Articles scientifiques

6 publications

1. Commissioning-phase hydraulic loading and influent quality of a market-fed wastewater plant in Parakou, Bénin: baseline data for adaptive management

   Cossi Semevo Jean-Oreste Gnimagnon, Mohamed Moukorab Arêmou Daouda, Romaric Christian Marc Hekpazo, Akuegnon El-Bachir Cassaï Tobossi, Juste Celestin Sehonou, Martin Pépin Aina, Daouda Mama · 2026

   DOI : https://doi.org/10.2166/wst.2026.277

   Résumé

   Wastewater plants (WWPs) are widely recognised as nature-based solutions for wastewater management in West Africa, yet empirical data from their commissioning phase remain limited. This study investigates the rehabilitated Arzêké WWP in Parakou, Bénin, which treats wastewater from a large urban market through a three-stage anaerobic, facultative, and maturation pond system. A two-phase monitoring protocol compliant with ISO 5667-10 was applied, yielding 293 flow observations and 70 concurrent flow -quality pairs. Eleven physicochemical parameters were analysed: pH, electrical conductivity, total suspended solids (TSS), chemical oxygen demand (COD), 5-day biochemical oxygen demand (BOD5), total nitrogen, total phosphorus, iron, copper, lead, and zinc. Phase 1 yielded a mean daily flow of 6.14 m³/d (27.4% of design); Phase 2 confirmed underloading at 5.47 m³/d (24.4%). This resulted in a nominal hydraulic retention time of 549 days, 4.1 × the design value, with implications for sludge accumulation and greenhouse gas emissions. The influent was concentrated but biodegradable (median BOD5 983 mg/L; COD/BOD5 1.85). Trace metals were strongly associated with TSS (rs = 0.68–0.81), with inter-week increases exceeding 100% for Pb and Zn. This study provides the first commissioning-phase reference dataset for a market-fed WWP in West Africa.

   Key words : commissioning phase, hydraulic loading, influent characterisation, market wastewater, wastewater plant, West Africa

2. Characterizing nutrient pollution in urban well water using hydrochemical, statistical, and remote sensing methods.

   Hekpazo, R. C. M., Djihouessi, M. B., Elegbede, B. M., Azon, N. B. N., Alle, I. C., Nongnide, A. F. E., & Aina, M. P. · 2026

   DOI : https://doi.org/10.1007/s44274-026-00556-z

   Résumé

   In peri-urban areas where access to potable water is limited, groundwater from water table frequently constitutes the primary source of water, despite its susceptibility to contamination from human activity. The present study, conducted in Abomey-Calavi (Benin Republic), sought to identify the key factors influencing nitrogen and phosphorus pollution in well water. Thirty wells were sampled over three campaigns between June and December 2021. The analysis of nutrient concentrations was conducted, and a comparison was made with national and international drinking water standards. A multidimensional approach was adopted, combining multivariate statistical analyses (Principal Component Analysis and Hierarchical Cluster Analysis), nutrient pollution indices, and remote sensing data derived from Sentinel-2 imagery. Satellite data were processed to compute the Normalized Difference Built-Up Index (NDBI), Normalized Difference Vegetation Index (NDVI), and Normalized Difference Water Index (NDWI) to characterise land-use, vegetation cover, and surface moisture around the sampled wells. Results revealed acidic conditions (pH: 3.15–5.81) and low mineralisation (272.86 µS/cm). More than 40% of the wells exhibited nitrate levels exceeding the 50 mg/L limit. Statistical analyses indicated that proximity to inadequate sanitation, human pressure, and seasonal variability significantly contributed to nutrient enrichment. Remote sensing results showed a positive correlation between NDBI and nitrate concentration (r = 0.39), while NDVI and NDWI exhibited inverse relationships, suggesting that urban expansion and reduced vegetation cover promote pollutant infiltration. Overall, the study underscores the need for integrated monitoring systems combining field data, statistical tools, and satellite indicators to assess and mitigate groundwater pollution in rapidly urbanising regions.

3. Sediment-mediated eutrophication in coastal lagoons: A systematic review of benthic nutrient fluxes and management implications

   Socohou, A. A., Djihouessi, M. B., Tigo, B. A., Hekpazo, R. C. M., Adounpke, J., & Aina, M. P. · 2026

   DOI : https://doi.org/10.1016/j.ijsrc.2026.03.004

   Résumé

   Shallow coastal lagoons are among the ecosystems most vulnerable to eutrophication; their sediments, which act as nutrient reservoirs under oxic conditions, become diffuse sources of nitrogen and phosphorus in hypoxic conditions, prolonging the eutrophic state despite reduced external inputs. This systematic review, conducted according to PRISMA-ScR, searched Web of Science, Scopus and Google Scholar; 82 publications were selected after double screening, and quantitative data from 26 lagoon/estuary ecosystems across four continents were analyzed. The meta-synthesis of 327 benthic flux measurements shows a median of 1.9 mmol·m−2·d−1 for phosphorus in lagoons (approximately × 2 of shallow lakes) and 3.8 mmol·m−2·d−1 in tropical environments, revealing a high thermal and redox sensitivity of P–Fe locks. For nitrogen, the median net flux reaches 11 mmol·m−2·d−1; 85% of positive fluxes come from NH4+ efflux (DNRA dominant in eutrophic contexts), while 34% of cases indicate a sink via N2 production (denitrification/anammox). Analysis of the controlling factors identifies background oxygen as the main lever, followed by organic load, NO3− availability, bioturbation, salinity and temperature, which together explain 82% of the variance in PO43− and NH4+ fluxes. On this basis, effective management of eutrophic lagoons rests on two inseparable pillars: (i) sustainable reduction of external inputs (a prerequisite) and (ii) treatment of internal stocks (targeted dredging, reactive capping, oxygenation/mixing) in a diagnostic-dependent and adaptive framework, complemented by nature-based solutions to stabilize gains. This summary highlights the operational importance of sediments in eutrophication trajectories and provides orders of magnitude and levers to guide the restoration of coastal lagoons. 

   Keywords Coastal lagoons; Eutrophication; Sediments; Internal nutrient recycling; Benthic fluxes

4. Multivariate assessment of riverine water quality in the Ouémé Delta (Benin): ecological suitability and adaptive monitoring design.

   Djihouessi, M. B., Tigo, B. A., Socohou, A. A., Hekpazo, R. C. M., Okpeitcha, O. V., Mahunon, S. E. R., Aina, M. P., Youssouf, A., & Mama, D. · 2026

   DOI : https://doi.org/10.1007/s10661-026-15300-6

   Résumé

   Riverine water quality in data-limited deltas is a subject that has received little attention, despite their high biodiversity value and rising agricultural and urban pressures. The present study establishes a baseline for the lower Ouémé Delta (Benin), focusing on the Ouémé and Sô rivers, and tests compact indicators for management. Monthly sampling over 24 months was conducted at ten stations. The in situ parameters encompassed temperature, pH, electrical conductivity (EC), salinity, dissolved oxygen (DO), and turbidity. The laboratory variables included total nitrogen (TN), total phosphorus (TP), orthophosphate (PO4-P), nitrate (NO3-N), ammonia (NH4-N), and chlorophyll-a. These measurements were integrated with discharge estimates from an Acoustic Doppler Current Profiler (ADCP). Multivariate analyses comprised partial correlations, regression, and regularised linear discriminant analysis. The Sô River demonstrated chronic wet-season hypoxia (DO < 1 mg/L during > 80% of July–October), whereas the Ouémé generally maintained higher DO (≥ 2 mg/L); turbidity peaked during floods (Ouémé 250 NTU; Sô 100 NTU). Nutrient levels reached 21 mg/L TN (Sô) and 344 µg/L TP (Ouémé). River identity was moderately separable, with DO, turbidity, TP, and salinity emerging as the main discriminating variables. TP was consistently explained by turbidity and PO₄–P (R2 up to 0.92 in the Ouémé; 0.72 in the Sô), whereas TN remained weakly predictable. Overall, biological suitability was primarily constrained by DO deficits, high turbidity, and nutrient pressure, with salinity acting as a seasonal filter in the Sô. A stratified–adaptive monitoring design, combining control nodes along major gradients, a parsimonious marker set and high-risk seasonal intensification, can maximise information yield under budget constraints and support SDG 6.3.2 reporting through station–season classification of ambient water quality.

5. Ecological patterns in the Porto-Novo Lagoon (Benin, West Africa): A review with implications for SDG 6.3.2 and EU-WFD readiness toward ecological status classification.

   Djihouessi, M. B., Djidohokpin, G., Hekpazo, R. C. M., Socohou, A. A., Zounon, L., Sohou, Z., Youssouf, A., & Aina, M. P · 2026

   DOI : https://doi.org/10.1371/journal.pone.0348113

   Résumé

   Urban tropical lagoons provide vital services yet face nutrient loading, macrophyte blooms, and episodic hypoxia. This scoping review compiles four decades of hydromorphological, physico-chemical, and biotic evidence for Benin’s Porto-Novo Lagoon and assesses readiness for UNEP SDG 6.3.2 Level 1 reporting and a EU Water Framework Directive (WFD)-oriented exploratory gap analysis rather than an operational assessment. The lagoon is strongly seasonal, shaped by Ouémé inflows and marine intrusions; eutrophication symptoms and floating macrophytes have intensified since the 1980s. Core SDG 6.3.2 Level 1 parameter-group proxies (pH, DO, EC, PO₄-P) are moderately covered, but nitrogen evidence is scarce and time-continuous station series are rare. A completeness audit indicates about 60% coverage for SDG-related variables and lower maturity for WFD biological elements, meaning that formal SDG compliance-based classification (“good and not good”) and any Ecological Quality Ratios based WFD ecological status classification cannot be derived from the compiled literature. Near-term priorities are therefore framed as readiness building, focused on strengthening consistent in situ series for the SDG core groups; context-adapted complementary sources may support gap filling and interpretation but do not substitute Level 1 requirements. For WFD, the evidence base highlights prerequisites for future work, including transitional water-body delimitation and type-specific reference conditions, standardised Biological Quality Elements monitoring, and priority-substance surveillance. Comparison with Ébrié, Lekki, and Sakumo II underscores the value of governance and monitoring designs that avoid data fragmentation.

6. Assessment of benthic nutrient fluxes from muddy sediments: The case of Lake Nokoué in West Africa.

   Socohou, A. A., Tigo, B. A., Djihouessi, M. B., Lagomasino, D., Zoumenou, G. Berny’s. M. Y., Viainon, V. M., Hekpazo, R. C. Marc., Chabi, L. C., Akpassou, B. C. R., Adounpke, J., Daouda, M., & Aina, M. P · 2026

   DOI : https://doi.org/10.1016/j.nxsust.2026.100316

   Résumé

   The muddy sediments of shallow coastal lakes, such as Lake Nokoué, are important reservoirs of organic matter and nutrients, playing a key role in eutrophication processes. This study assesses the benthic flux of nitrogen and phosphorus from these sediments by incubating core samples in the laboratory for 24 h under different light and hydrodynamic conditions. The average 24-hour fluxes recorded under static conditions reached 0.20 ± 0.026 mmol m⁻² h⁻¹ for dissolved inorganic nitrogen (DIN) and 0.014 ± 0.005 mmol m⁻² h⁻¹ for dissolved inorganic phosphorus (DIP). Ammonium (0.145 ± 0.024 mmol m⁻² h⁻¹) is the main form of nitrogen released (72%), indicating high ammonification activity due to bacterial degradation of organic matter. Nitrates (0.053 ± 0.01 mmol m⁻² h⁻¹) and nitrites (0.004 ± 0.001 mmol m⁻² h⁻¹), totalling 28%, indicate the contribution of nitrification and denitrification processes modulated by redox potential. The fluxes measured under static and dynamic conditions did not show a statistically significant difference, suggesting that nutrient exchanges are mainly governed by molecular diffusion rather than advective transport. Nocturnal fluxes, representing 75% of DIN and 55% of DIP, reveal an intensification of remineralisation and nutrient mobilisation when photosynthetic assimilation decreases. DIP flux increases under reducing conditions, due to the desorption of iron- and phosphorus-rich organic matter. These results confirm the role of the muddy sediments of Lake Nokoué as an internal nutrient pump, maintaining the enrichment of the water column even in the absence of external inputs.