Lærke Thorling

This data set provides a comprehensive assessment of groundwater partial pressure CO2 (pCO2) across Denmark, spanning more than three decades (1988–2024). Using groundwater chemistry data from the national borehole database, Jupiter, pCO2 levels were calculated and compared using two distinct approaches: (1) geochemical modelling via PHREEQC; and (2) simplified carbonate equilibria using only pH and alkalinity. While the latter is widely used for its simplicity, the PHREEQC method offers more accurate estimates by accounting for ionic strength and complexation. Despite Denmark’s extensive public archive of groundwater chemistry, pCO2 values have remained largely unavailable to the scientific community due to data complexity. This data set addresses this data gap through rigorous cleaning and the flagging of potentially contaminated samples to ensure high data fidelity. The resulting long-term, large-scale overview is essential for elucidating the role of groundwater in the carbon cycle and its spatiotemporal dynamics. Furthermore, these data establish critical baseline conditions necessary for monitoring and protecting groundwater resources in the context of the future CO2 injection and storage in deep geological reservoirs.

Denitrification is the most important process for nitrate removal in groundwater. Although carbonate aquifers are important for drinking water supply, the denitrification process in these systems is less understood than for unconsolidated sedimentary aquifers. We reviewed studies (1) providing evidence for denitrification in carbonate aquifers, (2) discussing the specific location where it occurs, and (3) quantifying the denitrification rates. Literature sources were identified by systematic search of scientific databases and follow-up reference tracking. The key finding is that denitrification in carbonate aquifers is possible, supported by microbiologic and/or isotopic evidence for several locations. Pore-size and connectivity are major limiting factors for denitrification within the matrix. Denitrification in fissures/fractures would be limited by electron donor availability and unfavourable redox conditions. However, it has also been hypothesised that denitrification could occur in micro-anaerobic environments and biofilms. Denitrification rates for carbonate aquifers varied several orders of magnitude (0.01–36,792 mg N/L/y). High and reactive organic carbon from waste-water contamination and high groundwater temperatures are characteristic for some of the studies. Thus, the rates may not be transferable to cooler oligotrophic conditions. Future research is needed to fill identified knowledge gaps.

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 drinking 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-sensitive 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 municipality 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 identified knowledge gaps and discussed a potential way forward with a more integrated management of groundwater protection in Denmark.

Globally, nitrogen (N) pollution of the environment is a widespread consequence of intensive agriculture. Denmark has combated this challenge from mid-1980 by implementing effective national N regulation of agriculture. Further reduction is needed to meet legislative objectives, and more targeted farm-level regulation schemes are under development. Assessing the necessary farm-level reduction of agricultural N losses to the aquatic environment such as groundwater requires detailed knowledge of denitrification in the groundwater system. In this paper we demonstrate how determinations of the extent of the anoxic nitrate-reducing zone and associated denitrification rates can be done by combined use of national and local level multiscreen groundwater monitoring data representing different agro-hydrogeochemical conditions across Denmark. A new concept for evaluating groundwater denitrification both nationally and locally is presented. The new concept relies on the assessment of nitrate trends in oxic and anoxic nitrate-reducing groundwater using up to 35 years of groundwater monitoring data on nitrate content and groundwater age, dated with CFC or the ³H/³He method. The results from the anoxic nitrate-reducing zone indicate that the probability of denitrification increases with groundwater residence time. On average, the national level assessment shows that in the anoxic nitrate-reducing zone approximately 47% of the nitrate is being denitrified while in the local areas the assessments show variation between 14%–59%, and estimations of denitrification rates range from 0.008–0.032 µmol l⁻¹ day⁻¹. Results show that the variation in the thickness of the anoxic nitrate-reducing zone can be up to 16 m, indicating that the denitrification rates can vary from very fast (thin zone or sharp interface) to slow (thick zone) in a homogenous aquifer layer with the same magnitude of residence time. These results are important for groundwater modeling of nitrate transport from agricultural fields to streams and coastal areas through the groundwater system.

Three spatiotemporal data sets of drinking water hardness in Denmark (version 1) are presented here: (1) annual drinking water hardness at public waterworks (1905–2023); (2) annual drinking water hardness at their water supply areas (1978–2023) and (3) the latest drinking water hardness at the water supply areas (1980–2023). Raw data were extracted from the Jupiter database for groundwater and drinking water data in Denmark, and were quality-assured. Hardness was calculated after semi-automatic outlier exclusion based on Ca and Mg, or if not available, the reported total hardness. Data were further aggregated at the waterworks level by the annual mean and at the supply area level by the weighted mean (weighted to waterworks annual abstraction volumes). Temporal and spatial gaps were filled prior to these aggregations. Various stakeholders could benefit from these open access data. They provide a societal service in response to increased public interest in drinking water hardness. The research community could use the data in environmental, exposure or epidemiological assessments. Finally, the water supplies and the public sector could benefit from these data as they provide a nationwide overview of current and past drinking water hardness in Denmark and highlight the geographic areas that lack recent data, most probably due to de-regulation.

Quantitative assessments of long-term, national-scale responses of groundwater quality to pesticide applications are essential to evaluate the effectiveness of pesticide regulations. Retardation time in the unsaturated zone (R_u) was estimated for selected herbicides (atrazine, simazine, and bentazon) and degradation products (desethylatrazine (DEA), desisopropylatrazine (DIA), desethyldesisopropylatrazine (DEIA), and BAM) using a multidecadal time series of groundwater solute chemistry (∼30 years) and herbicide sales (∼60 years). The sampling year was converted to recharge year using groundwater age. Then, R_u was estimated using a cross-correlation analysis of the sales and the frequencies of detection and exceedance of the drinking water standard (0.1 μg/L) of each selected compound. The results showed no retardation of the highly polar, thus mobile, parent compounds (i.e., bentazon), while R_u of the moderately polar compounds (i.e., simazine) was about a decade, and their degradation products showed even longer R_u. The temporal trends of the degradation products did not mirror those of the sale data, which were attributed to the various sale periods of the parent compounds, sorption of the parent compounds, and complex degradation pathways. The longer R_u in clayey/organic sediments than in sandy sediments further confirmed the role of soil-specific retardation as an important factor to consider in groundwater protection.

Chalk and limestone aquifers contribute one-third of the drinking water supply in Denmark, and one-sixth of that national groundwater resource is assessed as having a ‘Poor’ status in terms of the quantitative Water Framework Directive due to intensive abstraction. This paper describes the national groundwater level monitoring network with regard to the following three applications: (1) when used for the annual surveying and reporting of groundwater resources and impacts from climate and groundwater abstraction; (2) as part of real-time monitoring and modelling for daily and seasonal forecasting; and (3) for tracking long-term climate change impacts on groundwater levels. Groundwater level monitoring provides a particularly important indicator of abstraction pressure and sustainable balance compared with recharge. Many larger chalk and limestone groundwater bodies in Denmark are only monitored by local water companies and not represented in the national groundwater level network. This raises the concern that current national groundwater level monitoring does not fully support integrated modelling and assessment purposes for chalk and limestone groundwater bodies. This also implies that, for tracking long-term climate change and anthropogenic impacts on groundwater levels, the national groundwater monitoring network especially lacks long-term records with complete 30-year time-series for many intensively exploited large chalk and limestone aquifers.

Determining natural background levels (NBLs) is a fundamental step in assessing the chemical status of groundwater bodies in the EU, as stipulated by the Water Framework and Groundwater Directives. The major challenges in deriving NBLs for trace metals are understanding the interaction of natural and anthropogenic processes and identifying the boundary between pristine and polluted groundwater. Thus, the purpose of this paper is to present a roadmap guiding the process of method selection for setting meaningful NBLs of trace metals in groundwater. To develop the roadmap, we compared and critically assessed how three methods for excluding polluted sampling points affect the NBLs for As, Cd, Cr, Cu, Ni, and Zn in Danish aquifers. These methods exclude sampling points based on (1) the primary use of the well (or sampling purpose), (2) the dominating anthropogenic pressure in the vicinity of the well, or (3) a combination of pollution indicators (NO₃, pesticides, organic micropollutants). Except for Ni, the NBLs derived from the three methods did not differ significantly, indicating that the data pre-selection based on the primary use of the wells is an important step in assuring the removal of anthropogenically influenced points. However, this pre-selection could limit the data representativity with respect to the different groundwater types. The roadmap (a step-by-step guideline) can be used at the national scale in countries with varying data availability.

Cost-efficient targeted nitrogen (N) regulation of agriculture with low impact on the environment is the new N regulation paradigm. It requires detailed knowledge on the geological and geochemical conditions of the subsurface that is crucial for assessing the nitrate flowpaths and reduction processes. An integral part of this is analysis of the subsurface redox structures to determine the locations of nitrate reduction. This knowledge has so far not been easy to access because of lacking technology. Here we present a new concept consisting of integration and interpretation of data from the geophysical towed transient electromagnetic method, borehole information on lithology, sediment colour descriptions, geochemistry and groundwater chemistry. The concept is demonstrated in three small first-order hydrological catchments. National GIS screening analyses show that the new concept is highly needed in large parts of Denmark where the redox structures are complicated e.g. in marine landscape types and in glacial moraine landscapes but less needed in areas dominated by homogeneous meltwater plains. Providing subsurface knowledge for locally targeted N regulation of agriculture is paramount in many developed countries with intensive agriculture to lower the environmental impact, and it could also be critical in developing countries to support sustainable economic and environmental development.