Evaluation of the origin of nitrate influencing the Ključ groundwater source, Serbia

This paper describes the use of the dual isotope method involving δ¹⁵N and δ¹⁸O measurements of dissolved nitrates to assess the origin and fate of groundwater nitrate at the Ključ groundwater source, Serbia. A sampling campaign was conducted in September 2007 during flow conditions obtaining groundwater from observation wells and river water fed by a shallow aquifer hosted in alluvial (sandy-gravel) sediments. Nitrate isotope ratios ranged from +5.3 to +16.9‰ and δ¹⁸O_NO3 values varied from −2.3 to +5.0‰. Two major contamination sources were identified with isotopic compositions characteristic for nitrate derived from nitrification of soil organic nitrogen (+5.3 to +7.8‰ for δ¹⁵N) resulting in nitrate concentrations of 33.6 and 78.8 mg/L and nitrate derived from animal wastes or human sewage, e.g. via septic systems, yielding δ¹⁵N values of +9.9 to +11.9‰ and elevated nitrate concentrations of 31.2–245.8 mg/L. The occurrence of nitrification and denitrification was also revealed based on concentration and isotope data for dissolved nitrate.