Deciphering Groundwater Nitrate Origins in a Hypersaline Inland Wetland through Isotopic Fingerprinting

Nitrate (NO 3 - ) contamination in groundwater represents a critical environmental challenge due to its persistence in the nitrogen cycle. While nitrate sources and transformations are being well-characterized in freshwater systems, their dynamics in hypersaline inland wetlands remain poorly understood due to complex hydrogeochemical interactions. Sambhar wetland, located in a semi-arid region of the Thar Desert in the Northwestern part of India, serves as a wintering habitat for lesser flamingos. Apart from being an extremely ecologically sensitive wetland, the evaporative processes of the wetland are often studied as a terrestrial analogue for the Martian surface. This study uses dual isotopes (δ¹⁵N and δ¹⁸O) of dissolved nitrate in water to identify the sources and transformation pathways of groundwater nitrate in the wetland ecosystem. The average depth of groundwater in the region is 40m, and the average salinity of 3.8 ppt (n=34). The nitrate concentration of the region varies from 0 to 54 ppm, exceeding the WHO permissible limit. A wider range of data from the δ¹⁵N and δ¹⁸O analysis represents a mixed source contribution. Quantitative source apportionment identified animal waste, sewage inputs, soil-derived nitrate, and ammonium fertilizers as the dominant contributors to dissolved nitrate in the groundwater system. The isotopic enrichment of δ¹⁵N suggests an active denitrification in the region, probably from the salinity-dependent microbial processes. This study establishes the first isotopic baseline for groundwater nitrate in the Sambhar system, demonstrating that despite the arid environment’s limited hydrologic connectivity, anthropogenic nitrogen inputs significantly impact groundwater quality. The findings have important implications for managing nutrient loads in this ecologically sensitive wetland, particularly given the groundwater’s dual role as both a potential pollution vector and a local water resource.