Sediments Reveal Pollution Risks in Mozambique’s Bons Sinais Estuary

On the coast of central Mozambique, just north of the Zambezi Delta, the Bons Sinais Estuary funnels fresh water toward Sofala Bank through a sinuous, braided channel studded with four large islands. A new study of the estuary’s bottom sediments shows that this seemingly sandy, high-energy environment is far more vulnerable to pollution than its coarse appearance suggests. The research, published in Estuarine Management and Technologies, combined classical grain size analysis with statistical parameters and a Pejrup diagram to infer the sedimentary dynamics of an estuary whose sediment characteristics had never before been systematically explored.

The team collected 15 bottom sediment samples along the estuary using a Van Veen sampler, working during the low tide of spring tides in October 2022. Each sampling point was georeferenced with GPS, and the samples were analysed in the laboratory of civil engineering in Quelimane by conventional sieve and pipette methods. Data processing included the computation of statistical parameters such as mean, sorting, skewness and kurtosis, and the plotting of Shepard and Pejrup diagrams in the software SYSGRAN 3.0 to infer facies distributions and sediment dynamics along the estuary.

The textural results reveal a three-class composition. Sand is the dominant class at 51 percent, followed by silt at 25 percent and clay at 24 percent. Although sand prevails, the abundance of thinner sediments is noteworthy, and it is precisely this muddy fraction that carries the greatest pollution implications. According to decades of sedimentological research, pollutants from the water column accumulate mainly in thinner rather than coarser sediments, because smaller particles offer a much larger specific surface area for surface adsorption and ionic attraction. Trace metals, in particular, show strong correlations with fine grain size fractions below 63 or 125 micrometres.

The mean grain size values varied from 2.50 phi, corresponding to fine sand, in station B3, up to 7.30 phi, corresponding to very fine silt, in station C2. In most of the sampled stations the sediments were classified as very fine sand, indicating generally coarse material. This coarseness is consistent with the estuary’s high energy conditions. Environmental energy is closely related to mean grain size, with coarser sediments revealing high energy conditions and thinner sediments revealing the opposite, as earlier workers in Brazilian and Moroccan estuaries have documented.

The sediments were generally poorly sorted, with sorting values ranging from 0.70 phi in station D2 to 1.90 phi in station E1. Twelve of the fifteen stations showed poor sorting, while the remaining three were moderately sorted. Poor sorting can reflect limited reworking by waves and currents, deposition close to the sediment source, or the coexistence of many depositional processes. In the Bons Sinais Estuary, where fluvial and marine inputs mix continuously, all three explanations are plausible, and the mixing of different grain sizes is exactly what poor sorting records.

Skewness proved to be the most consequential parameter for pollution susceptibility. Values ranged from -0.30 phi in station C2 to 0.60 phi in stations B3 and E2, and nine stations were classified as very fine skewed, with three more classified as fine skewed. Higher skewness values are associated with thinner sediments, so even though the percentage of coarse material is high, the skewness reveals a considerable relative abundance of fine grains. Based on these values, the authors conclude that the estuary is susceptible to pollutant retention, because the fine fraction provides abundant binding sites for contaminants delivered by domestic and industrial sewage.

Kurtosis values spanned an unusually wide range, from 0.60 phi in stations A1, D2 and E1 to 3.70 phi in station B2. Stations B1, B2, B3 and E2 were extremely leptokurtic, while others were very leptokurtic, mesokurtic, platykurtic or very platykurtic. Variations in kurtosis reflect the degree of polymodality of the sediment distribution. When a single sediment source dominates, the distribution curve tends to be unimodal with kurtosis near one. When several sources contribute populations of different sizes, the curve becomes bimodal or polymodal and kurtosis falls below one. The pattern in the Bons Sinais Estuary indicates that although both continental and marine sediments reach the estuary, one source is dominant, most probably the marine one, given the estuary’s exceptional tidal amplitude.

The Pejrup diagram placed the stations in bands corresponding to hydrodynamic regimes from moderate to high. Stations A1, C2 and D3 fell in band II-D, with clay percentages between 50 and 80 percent and sand below 10 percent, while stations D2 and E1 occupied band II-C. Six stations, including A2, B2, B3, C1, E1 and E2, were located in band II-B, and the remaining stations fell in bands III-B and III-C. The Shepard diagram confirmed this heterogeneity, classifying stations as silty clay, sandy silty clay, clayey silt, sandy and silty sand. The high hydrodynamic regime is attributed to the semidiurnal tide, whose spring amplitudes of 3.5 to 4.0 metres are the highest of the Western Indian Ocean, and to the shallowness of the estuary, with a mean depth of about 10 metres.

A detailed reading of the Pejrup diagram reveals an inverse relationship between hydrodynamic energy and the concentration of finer sediments: the higher the hydrodynamics, the lower the vulnerability to pollutant accumulation. This means the estuary’s own vigorous tidal flushing offers some natural protection, yet the considerable muddy fraction, sustained by local morphology and by flocculation where fresh and salt water meet and pH changes promote aggregation, remains a significant sink. Pollutants retained in these fine sediments can interact with the water column, reduce contaminant mobility only slowly, and ultimately enter the trophic chain, affecting living organisms.

The estuary is a long, shallow system of 28 kilometres with an extensive floodplain, a positive salinity gradient and a well to partially mixed water column. It receives an estimated 500 to 840 cubic metres per second of freshwater from its tributaries, a flow that now depends entirely on precipitation in its own hydrographic basin since the estuary became independent of the Zambezi River. Average annual evaporation exceeds precipitation by roughly 500 millimetres, and southeast winds of 3 to 5 metres per second dominate the summer months. These climatic and hydrological settings shape the sedimentary dynamics that the grain size statistics so clearly record.

The findings carry direct implications for environmental management in the city of Quelimane and the surrounding region. Because many estuarine processes, including species distribution, pollutant transport and nutrient cycling, are intrinsically linked to bottom sediment distributions, understanding where fine sediments accumulate helps decision makers target pollutant treatment and prevent direct dumping of industrial and domestic wastes, as well as fertilizers from agricultural activities upstream. The authors propose preventing direct pollutant discharge into the estuary and establishing a pollution treatment plan for Quelimane, using the sediment maps as a guide for tracking contaminants.

For a major science audience, the study is a reminder that estuaries are sediment receivers whose evolutionary processes occur rapidly, and that their bottom sediments are a mixture of fluvial and marine materials whose texture reflects the geodynamic processes they were exposed to both at sea and on land. In the Bons Sinais Estuary, the interplay of a 4-metre tide, a 10-metre mean depth and continuous island construction and destruction at the mouth produces a sandy, high-energy seafloor that nonetheless harbours enough fine sediment to act as a pollution reservoir. The combination of statistical grain size parameters and the Pejrup diagram offers a transferable, low-cost toolkit for other data-poor estuaries, particularly along the eastern African coast, where growing urbanization raises the stakes for sediment-based pollution risk assessment.

Subject of Research: Estuarine sediment dynamics

Article Title: Understanding sedimentary dynamics and bottom sediment characteristics in Bons Sinais Estuary – Mozambique: Insights from statistical analysis and Pejrup diagram

Article References: Hoguane, C. A. M., Mativane, J., da Cruz, N. L., & Machaieie, H. A. (2024). Understanding sedimentary dynamics and bottom sediment characteristics in Bons Sinais Estuary – Mozambique: Insights from statistical analysis and Pejrup diagram. Estuarine Management and Technologies, 1(1), 7-18. https://doi.org/10.3897/emt.1.121901

Image Credits: AI Generated

DOI: 10.3897/emt.1.121901

Keywords: Understanding, sedimentary, dynamics, bottom, sediment, characteristics, Bons, Sinais, Estuary, Mozambique, Insights, statistical