engleski

Estimation of nitrate origin in Zagreb aquifer system

Nitrate pollution is a worldwide problem. A lot of studies showed high correlation between agriculture and nitrate concentration in groundwater, proclaiming extensive use of fertilizers as a main nitrate diffuse source. Zagreb aquifer system is the main source of potable water for the quarter of the Croatian population. It is designated as part of the country`s strategic water reserves, protected by the Croatian state. It covers the area of approximately 425 km2, where eight major well fields are pumping groundwater for water supply. According to the River Basin Management Plan (RBMP) it is at risk of not achieving good chemical status (EU WFD) due to nitrate concentrations. One of the most important things before the application of the groundwater protection measures is to define the nitrate origin in groundwater. Nitrate source can be estimated studying the correlation between NO3 - , Cl- and Na+ concentrations. Generally, if there is a good positive statistical correlation among these chemical parameters, one can conclude that nitrate origin is related with sources of pollution of urban and animal waste. If the statistical correlation is not significant, one can conclude that nitrate concentration is probably associated with fertilizer nitrogen as the only source, or as the nitrogen from the mixed agricultural and urban sources. Pearson and Spearman correlation coefficients were calculated to estimate the nitrogen origin in oxidized alluvial part of the Zagreb aquifer system. More than 10000 chemical analysis, mostly from 1994 to 2013, were analyzed regarding to NO3 -, Cl-, Na+ and O2 concentrations. Correlation was made only for the observation wells (about 90) where more than 30 water quality analysis were available and where average O2 was higher than 2 mg/l. Results showed different variations of positive and negative correlation, indicating that Zagreb aquifer system is very complex geochemical environment due to combination of natural and anthropogenic influence, but also because of its dynamic relationship with the Sava river. However, in few observation wells good positive correlation was identified. That indicates that nitrate origin in Zagreb aquifer is mainly from agriculture and mixed sources, even though there are some areas where nitrate concentrations are probably the consequence of the leaking sewage system, septic tanks, animal waste or landfills.

Nitrate origin; statistical correlation; Zagreb aquifer system

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