Igor Karlović, mag. ing. geol.
Over the last decades, high nitrate concentrations in Varaždin alluvial aquifer raised public concern regarding groundwater quality. The aquifer is the main source of drinking water for the local population in the Varaždin County in NW Croatia. Moreover, according to its hydrogeological characteristics, it represents one of the strategic groundwater resources in Croatia. For better understanding of nitrate distribution in groundwater and formulating appropriate management strategies for groundwater quality protection, it is necessary to investigate the origin, fate, and transport of nitrate within the Varaždin aquifer. Simply put, nitrates are formed by the nitrification process and disappear by the denitrification process. There are other nitrogen transformation processes, but these two are the main and best researched. The research conducted within the TRANITAL project (Origin, fate and TRAnsport modelling of NItrate in the Varaždin ALluvial aquifer) combined hydraulic, hydrochemical, isotope, microbiological, statistical, and modelling techniques which resulted in numerous findings about the alluvial aquifer, its interaction with surface water and precipitation, and nitrate behaviour within the aquifer. The alluvial aquifer is composed of gravel and sand with variable proportions of silt and clay. It consist of three layers: upper aquifer, semipermeable interlayer, and lower aquifer (Figure 1).
Figure 1 Three-dimensional model of the Varaždin aquifer (Karlović et al., 2022a)
Groundwater, surface water and precipitation sampling (Figure 2) was conducted on a monthly basis during four-year period (from June 2017 to June 2021) for hydrochemical and stable water isotope analyses. Hydrochemical analyses of groundwater samples identified main processes that influence the groundwater chemistry: dissolution and precipitation of carbonate minerals, silicate weathering, cation exchange, transformation of organic matter, and anthropogenic influence. Hydrochemical data suggested that nitrate in groundwater could be related to usage of manure and fertilizers in agricultural production and wastewater. The stable water isotopes indicated that groundwater and surface water are recharged by precipitation (Figure 3).
Figure 2 Monitoring network with groundwater, surface water and precipitation sampling points
Figure 3 The relationship between δ2H and δ18O in groundwater and surface water (modified according to Karlović et al., 2022b)
Analysis of head contour maps shows that aquifer is recharged from the Drava River and accumulation lake Varaždin for all hydrological conditions, keeping the groundwater flow in the quasi-steady state. The general direction of groundwater flow is from NW to SE (Figure 4).
Figure 4 Map of head contour maps (modified according to Karlović et al., 2021)
Nitrate origin was studied using combination of dual isotope approach (δ15N and δ18O in nitrate), chemical and bacterial data, and isotope mixing model. The results showed that manure is the main nitrate source in agricultural, wastewater in urban, and soil organic N in natural area. Nitrification was identified as the main nitrogen transformation process, while denitrification can occur locally, but does not have significant impact on regional scale. The calibrated groundwater flow and nitrate transport model was used to simulate nitrate concentrations in groundwater in the next two decades (Figure 5). Model simulations predict continued downward trend of nitrate concentrations in the central part, and steady low nitrate concentrations in the northern part of the model. The modelling results demonstrated that management of agricultural practices is the most important aspect to gradually reduce nitrate contamination in the Varaždin aquifer, but it takes decades for nitrate concentrations in groundwater to respond to changes in nitrogen input from the surface.
Figure 5 Nitrate distribution in year 2020 and simulated nitrate distribution in year 2040
References:
Karlović, I., Marković, T., Vujnović, T., Larva, O. (2021): Development of a Hydrogeological Conceptual Model of the Varaždin Alluvial Aquifer. Hydrology, 8, 19, 13. doi:10.3390/hydrology8010019
Karlović, I., Posavec, K., Larva, O., Marković, T. (2022a): Numerical groundwater flow and nitrate transport assessment in alluvial aquifer of Varaždin region, NW Croatia. Journal of Hydrology: Regional Studies, 41(3):101084. doi: 10.1016/j.ejrh.2022.101084
Karlović, I., Marković, T., Vujnović, T. (2022b): The groundwater recharge estimation using multi component analysis: case study at the NW edge of the Varaždin alluvial aquifer, Croatia. Water. 14, 42. doi: 10.3390/w14010042
Marković, T., Karlović, I., Perčec Tadić, M., Larva, O. (2020): Application of Stable Water Isotopes to Improve Conceptual Model of Alluvial Aquifer in the Varaždin Area. Water, 12, 2; 1-13. doi:10.3390/w12020379
Igor Karlović, mag. ing. geol. is a research assistant at the Department of Hydrogeology and Engineering Geology at the Croatian Geological Survey. He enrolled in the PhD study program of Applied Geosciences, Mining and Petroleum Engineering at the Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb in 2018 with a topic entitled Origin, fate and transport modelling of nitrate in the Varaždin aquifer.
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