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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

Abstract

Texte intégral

1The great successes achieved by the Onchocerciasis Control Programme in West Africa (OCP) in the large-scale control of the blinding savanna form of onchocerciasis (river blindness) have allowed vast tracts of previously abandoned valleys to be resettled and developed. Among the many issues and problems that have to be addressed, to ensure the most effective long-term management of settlement and development in these onchocerciasis-controlled areas, is that which relates to the potential impact of those development activities on the environment.

2Despite the fact that for many years anti-blackfly larvicides were regularly applied to the rivers of the OCP area, the quality of the aquatic environment was preserved, partly because of the precautions taken by the OCP, but largely because of the absence of human population pressure in the areas most severely affected by onchocerciasis. However, the situation is now changing—rapidly in parts of the OCP area—and there is increasing concern for the welfare of both aquatic and terrestrial environments.

3In this context, it was recognized that there was a need for the formulation of an appropriate methodology for environmental impact assessment. In 1991 the OCP Committee of Sponsoring Agencies (CSA) started to address this issue, and in 1993 launched a pilot project in one selected basin (the Upper Léraba Basin), the results of which are described in this report.

4The primary objectives of the Pilot Project were i) assessment of the present environmental situation in the Basin, with a view to determining potential sources of impact on the aquatic environment, ii) quantification of Chemical loads, and assessment of modification to the physical environment, and iii) identification of simple study methods which would be applicable to other areas and which could be offered as examples to predict environmental impacts of settlement and development, and offer ways to minimize those impacts.

5Through the collection of geographic information and data on land utilization, as well as by an assessment of loads (fertilizers, pesticides, human and livestock wastes), an ecotoxicological impact evaluation of human activities was made, at basin scale, in the Upper Léraba Basin. More detailed evaluations of changes in the physical environment were made at medium and local scales of resolution, with special attention being given to analyses of changes in land utilization patterns between 1972 and 1993, and to determining the ecological significance of the observed changes.

6It was concluded that organic loads and nutrients were only of relevance along limited stretches of the main Léraba rivers, where settlements were located close to the river banks and provided point sources of contamination. The contamination of river water by pesticides used for cotton protection was calculated by means of simple models which also permitted the theoretical concentrations of pesticides in river water to be estimated. Although the predicted concentrations of pesticides in river water were not at levels that could give cause for alarm, they could however be considered as early warning signals.

7With regard to physical changes in the environment, it was concluded that about 75 per cent of the original savanna woodland had been cleared for settlement and for agricultural development (most of it over the last decade). The riverine forests of many of the smaller rivers and streams had been destroyed, and on some cleared river banks the first signs of soil erosion were detectable.

8On the positive side, it was concluded that there had not been any significant disturbance of the riverine forests and associated floodplain grasslands of the main rivers.

9The overall conclusion from the Pilot Project was that, on the basis of studies which were limited both in 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. In relation to the Pilot Project’s third main objective, the investigations clearly demonstrated that the techniques employed—using input data that were very basic/but readily available from national agricultural and development authorities, and by the application of standard hydrobiological techniques and simple models—can be used as examples of quick and reliable environmental impact assessment methodologies, which are applicable to other, similar areas.

© IRD Éditions, 2003

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