engleski

Origin, fate and transport modelling of nitrate in the Varaždin aquifer

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. 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. The conducted research combined different methods (hydraulic, geochemical, isotope, microbiological, statistical, modelling), which resulted in numerous findings about the alluvial aquifer, its interaction with surface water and precipitation, and nitrate behaviour within the aquifer. The stable water isotopes (δ18O and δ2H) indicated that groundwater and surface water are recharged by precipitation. The average estimated recharge from precipitation using Wetspass-M model was 34% of total precipitation. Analysis of head contour maps showed that aquifer is recharged from the Drava River and accumulation lake Varaždin, which was supported by stable water isotopes, and quantified by water budget analysis: surface waters participate in groundwater recharge with 68%, and precipitation infiltration with 32%. Dual isotopes of nitrate (δ15N and δ18O in NO3) indicated that manure, wastewater, soil organic N, and ammonia fertilizers are the possible sources of nitrate in groundwater. Chemical, isotope, bacterial, and hierarchical cluster analysis displayed grouping of wells in agricultural, urban, and 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 results of isotope mixing model showed that manure is the dominant nitrate source in agricultural, wastewater in urban, and soil organic N in natural group. The calibrated groundwater flow and nitrate transport model was used to simulate nitrate concentrations in groundwater in the next two decades. 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.

nitrate ; water and nitrate stable isotopes ; bacteria ; statistical analyses ; mixing model ; numerical model ; Varaždin alluvial aquifer

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