Evaluation of Hydrogeochemical Processes for Irrigation Use and Potential Nitrate Contamination Sources in Groundwater Using Nitrogen Stable Isotopes in Southwest, India: A Case Study

The current research objective is to evaluate the appropriateness of groundwater for irrigation use and identify the sources of nitrate (NO₃⁻) contamination in groundwater using stable isotopic signatures (N¹⁵ and O¹⁸). Groundwater samples (n = 50) from Southwest Delhi were analyzed pre- and post-monsoon for irrigation quality. The pH, electrical conductivity (EC), total dissolved solids (TDS), sodium adsorption ratio (SAR), sodium percentage (Na%), and other parameters were measured. EC exceeded 3000 μS/cm in 32% of pre-monsoon and 58% of post-monsoon samples. TDS categorized 32%, 52%, and 16% pre-monsoon and 68%, 28%, and 4% post-monsoon samples as non-saline, slightly saline, and moderately saline, respectively. SAR classified 72%, 20%, 6%, and 2% pre-monsoon and 62%, 12%, and 26% post-monsoon samples in low, medium, high, and very high sodium categories. Na% was unsuitable in 4% pre-monsoon and 4% post-monsoon samples. Kelly’s ratio showed 6% pre-monsoon and 20% post-monsoon samples unfit for use. The magnesium ratio indicated 34% pre-monsoon and 32% post-monsoon samples exceeding the limit. On Gibbs plots, 80% pre-monsoon and 67% post-monsoon samples indicated the dominance of evaporation and mineral dissolution processes influencing the resulting chemistry of irrigation water. Piper diagram suggested the majority of the samples belonged to mixed Ca-Mg type, Na-Cl type, and Ca–Mg–SO₄–Cl, representing temporary hard water and permanent hard water type in both seasons. Additionally, the isotopic signature serves as additional confirmation that the primary contributors of NO₃⁻ in the groundwater within the study area are a combination of ammonium fertilizers, soil nitrogen, and manure/septic waste in both seasons.