Assessing sources of nitrate contamination in the Shiraz urban aquifer (Iran) using the δ15N and δ18O dual-isotope approach

Nitrate(NO33-) is one of the major threats to the quality of the drinking water taken from the Shiraz aquifer. This aquifer undergoes high anthropogenic pressures from multiple local urban (including uncontrolled sewage systems), agricultural and industrial activities, resulting in NO3- concentrations as high as 149 mg L-1, well above the 50 mg L-1 guideline defined by the World Health Organisation. We coupled here classical chemical and dual isotope (N-15 and O-18 of NO3-) approaches trying to characterize sources and potential processes controlling the budget of this pollutant. Chemical data indicate that nitrate in this aquifer is explained by distinct end-members: while mineral fertilizers isotopically show to have no impact, our isotope approach identifies natural soil nitrification and organic NO3- manure and/or septic waste) as the two main contributors. Isotope data suggest that natural denitrification may occur within the aquifer, but this conclusion is not supported by the study of other chemical parameters.