RESUMO
Nitrates are bioavailable compounds, which are necessary for every living organisms. However, their excess could be harmful, because it could lead to, i.e., methemoglobinemia, formation of carcinogenic nitrosamines and reservoir eutrophication. Isotopic analysis of δ15N and δ18O in nitrates allows to distinguish their different sources. To simplify the way of nitrate source monitoring, we developed a less complex and cheap "off-line" method for nitrate isotopic analysis, in which we obtain both gases required for isotopic analysis of NO3-, N2 for δ15N and CO2 for δ18O measurements, in one procedure. The developed method is based on simultaneous conversion of AgNO3 (mixed with C and placed into PtIr boat) to N2 and CO2 in a glass vacuum apparatus. A small amount of sample (8 mg of AgNO3) required for this procedure and a good reproducibility of the isotopic analysis (better than 0.3 for δ18O and 1 for δ15N) make this method a useful tool for environmental research. Using the developed method, we analyzed the isotopic composition of N and O in nitrates in the Zemborzycki Reservoir near Lublin (Poland), in 2015. The results indicate a large variation of δ15N (from -1.4 to +37.7) and δ18O (from +5.5 to +25.9) values in NO3- ions, while the concentration of NO3- varied from 0.2 to 15.3 mg/dm3. These preliminary results suggest that nitrate fertilizers probably are the main source of NO3- pollution. Our study points out the input of manure and/or sewage to the analyzed reservoir in the subsequent periods of sample collection.
