Isotopic Evidence of Denitrified Nitrate in Groundwater beneath Intensive Agriculture and Exchange with Estuary (Godavari, India): Fid-Bau Portal

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

Kumar, B. S. K. / Sridevi, B. / Sarma, V. V. S. S. / Cardinal, D. / Sebilo, Mathieu

Abstract

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.

Manure/septic waste is the major source of nitrogen in groundwater and it is denitrified and may exchange with estuarine waters. Higher concentrations of nitrate was observed during wet than dry period but δ¹⁵N and δ¹⁸O values of nitrate did not vary significantly. Nitrate is denitrified within the groundwater due to low oxygen levels. Denitrified nitrate in the estuarine water may be transported from groundwater.