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Trente ans de lutte contre l’onchocercose en Afrique de l’Ouest. Traitements larvicides et protection de l’environnement

 | 
Laurent Yaméogo
, 
Christian Levêque
, 
Jean-Marc Hougard

Environmental Impact Assessment of Settlement and Development in the Upper Léraba Basin

Conclusions

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1It is concluded that, at least for the time being, organic loads and nutrients in the upper Léraba basin are only of relevance along limited stretches of the main Léraba rivers, where settlements are located close to the river banks and provide point sources of contamination, with localized effects on river water quality and thus on fish and aquatic invertebrates.

2Assuming that the input data for the SoilFug model were reasonably correct, it can be concluded that contamination of river water by pesticides used for cotton protection, could be clearly detected. From an ecotoxicological viewpoint, it is deduced that the predicted concentrations of pesticides in river water are not yet at levels that could give cause for alarm. However, if for example, the predicted concentrations are considered in relation to the stricter EEC standards for river water quality, one is obliged to take a more serious view of the fact that acceptable levels of concentration were exceeded for several days each year.

3With regard to the physical environment it can be concluded that about 75% of the original savanna woodland has now been cleared for settlement and for agricultural development (most of it during the last twenty years), or is in the process of being destroyed. Most probably the remaining woodland will disappear in the not too distant future, resulting in overall environmental deterioration and the risk of increased albedo. Likewise, the riverine forests of many of the smaller rivers and streams have been or are being destroyed, resulting in greater risks of increased runoff of chemical substances (pesticide, nutrients, etc.) into the main rivers.

4On the positive side, it is concluded that there has not been any significant disturbance of the riverine forests and associated floodplains of the main rivers. However, it is to be expected that before long, when wood becomes progressively less readily available in the woodland savanna, the main riverine forests will become threatened.

5The overall conclusion from the Pilot Project is that on the basis of studies that were limited in both time and scope, it was possible to make valid and meaningful assessments of the Upper Léraba basin in terms of Chemical contamination of river water, of localized changes in the biological condition of the aquatic ecosystem, and of physical degradation of the woodland savanna component of the terrestrial environment.

6By inference, it is concluded that the first two main objectives of the Pilot Project have been attained, it being understood that it was never the intention of the Project to conduct very precise and exhaustive quantitative surveys of the study area, but rather to identify and briefly explore those elements of the Léraba situation that were most pertinent for further, more intensive investigations, and for management purposes.

7While it cannot be claimed that the SoilFug runoff model was expected to provide very precise information on river water contamination, the authors concluded that the output data from the model enabled them to obtain a good overall impression of river water contamination in relation to local pesticide use. And here it is important to report that in a very recent study, Di Guardo et al. (1994b) provided evidence to support the authors’ conviction that their choice of the SoilFug model was well-founded. In a detailed investigation, in which the model was applied to a farm situation, Di Guardo and his colleagues clearly demonstrated that there was a close correlation between modelled and measured concentrations of several pesticides in surface water.

8Finally, the authors consider that in relation to the Pilot Project’s third main objective, their investigations have clearly demonstrated that the employed techniques, using input data that were very basic, but readily available from national agricultural and development authorities and from the application of standard hydrobiological techniques, can be used as examples of quick and reliable environmental impact assessment methodologies for use under similar circumstances in other areas.

© IRD Éditions, 2003

Conditions d’utilisation : http://www.openedition.org/6540