Hydrogeochemical and isotopic assessment of the origin of NO3− and N–NH3 contents in the aquifer located in a closed lacustrine volcano-sedimentary basin in the metropolitan area of Mexico City

To explain the presence and spatial distribution of NO₃⁻ and N–NH₃ in the Aquifer of the Metropolitan Area of Mexico City (AMAMC), a hydrogeochemical and isotopic analysis using ¹³C_DIC (as well as the stable isotopes ¹⁸O and ²H) in groundwater was conducted. This aquifer is located in an old closed lacustrine volcano-sedimentary basin; some wells hosted in the semi-confined zone contain high N–NH₃ concentrations, while others present NO₃⁻ contents in the recharge zones (hosted in an oxidizing environment). In this study, a change in the isotopic signature (primarily in ¹⁸O and ²H) was observed from the recharge zones to the basin center in some of the wells with high NO₃⁻ concentrations, this behavior can be attributed to evaporation during the incorporation of recently infiltrated water. In addition, the results for ¹³C (along with ²H) in wells with the highest N–NH₃ concentrations exhibited an atypically broad range of values. Results indicated the occurrence of hydrogeochemical and/or biochemical processes in the aquifer (in an oxidizing or reducing environment), such as organic degradation, bacterial decomposition (primarily in the ancient Lake Texcoco and which acts as a natural sink for carbon, nitrogen, sulfur, and phosphorus), besides rock weathering and dissolution, which may be responsible for a very marked isotopic modification of the ¹³C (and, to a lesser extent, ²H). Methanotrophic bacterial activity and methanogenic activity may be related to N–NH₃ removal processes by oxidation and residual water incorporation respectively, whereas the increase in the NO₃⁻ content in some wells is due to the recent contribution of poor-quality water due to contamination.