Sewage Leaves a Lasting Isotopic Stamp on Tropical City Rivers

In the rapidly growing coastal city of Dar es Salaam, Tanzania, three rivers carry the chemical memory of everything that has been flushed, dumped, and washed into them. A new study published in Environmental Science and Pollution Research shows that the sediments of the Kizinga, Mbezi, and Msimbazi rivers bear an unmistakable isotopic fingerprint of chronic sewage contamination, one that persists through both wet and dry seasons. The research, led by Zakaria Mhande of the Open University of Tanzania and the Tanzania Bureau of Standards, together with colleagues at the Vrije Universiteit Brussel, offers some of the most detailed evidence yet that urban wastewater dominates the organic matter cycling of Sub-Saharan African rivers, even when seasonal floods might be expected to sweep those signatures away.

The team focused on sediment organic matter, the complex mixture of plant debris, microbial biomass, and human-derived waste that accumulates on riverbeds. Rather than measuring pollution in the water column, which fluctuates hour by hour and storm by storm, the researchers interrogated the riverbed itself. Sediments act as long-term integrators of what flows through a catchment, recording weeks and months of inputs in their carbon and nitrogen chemistry. By reading that record, the scientists could distinguish between organic matter derived from natural vegetation, such as mangrove and terrestrial plant litter, and organic matter derived from human waste, which carries a distinctly different isotopic signature.

The analytical approach rested on two complementary measurements. The first is elemental: the concentrations of total organic carbon and total nitrogen in the sediment, and the ratio between them. Terrestrial plants are carbon-rich and nitrogen-poor, producing high carbon-to-nitrogen ratios, typically well above twelve. Sewage and other nitrogen-enriched waste streams push that ratio sharply downward. The second measurement is isotopic. The stable isotope ratios δ13C and δ15N express, in parts per thousand deviations from international standards, the relative abundance of heavy carbon-13 and nitrogen-15 isotopes in the organic matter. Because different sources fractionate these isotopes in characteristic ways, they function as fingerprints: untreated human waste and wastewater effluent tend to be strongly enriched in nitrogen-15, while natural plant material is comparatively depleted.

Sediment samples collected across the three rivers revealed striking patterns. Total nitrogen ranged from 2.18 to 5.18 milligrams per gram and total organic carbon from 24.47 to 50.31 milligrams per gram, yet neither showed statistically significant seasonal differences. That absence of seasonal variation is itself a finding. In less disturbed rivers, wet-season flushing typically dilutes and redistributes organic loads, producing measurable contrasts between hydrological regimes. Here, the loading of anthropogenic organic matter is so persistent and so continuous that the sediment composition remains essentially stationary throughout the year, regardless of rainfall.

Each river told its own isotopic story. The Msimbazi River, which drains some of the most densely populated and poorly sewered neighbourhoods of the city, showed the highest organic enrichment and the lowest molar carbon-to-nitrogen ratios, ranging from roughly 9.5 to 10.5. Low ratios of this kind point to nitrogen-rich inputs, the hallmark of human waste. The Kizinga River presented a different but equally diagnostic picture: its sediments carried the most enriched δ15N values measured in the study, reaching up to 10.3 plus or minus 1.0 per mille. Values of this magnitude are consistent with wastewater-derived nitrogen that has been further concentrated by fractionating transformations in the riverbed, including ammonia volatilisation, in which the lighter nitrogen-14 isotope escapes preferentially to the atmosphere, and denitrification, in which microbes convert nitrate to gaseous nitrogen while preferentially consuming the lighter isotope and leaving the residual pool progressively heavier.

To move from qualitative fingerprinting to quantitative source apportionment, the researchers employed a Bayesian mixing model known as FRUITS, short for Food Reconstruction Using Isotope Transferred Signals. Originally developed for reconstructing ancient diets from bone collagen, the model treats each measured isotope and elemental value as a mixture of contributions from candidate sources, each with its own characteristic signature and associated uncertainty. By sampling the space of possible mixing solutions, FRUITS generates posterior probability distributions for the contribution of each source. In this application, the model estimated that sewage-derived organic matter accounts for a posterior median of 41 percent of sediment organic matter in the Msimbazi River, with a 95 percent Bayesian credible interval spanning 28 to 52 percent, and 43 percent in the Kizinga River, with a credible interval of 30 to 54 percent.

Those numbers deserve careful interpretation. A credible interval of this width reflects genuine uncertainty in the exact end-member signatures of sewage, soil organic matter, and plant detritus, which vary across space and are altered by diagenetic processing in the sediment. Yet even the lower bounds of the intervals are remarkable: in both rivers, at least roughly three in ten units of sediment organic matter can be attributed to sewage, with the most probable value close to four in ten. For rivers that also receive runoff, agricultural inputs, and natural litterfall, this confirms that wastewater is not one contributor among many but the single dominant anthropogenic driver of sediment composition in these urban tropical catchments.

The concept that best captures the study’s central conclusion is biogeochemical stationarity, an idea borrowed from earlier work on managed catchments in which nutrient export becomes nearly constant despite variable hydrology. When chronic inputs overwhelm natural variability, the river stops responding to seasons and instead reflects the steady pressure of its pollution sources. In Dar es Salaam’s rivers, that pressure is structural: much of the city lacks adequate sewerage, and untreated or partially treated wastewater enters the channels continuously. Seasonal flushing, the study concludes, is simply insufficient to remove the accumulated anthropogenic signature from the riverbed. The sediments function as both a record and a reservoir, holding nitrogen and organic carbon that can be remobilised into the water column and ultimately delivered to the sensitive coastal waters of the Indian Ocean, where mangroves, seagrasses, and fisheries face eutrophication risk.

The implications extend well beyond Tanzania. Sub-Saharan Africa is urbanising faster than almost any other region, and its coastal cities frequently lack the wastewater infrastructure that industrialised nations built over decades. Stable isotope fingerprinting of sediments offers these data-scarce environments a practical monitoring tool: it requires relatively few samples, integrates pollution over time, and does not depend on continuous instrumental records. The technique can identify which catchments are most affected, quantify the sewage contribution, and provide a baseline against which future interventions can be judged. The authors argue that their findings underscore the need for structural interventions, including expanded sewerage coverage, improved treatment capacity, and nature-based solutions such as constructed wetlands that can strip nutrients from effluent before it reaches river channels.

What makes the study resonate beyond its technical contribution is the clarity of its message: in these tropical urban rivers, the sewage signal never goes away. Rainy seasons come and go, floodwaters scour the channels, and yet the isotopic composition of the riverbed remains anchored to its human source. For the millions of people living along the Kizinga, Mbezi, and Msimbazi, and for the coastal ecosystems downstream, the rivers are effectively running at a permanently elevated baseline of wastewater influence. Reversing that baseline will require more than waiting for the rains; it will demand deliberate, engineered changes to how a growing city handles its waste. The isotopes, patient and precise, will be there to verify whether those changes work.

Subject of Research: Isotopic tracing of sewage-derived organic matter in urbanised tropical river sediments in Dar es Salaam, Tanzania

Article Title: Isotopic fingerprinting of sediment organic matter reveals pervasive sewage influence in urbanised tropical rivers

Article References: Mhande, Z., Mihale, M., Hellar-Kihampa, H., Brion, N., & Baeyens, W. (2026). Isotopic fingerprinting of sediment organic matter reveals pervasive sewage influence in urbanised tropical rivers. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38287-6

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38287-6

Keywords: sewage pollution, stable isotopes, sediment organic matter, tropical rivers, Dar es Salaam, Tanzania, biogeochemical stationarity, Bayesian mixing model, FRUITS, nitrogen cycle, urbanisation, water quality