Sources and dynamics of nitrate in groundwater in a crystalline terrain of tropical Sri Lanka based on a dual isotope approach

Abstract

Nitrate pollution of groundwater is one of the major environmental problems in many parts of the world, mainly due to intensive agriculture and waste disposal. Therefore, understanding the fate of nitrate in aquifer systems is important for sustainable groundwater management. In Sri Lanka, a tropical terrain largely covered with high-grade metamorphic rocks, non-point source pollutants from agricultural lands are the main sources of nitrate in groundwater. This study presents nitrate dual-isotope signatures (δ¹⁵N_NO3 and δ¹⁸O_NO3) of groundwater samples collected in the pre- (n=60) and post-monsoon (n=71) periods from the Yan Oya River basin in the dry zone of Sri Lanka. The nitrate concentrations (as NO₃^--N) of the groundwater samples varied between 0.10 and 5.60 mg/L. The δ¹⁵N_NO3 and δ¹⁸O_NO3 values were between -7.0 and +37.0‰ and between -4.4 and +32.2‰, respectively. The large coefficient of variation (CV) values (>100%) of nitrate concentrations indicated higher spatial variability and the influence of non-point source pollutants on groundwater nitrate concentrations. Land use was the primary factor influencing spatial heterogeneity. The isotopic evidence demonstrated that nitrate in groundwater originated through nitrification and mixing sources, e.g., manure, sewage, soil nitrogen, and ammonium in fertilizer. The fractionation during nitrate transformation processes and mixing of sources have altered the nitrate isotopic signatures of the sources in the region. Denitrification, assimilation, and ammonia volatilization reduced available nitrate for leaching in soil, resulting in lower nitrate concentrations with higher isotopic values in groundwater. The present study highlights the importance of applying dual isotopes to identify the sources and fate of groundwater nitrate in a crystalline aquifer in the dry zone of Sri Lanka.