Contribution de la géomatique à l’analyse de la qualité des nappes souterraines : Approche intégrée des indices spectraux appliquée au site minier de Tortiya (Nord de la Côte d’Ivoire)
Abstract
L’exploitation minière exerce une pression significative sur les écosystèmes et les ressources en eau souterraine, particulièrement dans les zones de socle cristallin fissuré comme le site minier de Tortiya (nord de la Côte d’Ivoire). Cette étude évalue la contribution de la géomatique et de la télédétection satellitaire à l’analyse de la qualité des nappes souterraines à travers le rapport surface-subsurface dans un contexte minier. Les indices spectraux NDVI (végétation), NDWI (eau de surface et recharge de la nappe) et NDTI (turbidité et sols exposés) ont été calculés à partir des images Sentinel-2 en bandes visibles uniquement, grâce à une extraction automatique via Google Earth Engine (GEE). Le croissement de ces indices intégrés à QGIS a permis d’établir une carte vulnérabilité environnementale pour l’identification des zones d’activité minière, haldes et zones de référence. Le NDVI varie entre -0.19 et 0,90, le NDWI quant à lui varie de -0.97 à 0.09 et le NDTI de -0.14 à 0.30. Les résultats obtenus mettent en évidence une hétérogénéité spatiale marquée par une vulnérabilité accrue de tout le site. Quatre (04) classes de vulnérabilité caractérisent le site minier de Tortiya parmi lesquelles les fortes et très fortes vulnérabilités observées étouffent respectivement le Nord et le Sud. Enfin, l’approche intégrée des indices spectraux offre un outil efficace pour la gestion durable des ressources en eau et la surveillance environnementale dans les sites miniers. Cette méthodologie peut être appliquée à d’autres régions minières et étendue à des séries temporelles de contrôle et suivi de l’évolution des impacts environnementaux.
Mining activities exert significant pressure on ecosystems and groundwater resources, particularly in fractured crystalline basement settings such as the Tortiya mining site in northern Côte d’Ivoire. This study assesses the contribution of geomatics and satellite remote sensing to the analysis of groundwater quality through the surface–subsurface relationship in a mining context. The spectral indices NDVI (vegetation), NDWI (surface water and groundwater recharge), and NDTI (turbidity and exposed soils) were computed from Sentinel-2A imagery using visible bands only, through automated extraction via Google Earth Engine (GEE). The integration and cross-analysis of these indices in QGIS enabled the development of an environmental vulnerability map for identifying mining activity zones, waste rock dumps, and reference areas. The NDVI varies between -0.19 and 0.90, the NDWI varies from -0.97 to 0.09 and the NDTI from -0.14 to 0.30. The results reveal pronounced spatial hétérogénéité with elevated levels of vulnerability across the entire site. Four (04) vulnerability classes were identified at the Tortiya mining site with high and very high vulnerability levels, predominantly affecting the northern and southern sectors, respectively. Overall, the integrated use of spectral indices provides an effective tool for sustainable water resources management and environmental monitoring in mining areas. This methodology can be applied to other mining regions and extended to time-series analyses for monitoring and tracking the evolution of environmental impacts.
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