Geochemical data collection in Villestrup Nret24, 2021-2025

The field work in the Villestrup area was designed to address the following working hypotheses for the biogeochemical investigations, within the framework of the Nret24 project:

-Infiltration of nitrate to the chalk aquifer occurs through sandy parts of the Quaternary top layers or though faults in the marl layer
-Nitrate reduction occurs in the chalk aquifer, and redox interfaces can be identified.
-The nitrate reducing zone (redox zone B) has a large extent in chalk aquifers.
-The double porosity of the chalk aquifers (and the impact on the 3D pattern of the redox conditions) plays creates a significant pattern of oxic fractures and reduced matrix.
-Rate experiments using the acetylene block method on undisturbed unconsolidated sediment core samples can give reliable N-reduction rates.
-In some areas nitrate reduction takes place in Quaternary sediments where we know that there is nitrate in the underlying chalk aquifer

The specific target for making new wells was to elucidate these hypotheses:

-The nitrate reduction capacity of the top Quaternary layer in some areas has been depleted; thus, the underlying chalk/limestone aquifers are vulnerable.
-The clay layer between the Quaternary layer and the limestone/chalk layer is deformed and not necessarily thick or continuous; thus, it does not necessarily play a role as a protective layer.
-Denitrification rates in the chalk/limestone are extremely slow; thus, the redox interface can be very deep and often redox type B water is developed.

This report summarizes the procedures for the fieldwork and the resulting field and laboratory
data from groundwater samples, sediment chemistry, and reaction rates for denitrification.