Variabilite hydroclimatologique et qualité des eaux dans un bassin aurifère en exploitation : cas du bassin versant de la Kadey (Est-Cameroun)

Abstract

The objective of this work was to study the hydroclimatological, hydrodynamic and hydrogeochemical functioning of the Kadey watershed (BVK) in a context of anthropogenic variability and/or change and forcing. The BVK is located on the South-Cameroon forest plateau in the south-east of the East Cameroon region. It is drained by the Kadey and Doumé rivers that converge to flow into the Sangha one of the tributaries of Congo in Cameroon. Its geological substrate consists of a gneissique granito base belonging to the North Equatorial Pan-African Chain (CPNE) of Cameroon, on which thick layers of ferralitic soils and hydromorphic soils are based in places. Its vegetation consists of dense forest and/or degraded forest. The main economic activities are agriculture; Livestock; logging and mining. The fieldwork collected: (i) climatic parameters (rain and temperature) at some weather stations and specialized centres of the BVK; (ii) hydrological data (flows) from the Kadey to Batouri and Pana; iii) hydrodynamic parameters (static levels) at the level of 72 hydraulic structures. In addition, 25 sampling points were selected for hydrogeochemical monitoring and spot sampling of surface and groundwater in dry and wet seasons during the period 2015 to 2016. The laboratory's work consisted of treatments, counting, analyses and interpretations of hydro-climatic and physical-chemical parameters in different laboratories in Cameroon and France, using different techniques of treatment and classical analysis and comparative statistics. The results show that, from a climatological point of view, average annual Temperatures vary at the level of rainfall stations between 22.9 and 26.1°C in the BVK. The Pettit test does not indicate any significant disruption for annual and monthly temperatures. Annual rainfall ranges from 863 to 2362 mm over the same period. They evolve differently from year to year with alternating wet and dry periods. The Pettit test reveals significant ruptures at all BVK stations with annual rainfall drops of between -9.5 and -13%. The variations in monthly and seasonal rains are characteristic of the transitional equatorial regime south of the basin and move to the two-season tropical transition regime north of the basin. Hydrologically, the annual flows of the Kadey to Batouri (8,749 km2) range from 97.9 m3/s to 218 m3/s during the period between 1954 - 2015; those of the Kadey in Pana (20,372 km2), vary between 148.4m3/s and 317 m3/s between 1964 - 2015 without taking into account the observation gap between 1981 and 1998. This, for year-to-± year-±25.6 m3/s modules (Qe - 16.80 L/s/km2, 529 mm, Ke - 30.08 percent) in Batouri and 304±28.1 m3/s (Qe - 15 L/s/km2, 470.59 mm, Ke - 28.81 per cent) Pana. When comparing the flows of the Kadey to Batouri before the gap (120 m3/s) and after the gap (104.5 m3/s) as well as those of Pana before (247 m3/s) and after the gap (215 m3/s); overall, there was a decrease in flows of about -13% on the Kadey at Batouri and Pana during the current periods compared to the old periods before the observational deficiencies. On the hydrodynamic level, there is a synchronous evolution, throughout the year, of the piezometric surface in the various catchment structures that could be the result and expression of the existence of a single and continuous subse surface aquifer throughout the basin as is often the case in a fractured crystalline base area. Hydrogeochemically, the waters of the BVK are acidic and very weakly mineralized. Several environmental factors control the water quality of the BVK: lithology, human activities and climate. Gold mining is mainly manifested by large fluctuations in turbidities and total ESM. It is also responsible for the high-profile presence of ETMs (Fe > Mn > Pb > Cr > Cu > Zn > CD > As > CN) in BVK waters.