Nitrate-containing groundwater in Denmark. Exploratory data analysis at the national scale

Here are reported the results of an exploratory data analysis conducted to understand groundwater (GW) redox conditions across Denmark, with a focus on anoxic, nitrate-reducing groundwaters (B redox type), compared against oxic, nitrate-containing groundwaters (A redox type) and further the more general classification into nitrate-containing vs reduced groundwaters (AB and CD redox type). Particular attention was paid to carbonate aquifers due to limited studies on redox conditions in these systems in Denmark. The exploratory data analysis addressed three primary research questions: (1) the spatial and depth distribution of redox type B groundwater, (2) the relationship between redox state and groundwater age, and (3) the temporal stability of redox conditions. This work was done as part of the project “Improved nitrogen retention mapping for new regulatory model of agriculture” (Nret24).

Spatial analysis revealed that redox type B well-screens were widely distributed across Denmark, with the highest densities in Weichselian moraine landscapes and a notable presence in carbonate aquifers. Depth distribution varied by lithology, with carbonate aquifers exhibiting the deepest median (28 m, Q90 = 60 m) and a bimodal depth distribution linked to differences in lithological properties such as porosity and permeability. This contrasted with shallower depths in Quaternary sand aquifers. Lithology-specific analysis highlighted significant differences in depth-distributions, further supporting the influence of geological factors. Additional literature review revealed that there was a large variation in B zone thickness across the country (1-16 m), if present at all; and the variability within study sites (between wells) was in the range 2-15 m (respectively at LOOP 6 and Javngyde).

Groundwater age analysis demonstrated a clear relationship with redox conditions. Redox type B groundwater was generally found to be older than type A, with median ages of 25 and 15 years, respectively. This trend persisted across aquifer types when modelled transit times were used and the focus was AB and CD redox. Particularly for carbonate aquifers, transit times were significantly longer for CD (reduced) compared to AB (nitrate-containing) redox types. A positive correlation was found between modeled transit times and groundwater age, though limited data coverage and spatial inconsistencies presented challenges for a more granular analysis with focus on specific carbonate lithology.

Temporal redox stability analysis indicated frequent transitions between redox states (e.g., A↔B and AB↔CD), with a trend toward oxidizing conditions, possibly influenced by measurement uncertainties. Most well-screens (79.1%) remained in a consistent AB state, however almost half of these had a single-event sampling, complicating interpretations. A significant subset displayed dynamic or uncertain AB redox conditions, with stability varying by well-screen type (data origin).

The findings contribute to understanding groundwater redox conditions in Denmark and the factors influencing them, including geology, depth, age, and temporal variability. This work supports further applications such as mapping nitrate pollution risks in carbonate aquifers and evaluating the temporal dynamics of redox states in Denmark. Future work should expand the dataset to enhance the representativity of redox state analyses and address gaps related to temporal dynamics and flow system changes.