Geochemistry of the Lake Chad Tributaries Under Strongly Varying Hydro-climatic Conditions

Abstract

The Lake Chad Basin (LCB) is one of the main endorheic basins in the world and has undergone large level and surface variations during the last decades, particularly during the Sahelian dry period in the 1970s and the 1980s. The Chari-Logone River system covers 25% of the LCB but accounts for up to 82% of the Lake Chad water supply. The aim of this study is to investigate the dissolved phase transported by the Chari-Logone system, in order (i) to elucidate the origin and the behavior of major elements and the weathering processes in the watershed; (ii) to estimate the total dissolved flux, its variability over the last decades and the driving factors. To do so, samples were collected monthly between January 2013 and November 2016 at three representative sites of the basin: in the Chari River in "Chagoua", in the Logone River in "Ngueli" just before the confluence of both rivers, and at a downstream site in "Douguia", 30 km after the confluence. Concentrations in major elements displayed significant seasonal variations in the Chari and Logone waters. At the seasonal time scale, the comparison between the concentrations of chemical elements and the flow rates showed a hysteresis loop. This hysteresis behavior correspond to a variable contribution over time of two water bodies, fast surface water, and slow groundwater, the latter carrying higher concentrations and Ca/Na ratio, which may result from the contribution of pedogenic carbonate weathering to the dominant signature of silicate weathering. On the other hand, similar average concentrations are observed in the Chari and Logone rivers, despite contrasted annual runoff. In addition, an interannual stability of ionic concentrations was observed in the Chari-Logone river during the flood regime, both during the years covered by our monitoring (2013-2016) and during the predrought period (1969, 1972 et 1973). This situation corresponds to a chemostatic behavior, where the annual river discharge is the main factor controlling the interannual variation of chemical fluxes.

Trace Metal Enrichments in Water of the Okavango Delta (Botswana): Hydrological Consequences

“…In addition to these natural causes of changes, the anthropic activities can drastically modify this fragile equilibrium (Markofsky et al., 2017), by changing the amount of inflow (pumping, dams), and/or changing its chemical composition (agriculture, industry, city). Inside these megafans, biological activity and evaporation are the most important process that controls on water chemistry in such a confined context (Atekwana et al., 2016; Bauer et al., 2004; Frondini et al., 2019; Hammer, 1986; Mahamat Nour et al., 2020; Ramberg & Wolski, 2008; Tweed et al., 2011; Wang et al., 2018). The Okavango Delta in Botswana is poorly disturbed by anthropic activities both in the wide watershed and inland: also, it offers a good opportunity to constrain the natural process driving the water quality within an endorheic megafan.…”

A Forty-Year Karstic Critical Zone Survey (Baget Catchment, Pyrenees-France): Lithologic and Hydroclimatic Controls on Seasonal and Inter-Annual Variations of Stream Water Chemical Composition, pCO2, and Carbonate Equilibrium

“…Lastly, the rise in ion concentrations observed in stream water during autumn with the first stormflow events occurring after the strong summer evaporation period, is supposed to come from soil horizons and epikarst drainage. This "flushing effect" was evidenced by Walling and Foster [145] and later on by several authors on catchment draining various lithologies and under various climate conditions: on silicate basins such as the Mont-Lozère, the Strengbach and Ringelbach [72,111], the silicate-rich Chari-Logone River catchment in Chad [146], the Vantaa River boreal catchment in southern Finland [147], the Heidingzi agricultural catchment in China [137], and also on calcareous basins such as the Oka River forested catchment in Spain [148], the Middle Bussento Karst System in Italy [149], and the Chenqi agricultural karstic catchment [150].…”