Application of a new hydrogeological conceptual model of basement aquifers: Structuring and distribution of hydrodynamic properties. Critique and revision of piezometric mapping methods in a sub-basin of the Cavally River in western Côte d'Ivoire

Abstract

<p>The Montagnes District is a metamorphic rocks area from Western Côte d’Ivoire covering an area of about 31,000 km<sup>2</sup>. Its capital is the city of Man. It is home to the largest industrial gold mine in Côte d'Ivoire, at Ity, with an annual production of 6 to 7 tons. Artisanal mining activities are also carried out in several places in the region. These activities can threaten groundwater, both in terms of quantity and quality. Therefore, a good understanding of the structure and functioning of aquifers in these mining areas is needed to prevent these impacts and to develop mining activities that are as environmentally friendly as possible.</p><p>This problem raises more generally the question of the validity of a hydrodynamic model explaining the origin of the permeability of hard rock aquifers in order to optimize groundwater resource management methods and better understand their vulnerability. To address this question, this work was able to rely on a statistical analysis of a database of 1654 boreholes.</p><p>Results shows that the structure of the aquifer is similar to that observed in several other hard rock areas in the world: it developed due to weathering processes, comprises the capacitive saprolite, 10–20 m thick on average, and an underlying transmissive fractured layer, overlying the unweathered impermeable hard rock. The fractured layer is about 80 m thick, the first 40 to 45 metres being its most productive zone, with a 11 m<sup>3</sup>/h median productivity.</p><p>The characterization of groundwater flow has led to the proposal of a new approach in the context of scarce piezometric data.</p><p>Finally, this research shows that the impacts of mining activities are local, limited mainly to the downstream part of the topographic watersheds where the mines are sited. There, groundwater quality and quantity may be affected, as well as the streams that drain these aquifers. In fact, surface water drains groundwater and therefore collects possible contaminants, downstream of the mines. A new approach to assessing the vulnerability of hard rock aquifers to contamination from mining has therefore been proposed.</p>