Sources and Biogeochemical Processes of Nitrate in the Laolongdong Karst Underground River Basin, Chongqing

Abstract

Samples of sewage, well water, and underground river water of the urbanized Laolongdong karst underground river basin in Chongqing, China were collected during July 2019 and October 2020 and measured to determine the nitrate origin and biogeochemical processes based on geochemistry and dual nitrate isotope (δ15N-NO3- and δ18O-NO3-) data. The results showed that:① the isotopic nitrate compositions of sewage ranged from -3.3‰ to 14.6‰ for δ15N-NO3- and from -5.2‰ to 20.6‰ for δ18O-NO3-, which indicated that nitrate originated from manure and sewage, fertilizer, and soil organic nitrogen. The δ15N-NO3- and δ18O-NO3- of well water varied from 3.1‰ to 12.6‰ and 2.9‰ to 8.9‰, respectively, suggesting nitrate was mainly from soil organic nitrogen and manure and sewage. For the underground river water, the δ15N-NO3- and δ18O-NO3- ranged from 5.6‰ to 28.6‰ and from -2.0‰ to 15.7‰, respectively, suggesting that municipal sewage and manure were the dominate nitrate sources. ② Based on the MixSIAR model, manure and sewage were the primary nitrate source of the underground river water, accounting for 89.1% of the total contribution, whereas the contributions of soil organic nitrogen, fertilizer, and atmospheric precipitation were 4.4%, 3.4%, and 3.1%, respectively. ③ In the basin, the concentration ratios of COD:ρ(NO3-) from low to high were as follows:well water (0.14-5.15), underground river water (0.50-9.36), and sewage (4.08-89.50). Only 50% of well water samples with COD:ρ(NO3-) were slightly higher than 0.65, which is the minimum stoichiometric ratio for denitrification occurrence. This indicated that there were insufficient COD concentrations to support that denitrification occurred in the well water. This was further verified by no significant enrichment of nitrogen and oxygen isotopes. As much as 90% of underground river water samples had a COD:ρ(NO3-) higher than 0.65, and the dual nitrate isotopes were simultaneously enriched with a δ15N:δ18O of 1.8, which is within the ratios ranging from 1.3 to 2.1, indicating that denitrification occurred. The COD:ρ(NO3-) for all wastewater samples was much higher than 0.65, of which 25% were higher than the stoichiometric ratio (29.34) for the occurrence of dissimilation reduction nitrate to ammonium (DNRA). The δ15N-NO3- and ρ(NH4+):ρ(NO3-) of sewage increased simultaneously, indicating that DNRA may have occurred in the sewage.