Integrating nitrate isotopes and organic contaminants of emerging concern to improve untreated wastewater tracing in urban groundwater

Untreated wastewater represents a major source of groundwater contamination in urban contexts with inadequate sanitation strategies. Identifying the origins and subsurface pathways of this contamination is essential for effective groundwater management, yet it remains challenging in environments influenced by multiple anthropogenic inputs. Although nitrate isotopes (δ¹⁵N-NO₃⁻, δ¹⁸O-NO₃⁻) are commonly used to trace contamination sources, they have their own limitations for distinguishing overlapping isotopic signatures, including those of wastewater and manure, or a mixture of untreated and treated wastewater inputs. A multi-tracer approach combining hydrogeochemical, isotopic and organic tools were used to accurately delineate wastewater impacts in mixed urban/peri-urban environments. Increasing nitrate concentrations in urban environments were attributed to an active anthropogenic stress, arising from population growth and widespread use of rudimentary on-site sanitation systems (soak-pits). Nitrate isotopes preliminarily confirmed that septic waste as the main source of pollution, but were unable to differentiate untreated wastewater inputs. In peri-urban settings, the potential interaction of domestic and agricultural influences in a heterogeneous residential landscape further complicated the interpretation of contamination processes. A strong correlation of acesulfame concentrations, a nearly ideal wastewater co-tracer, with those of labile and ubiquitous markers (saccharin, cotinine) indicated widespread contamination by recent untreated wastewater. The occurrence of non-ubiquitous pharmaceuticals confirmed the punctual impact of on-site sanitation systems. The sporadic detection of acesulfame in peri-urban areas underlines their vulnerability to man-made influences. The complementarity of nitrate isotopes and organic contaminants has significantly improved our ability to accurately identify the sources and timing of untreated wastewater inputs.