Linking urban sanitation practices to water resource quality: A multi-tracer approach in a humid tropical context using nitrate isotopes and emerging organic contaminants

Urban water resources in large cities of sub-Saharan Africa are increasingly impacted by anthropogenic activities, mainly due to the limited access to adequate sanitation systems. In these highly anthropized contexts surface water and groundwater contamination sources are multiple. However, the interactions between these contamination sources and urban water bodies remain poorly investigated, despite the need for such understanding to support sustainable water-resource management. This study applied a multi-tracer approach combining hydrochemistry, nitrate stable isotopes (δ 15 N-NO 3 and δ 18 O-NO 3 ), and Emerging Organic Contaminants (EOCs) to identify contamination sources and pathways, and to characterize the biogeochemical processes affecting shallow aquifers and surface waters in a tropical urban environment. High Cl/Br molar ratios (>1000), indicated that NO 3 – and Cl - primarily originate from domestic wastewater inputs. Enriched δ 15 N-NO 3 (6.6 to 25‰) and δ 18 O-NO 3 (+3.3 to + 9.9‰), further confirmed the predominance of wastewater-derived contamination and revealed the occurrence of partial denitrification processes. Intermediate isotopic ranges (soil organic nitrogen and sewage) suggested the influence of mixed contamination sources. Widespread occurrence of EOCs, particularly persistent contaminants such as fluconazole (FCZ), highlighted the significant impact of wastewater on both groundwater and surface water quality. Strong correlations of FCZ with Cl - , δ 15 N-NO 3 , together with their similar spatial distribution, identified FCZ as the most effective wastewater co-tracer in the study area. Furthermore, the co-occurrence of FCZ with non-persistent compounds such as paracetamol indicated recent and continuous wastewater inputs affecting urban water resources. Sporadic detection of non-ubiquitous compounds such as carbamazepine reflects localized wastewater infiltration from septic or pit latrine sources. These results underscore the high vulnerability of shallow urban aquifers and demonstrate the relevance of nitrate isotopes and EOCs coupling to better understand contamination dynamics and support sustainable urban water management.