Tracking the nitrogen cycle in a vulnerable alluvial system using a multi proxy approach: Case study Varaždin alluvial aquifer, Croatia.

At high concentrations nitrate is considered a serious environmental pollutant which degrades the quality of ground and surface waters. Such high nitrate concentrations (>50 mg NO/L) have been observed for decades in the alluvial aquifer in the Varaždin region of Croatia. Here we employ a novel cross disciplinary approach (dual isotopes, chemical, bacteria diversity and mixing modelling) to determine sources of nitrate and processes that can influence nitrate concentration within this vulnerable alluvial aquifer. Ten groundwater wells were sampled across the region and in different hydrological conditions for basic chemical, stable isotopes (δO-HO, δH-HO, δN-NO and δO-NO), and bacterial diversity analyses. In addition, solid samples, i.e. soil samples and fertilizers were collected and analysed for bulk δN. The primary nitrate sources were manure, sewage, soil organic N, and ammonia fertilizers, however we observe no clear evidence to indicate that synthetic fertilizers are a major contributor to groundwater nitrate concentrations. Whilst denitrification was observed in the parts of the study area with dissolved oxygen (DO) deficiency, i.e. anoxic conditions, nitrification has been identified as the major process responsible for nitrate behaviour within the aquifer system. Our results will facilitate the creation of a conceptual model of nitrate behaviour in the study area and from this, a numerical groundwater nitrate transport model. These data, understanding of nitrate dynamics and subsequent models will be critical for future sustainable water and agricultural management of the study area.