Isotopic Evidence of Denitrified Nitrate in Groundwater beneath Intensive Agriculture and Exchange with Estuary (Godavari, India)

Nitrate is accumulated in the groundwater, modified through nitrification/denitrification, and exchanged with coastal/estuarine water bodies. To examine the sources and modifications of nitrate, the concentrations and isotopic composition of nitrate (δ ¹⁵N and δ ¹⁸O_NO3) in the groundwater was monitored at 5 locations along the bank of Godavari estuary and in the estuarine waters for 7 months during wet (August–November) and dry (March–May) periods. Though the concentration of nitrate (NO₃^–) was higher during the wet than dry periods in both the groundwater and estuary, insignificant seasonal variability was observed in δ¹⁵N and δ¹⁸O_NO3 indicating homogenization through mineralization–immobilization turnover of NO₃^–. The range of δ¹⁵N to δ¹⁸O of NO₃^– indicates soil, manure, and septic waste may be the major source of NO₃^–. The mean ratio of δ¹⁵N/δ¹⁸O of 1.1 ± 0.3 indicates the occurrence of denitrification in the groundwater. Concentrations δ¹⁵N_NO3 and δ¹⁸O_NO3 of NO₃^– displayed a significant relation between groundwater and estuarine water suggesting that NO₃^– is possibly denitrified. This study suggests that denitrified NO₃^– (enriched δ¹⁵N and δ¹⁸O) reported in the Godavari estuary may be contributed through submarine groundwater discharge than it is hypothesized to flux from the watershed.