In this study, we focus on the recent export balance and monitoring of seasonal and annual variability of dissolved major elements transported over the last two decades (2006–2022) by the Congo River to the Atlantic Ocean at the Brazzaville station, which controls more than 98% of the total area of the CRB. The results obtained were compared with those of previous work, in order to identify potential changes in the chemical composition of the surface waters of the Congo River. The mean interannual fluxes of Total Dissolved Solid (TDS) transported by the Congo river, calculated at this station, towards Atlantic Ocean is 37 × 106 t yr −1, dominated by HCO3 (13 × 106 t yr −1), SiO2 (12 × 106 t yr −1) and Ca (3 × 106 t yr −1) for a mean flow rate of 40,465 m3 s−1. These results show that the flow rate of the Congo River is largely the main explicative variable to explain the chemical behaviour of the River water. Four distinct origins of dissolved chemical elements were identified: Ca, Mg, Na, SO4, HCO3 from subsurface and groundwater; K, Cl, SiO2 from surface runoff through soil leaching; DOC testifying the surface leaching through large flooding; and NO3 from local pollution marked by autochthonous origin. However a large increase of DOC by 35% and of K, Ca, SiO2 (by respectively 14, 10 and 12%) is recorded, probably due to both the increase of flooding period and also the local anthropogenic pollution nearby Brazzaville. To conclude, the chemical weathering is quite stable due to the weak change of the large floodplain forest and still to the very small anthropogenic inputs.The CRB is still a natural system. Graphical Abstract This study is the first after more than two decades to publish details of the chemical composition of the waters of the Congo River. It was carried out to analyze the current chemical composition of the surface waters of the Congo River at the Brazzaville/Kinshasa station. It investigates the seasonal variability of the main dissolved elements during the hydrological cycle, determines their origins and assesses their recent fluxes transported by the Congo River to the ocean; then detects potential changes in these waters. 16 hydrological cycles of dissolved major elements (determined using the SO-HYBAM protocol: <a href="https://hybam.obs-mip.fr/fr/protocoles/analyses-de-laboratoire/">https://hybam.obs-mip.fr/fr/protocoles/analyses-de-laboratoire/</a>) were processed using various statistical analyses (means, standard deviation, Principal Component Analyses, ANOVA, flux and correlation calculations) and graphs were designed using OriginPro version 9, Diagrams, Hydraccess, Minitab version 18 and Excel. The results obtained were then compared with those obtained in previous studies to identify potential changes in Congo River water chemistry over recent decades. These river waters are dominated by HCO3, SiO2 and Ca. The PCA reveals four origins: Ca, Mg, Na, HCO3 and SO4 in CRB waters come from the weathering of silicate rocks; SiO2 and K from soil leaching, NO3 from local anthropogenic pollution around Brazzaville and DOC from litter leaching in the Cuvette Centrale.

Analysis of Chronicles of Dissolved Major Elements Transported by the Congo River to the Atlantic Ocean to Detect Possible Changes in its Hydrochemistry

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发表日期：

2025年7月7日

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Sandra M. Bayonne Padou,
Guy D. Moukandi N’Kaya,
Edmond Malanda,
Ersilia I. Batsa Lahou,
Christelle Lagane

摘要

In this study, we focus on the recent export balance and monitoring of seasonal and annual variability of dissolved major elements transported over the last two decades (2006–2022) by the Congo River to the Atlantic Ocean at the Brazzaville station, which controls more than 98% of the total area of the CRB. The results obtained were compared with those of previous work, in order to identify potential changes in the chemical composition of the surface waters of the Congo River. The mean interannual fluxes of Total Dissolved Solid (TDS) transported by the Congo river, calculated at this station, towards Atlantic Ocean is 37 × 10⁶ t yr ⁻¹, dominated by HCO₃ (13 × 10⁶ t yr ⁻¹), SiO₂ (12 × 10⁶ t yr ⁻¹) and Ca (3 × 10⁶ t yr ⁻¹) for a mean flow rate of 40,465 m³ s⁻¹. These results show that the flow rate of the Congo River is largely the main explicative variable to explain the chemical behaviour of the River water. Four distinct origins of dissolved chemical elements were identified: Ca, Mg, Na, SO₄, HCO₃ from subsurface and groundwater; K, Cl, SiO₂ from surface runoff through soil leaching; DOC testifying the surface leaching through large flooding; and NO3 from local pollution marked by autochthonous origin. However a large increase of DOC by 35% and of K, Ca, SiO₂ (by respectively 14, 10 and 12%) is recorded, probably due to both the increase of flooding period and also the local anthropogenic pollution nearby Brazzaville. To conclude, the chemical weathering is quite stable due to the weak change of the large floodplain forest and still to the very small anthropogenic inputs.The CRB is still a natural system.

Graphical Abstract

This study is the first after more than two decades to publish details of the chemical composition of the waters of the Congo River. It was carried out to analyze the current chemical composition of the surface waters of the Congo River at the Brazzaville/Kinshasa station. It investigates the seasonal variability of the main dissolved elements during the hydrological cycle, determines their origins and assesses their recent fluxes transported by the Congo River to the ocean; then detects potential changes in these waters. 16 hydrological cycles of dissolved major elements (determined using the SO-HYBAM protocol: https://hybam.obs-mip.fr/fr/protocoles/analyses-de-laboratoire/) were processed using various statistical analyses (means, standard deviation, Principal Component Analyses, ANOVA, flux and correlation calculations) and graphs were designed using OriginPro version 9, Diagrams, Hydraccess, Minitab version 18 and Excel. The results obtained were then compared with those obtained in previous studies to identify potential changes in Congo River water chemistry over recent decades. These river waters are dominated by HCO₃, SiO₂ and Ca. The PCA reveals four origins: Ca, Mg, Na, HCO₃ and SO₄ in CRB waters come from the weathering of silicate rocks; SiO₂ and K from soil leaching, NO₃ from local anthropogenic pollution around Brazzaville and DOC from litter leaching in the Cuvette Centrale.