Inferring the heterogeneity, transmissivity and hydraulic conductivity of crystalline aquifers from a detailed water-table map

Abstract

Estimating the transmissivity or hydraulic conductivity field to characterize the heterogeneity of a crystalline aquifer is particularly difficult because of the wide variations of the parameters. We developed a new approach based on the analysis of a dense network of watertable data. It is based on the concept that large-scale variations in hydraulic head may give information on large-scale aquifer parameters. The method assumes that flux into the aquifer is mainly sub-horizontal and that the water table is mostly controlled by topography, rather than recharge. It is based on an empirical statistical relationship between field data on transmissivity and the inverse slope values of a topography-reduced water-table map. This relationship is used to compute a transmissivity map that must be validated with field measurements. The proposed approach can provide a general pattern of transmissivity, or hydraulic conductivity, but cannot correctly reproduce strong variations at very local scale (less than10 metres), and will face of some uncertainties where vertical flows cannot be neglected. The method was tested on a peridotite (ultramafic rock) aquifer of 3.5 km 2 in area located in New Caledonia. The resulting map shows transmissivity variations over about 5 orders of magnitude (average LogT:-5.2±0.7). Comparison with a map based on measured water-level data (n=475) shows that the comparison between LogT-computed values and LogT data deduced from 28 hydraulic tests is estimated with an error less than 20% in 71% of cases (LogT±0.4), and with an error less than 10% (LogT±0.2 on average) in 39% of cases. From this map a hydraulic-conductivity map has been computed showing values ranging over 8 orders of magnitude. The repeatability of the approach was tested on a second data set of hydraulic-head measurements (n=543); the mean deviation between both LogT maps is about 11%. These encouraging results show that the method can give valuable parameter estimates, and can characterize aquifer heterogeneity. The computed LogT and LogK maps highlight the spatial distribution of parameters that show a pattern clearly controlled by the fault network of this ultramafic massif. However, the faults are mainly characterized by low-permeability zones; this differs from results on other crystalline aquifers and may be due to the fact that weathering products of peridotite are clay-like materials. The resulting transmissivity map can be used as a starting point for modelling or to direct additional fieldwork.

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

“…Dewandel et al (2010Dewandel et al ( , 2017b show that the aquifer-scale vertical distribution of the drainage porosity (or specific yield) decreases with depth from 1.5% to 0.2% over a range of depth of 30-40 m below the base of the saprolite. Dewandel et al (2012) demonstrate, in a granite aquifer in India (see also Dewandel et al 2017c for similar results on a peridotite aquifer), the relatively good homogeneity of the SFL hydraulic conductivity at the scale of aquifer subareas. Hydraulic conductivity values range from 10 −7 to 10 −4 m/s in the whole aquifer, but are distributed over zones up to 1 km 2 in area with similar hydraulic conductivity.…”

“…Feldspars do not swell during weathering even if the transformation of plagioclase into clay minerals begins as early as the biotite transformation. In mafic and ultramafic rocks such as peridotites, weathering conduces swelling of olivine and pyroxene and consequently facilitates intense fracturing of the rock (Dewandel et al 2005(Dewandel et al , 2017cRudge et al 2010). Secondarily, pseudo-karstification may develop in such hard rocks below the saprolite cover mainly made up of limonite (Genna et al 2005).…”

Structural analysis and geophysical survey for hydrogeological diagnosis in uranium mine, Poços de Caldas (Brazil)

“…The energy released during detonations amplifies the existing structural arrangement in terms of continuity and connectivity of fractures and creating new ones. This peculiar aspect provides a structural framework that potentiates the underground flow both in the preexisting system and in the secondary one [52,53].…”

Applications of Aeromagnetic and Electrical Resistivity Data for Mapping Spatial Distribution of Groundwater Potentials of Dutse, Jigawa State, Nigeria

“…According to Obaje (2009), the basement complex of north central Nigeria is subjected to high degree of weathering which led to the formation of thick weathered basement in some places. Studies on borehole logs within the area revealed that the aquiferous zones mainly occur within this weathered and fractured zones (Dewandel et al, 2017;Ismail & Yola, 2012).…”