Nitrate in water: Understanding the sources using δ15N and δ18O values

Nitrogen is a crucial component of nutrient dynamics in the environment and exists in multiple oxidation states. Nitrate (NO^3-) is the most stable form of all the reactive nitrogen species and has a higher residence time in groundwater. Sources of nitrate include mainly fertilizers, sewage, manure, soil organic matter, and rain. In a country like India, where agriculture covers an area of about 60% of the total land and population with 2^nd rank globally, contributes a huge fertilizer and sewage component to the environment. Also, nitrate in groundwater deteriorates the potable water quality. So, optimization of nitrogen use and sources estimation of nitrate in groundwater and surface water is very essential. Hindon River basin in the western Indo-Gangetic plain provides an opportunity to study nitrate dynamics in a huge populated and extensive agricultural area. Nitrate concentration in groundwater has been found from 0.1 ppm to 80 ppm, far apart from the permissible limit. Pre-monsoon groundwater shows higher nitrate concentration than that of post-monsoon groundwater at most of the places suggesting the dilution effect of rainwater after monsoon. Fluctuations in δ¹⁵N and δ¹⁸O values seasonally suggest a rapid change in contribution of nitrate source in groundwater. Contribution from each source of nitrate was estimated by Stable Isotope Mixing Models in R (SIMMR). As the dual isotope plot shows denitrification trend, the actual fertilizers contribution shifted towards manure and sewage end members evidenced by higher sewage and manure contributions (60-75% in pre and post-monsoon respectively) need to be optimized for sustainability.