Analysis and numerical modelling of large-scale controls on aquifer structure and hydrogeological properties in the African basement (Benin, West Africa)

Abstract

The metamorphic basement units of the Upper Ouémé watershed in Benin have been investigated to identify the structural controls on aquifer properties, groundwater flow and water balance at large scale. Spatial analysis of borehole and hydrogeophysical data suggests that large-scale weathering profiles, aquifer transmissivity and storage properties are better correlated to a palaeo-weathering surface. Multi-model analysis, combined with assessment of nine transient numerical groundwater models against observations, suggests the best conceptualizations are those where hydraulic conductivity and specific yield are distributed within a weathered zone determined through interpolation of weathered zone thickness. When compared to previous studies, the general groundwater balance of simulated models suggests the groundwater system contributes, on average, 49.8 m3 s−1 to the river flow (mostly during the rainy season). The same volumetric flow would be lost to groundwater evapo-transpiration and deep/lateral drainage of the catchment. Borehole abstraction (about 7.5 m3 s−1) represents only 6% of the average groundwater recharge and 1% of the average rainfall. This suggests that despite relatively low borehole productivity, the basement aquifer system still has an important unused potential for rural to mid-scale water supply and that, at present, the main external drivers for groundwater resource sustainability are changes in climate and land use.

Review: Hydrogeology of weathered crystalline/hard-rock aquifers—guidelines for the operational survey and management of their groundwater resources

“…The demonstration that HRA are constituted by rather homogeneous layers (saprolite and SFL) at least at the scale of a few hundreds of meters allows their deterministic mathematical modelling with "classical models" similar to those used for sedimentary aquifers modelling (see for instance Lachassagne et al 2001a;Leray et al 2013;Baiocchi et al 2014;Durand et al 2017;Dickson et al 2018). This was performed for water resources evaluation and to propose management scenarios.…”

“…The role of weathering in creation of the fractured layer is also directly or indirectly confirmed by the data sets of most of the authors working on HRA. See for instance Davies et al (2014) and the countries cited previously, as well as Chilton and Smith-Carington (1984), Chilton and Foster (1995), Howard (1999, 2000), Chirindja et al (2017), Dickson et al (2018), Muchingami et al (2019) and Vassolo et al (2019) for Africa; Chambel (2014) for Portugal; Comte et al (2012) for Ireland; Tan et al (2017) for Denmark; Riber et al (2017) for Sweden; Francés et al (2014) for Spain; Krasny (1996) for Central Europe; Baiocchi et al (2014Baiocchi et al ( , 2016 for the Mediterranean Europe; Chou et al (2014) and Chuang et al (2016) for Taiwan; David et al (2014) for Australia; Riber et al (2017), Jahns (1943, Hart (2016), Sanford (2017), Meyzonnat et al (2018) and Setlur et al (2019) for North America, etc. Even if several authors have not attributed the fractures to weathering processes, some others keep an ambiguity on this genetic relationship, while others do not challenge or discuss this issue at all, at least for the uppermost crust (0-2 km; Achtziger-Zupancic et al 2017;Ingebritsen and Gleeson 2017).…”

Improving our ability to model crystalline aquifers using field data combined with a regionalized approach for estimating the hydraulic conductivity field

“…Despite this strong natural heterogeneity, even in the latest modelling studies on crystalline aquifers, a single property value is assigned to each layer or on large compartments (Join et al, 2005;Ahmed and Sreedevi, 2008;Rivard et al, 2008;Goderniaux et al, 2013;Leray et al, 2013;Kolbe et al, 2016;Marçais et al;Durand et al, 2017;Dewandel et al, 2017a;Dickson et al, 2018;Bianchi et al, 2020). Whether built to test conceptual models for the understanding of such complex aquifers, to assess the flow distribution or residence-times at various scales, to evaluate average aquifer properties, to predict aquifer productivity or to define a sustainable management of the water resource, these studies illustrate our difficulties to introduce the spatial variations of hydrodynamic properties in models.…”

In Situ Observations and Lumped Parameter Model Reconstructions Reveal Intra‐Annual to Multidecadal Variability in Groundwater Levels in Sub‐Saharan Africa

“…No pumping test data are available for the remaining three sites (Dingasso, Binde, and Ouda), and no data for unconfined storage coefficients are available for any of the sites. The resultant calibration ranges for K and S (Table 2) therefore represent a "best estimate," encompassing the K estimates derived above and considering the pumping test data quality and ranges reported for weathered basement aquifers in Africa (MacDonald et al, 2008;Ouedraogo et al, 2016;Wright, 1992) and West Africa specifically (Bianchi et al, 2020;Dickson et al, 2019;Koïta et al, 2017; SNC-Lavalin/INRS, 2011). Mackay et al (2014) and Jackson et al (2016) report significant interaction in AquiMod between hydraulic conductivity and the discharge elevation and aquifer length parameters.…”