitrat Fig. 2 Overview of the Køge municipality study site. a: Location of Køge municipality in Denmark. Th Fig. 3 Pesticide status of well screens in Køge municipality. a: shown on a map of the municipality Fig. 4 Cumulative distribution of pesticide concentrations by well type inside and outside of a gro Tables Table 1 Number and percent of well screens with at least one sample with detected or exceeding the Table 2 Summary statistics for length and depth to top of the different types of well screens in Køg Table 3 Number of well screens with at least one sample analysed for the seven pesticides and groups Table 4 Comparison of pesticide status in groundwater for Køge municipality and the entire country,