Improving the Methods for Processing Hard Rock Aquifers Boreholes’ Databases. Application to the Hydrodynamic Characterization of Metamorphic Aquifers from Western Côte d’Ivoire

Kouassi Aristide Aoulou,Benoît Dewandel,Severin Pistre,Yéï Marie Solange Oga,Patrick Lachassagne

doi:10.3390/w13223219

Abstract

Statistical analysis of a borehole database, linear discharges, and water strikes processing enabled an understanding of the structure, geometry and hydrodynamic properties of the metamorphic hard rock aquifers from the Montagnes District, Western Côte d’Ivoire. The database comprises 1654 boreholes among which 445 only were usable for this research work after its pre-processing. Analysis shows that the structure of the aquifer is similar to that observed in several other 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 80 m thick, the first 40 to 45 metres being its most productive zone, with a 11.3 m3/h median productivity. This research shows that metamorphic aquifers exhibit similar aquifer properties (thickness, hydrodynamic parameters) as plutonic ones and that there is interest in using such databases for research and other purposes. However, a rigorous pre-treatment of the data is mandatory, and geological data from published maps must be used instead of the geological data from the database. A previous methodology aiming at processing the boreholes’ linear discharges was improved. It notably appears that the slope method must be preferred to the percentile method.

Highlights

The present research was inspired by recent concepts of hard rock aquifers’ permeability which argue that the fractured layer is permeable due to the presence of minerals such as biotite which heal during the weathering processes, generate stresses in the rock that is at the origin of its fracturing, and at the origin of the fractured layer, causing the permeability of the hard rock aquifer [1,26]
Duplicates appeared to constitute about 40% of the database; From a methodological standpoint, our study definitely shows that the reference level for any computation in hard rock aquifer must be the base of the saprolite, and not at all the topographic surface; It shows that, when applying the method from [24], one must definitely use the slope method rather than the percentile method to define the useful thickness of the fractured layer
We observed that the structure of the aquifer is similar to the ones observed in several other areas in the world: the aquifer develops due to weathering processes, and it comprises the saprolite layer and an underlying fractured layer, respectively constituting the capacitive and transmissive parts of the aquifer, overlying the unweathered lowpermeable hard rock

Summary

Introduction

Water resources (surface and groundwater) are essential for life, ecosystems and all economic activities They are required for domestic, industrial, and agricultural needs (human food). Both for reasons of quantity, during periods of low water levels when surface water may not be available, and for reasons of quality (lower treatment costs than surface water), groundwater is being used more and more frequently throughout the world [1]. It is almost systematically utilised in African countries [2] and frequently in hard rock contexts. Hard rock aquifers generally occupy the first tens of metres below ground surface [3]

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Conclusion