Isotopic fingerprinting of sediment organic matter reveals pervasive sewage influence in urbanised tropical rivers.

Rapid urbanisation in tropical coastal regions is altering riverine biogeochemistry, yet specific drivers of sediment organic matter (OM) dynamics remain understudied in Sub-Saharan Africa. This study assessed spatial and seasonal variation in sediment total nitrogen (TN), total organic carbon (TOC), molar C/N ratio, and stable isotopes (δ13C and δ15N) in three coastal rivers draining Dar es Salaam, Tanzania (Kizinga, Mbezi, and Msimbazi). Across all rivers, TN (2.18-5.18 mg/g) and TOC (24.47-50.31 mg/g) concentrations showed no statistically significant seasonal differences (p > 0.05), indicating persistent anthropogenic loading across contrasting hydrological seasons. Msimbazi River exhibited the highest organic enrichment and lowest molar C/N ratios (9.5 ± 1.5-10.5 ± 1.0), consistent with N-rich organic inputs. In contrast, the Kizinga River displayed higher molar C/N ratios but the most enriched δ15N values (up to 10.3 ± 1.0‰), consistent with wastewater-derived nitrogen subjected to fractionating transformations such as ammonia volatilisation and denitrification. Bayesian source apportionment using the Food Reconstruction using Isotope Transferred Signals (FRUITS) model estimated that sewage-derived OM contributes a posterior median of 41% (95% Bayesian credible interval (BCI) 28-52%) in Msimbazi and 43% (95% BCI 30-54%) in Kizinga sediments. These findings demonstrate a state of biogeochemical stationarity in which chronic urban wastewater inputs dominate sediment composition across contrasting hydrological seasons. The results highlight that seasonal flushing is insufficient to remove accumulated anthropogenic signatures from riverbeds and underscore the need for structural interventions to reduce sewage inputs to urban tropical rivers.