Estimation of groundwater recharge in weathered basement aquifers, southern Zimbabwe; a geochemical approach

Abstract

Geochemical techniques have been used to estimate groundwater recharge and its spatial variability in basement terrain in a semi-arid area of southern Zimbabwe. Recharge rates estimated by chloride mass balance have been determined in the Romwe Catchment, a small (4.6 km 2 ) headwater catchment underlain by banded gneisses, with good hydrological and geological control. The results support the findings from piezometric monitoring that there are significant differences in hydrogeological properties of weathered basement derived from different primary lithologies. Annual recharge estimates for shallow weathered aquifers derived from melanocratic bedrock (dominated by pyroxene gneiss), 22 mm, and leucocratic felsic bedrock, 6.7 mm, are 3.7% and 1.1% respectively of the long-term mean annual rainfall. The significant uncertainties associated with the chloride mass balance recharge estimates are discussed. Groundwater derived from each lithology generally has a distinctive geochemistry (Na/Cl, K/Na, Mg/Ca, Na/Cl, B, Ba). The information from the Romwe catchment control area was then scaled up using information from remote sensing images (which defined areas of dark and light soils above the banded gneiss) to confirm the higher recharge rates in the melanocratic lithology in the unexplored Greater Romwe (225 km 2 ) area. It is concluded that properly calibrated, remote sensing images could be further regionalized to site groundwater sources in basement terrain, providing a relatively inexpensive development tool.

Water Balance Components for Sustainability Assessment of Groundwater‐Dependent Agriculture: Example of the Mendae Plain (Tigray, Ethiopia)

“…This made it possible to use the CMB for estimating aquifer recharge (Eriksson & Khunakasem, 1969). The CMB method has successfully been applied for estimation of groundwater recharge in weathered basement aquifers (MacDonald & Edmunds, 2014).…”

Investigating the Productivity and Sustainability of Weathered Basement Aquifers in Tropical Africa Using Numerical Simulation and Global Sensitivity Analysis

“…Furthermore, well performance data, particularly yield, are subject to a high degree of uncertainty, limiting the effectiveness of these relationships for well siting and productivity mapping (Chilton & Foster, 1995; Taylor & Howard, 2000). Recent studies addressed the issue of predicting the potential productivity of WBAs by focusing on upscaling and mapping of the hydrogeological properties with various approaches: geostatistical methods (Razack & Lasm, 2006); relationships between geophysical and hydrogeological properties (Chandra et al, 2008; Meju et al, 2001; Shishaye et al, 2019); multicriteria analysis using GIS (Lachassagne et al, 2001); large‐scale fluctuations in hydraulic heads (Dewandel et al, 2017) in combination with satellite data (Dewandel et al, 2017; Macdonald & Edmunds, 2014); the integration of borehole data and geological mapping (Courtois et al, 2010; MacDonald et al, 2012); and probabilistic and machine learning models (Ahmed & Mansor, 2018).…”

Hydrogeological characterisation of regional faults and dolerite dykes in the Precambrian Basement and Karoo Supergroup (Tete Province, Mozambique)

“…The weathered/ regolith zone (depending on the overall thickness) and the fractured Basement rock matrix form a composite aquifer unit. Basement aquifers are characterised by a complex network of fractures resulting from weathering, lithostatic decompression, and neotectonics (Bakundukize et al, 2016;Macdonald and Edmunds, 2014). Weathering depth is extremely variable and depends on numerous factors and may reach 80 m, even in semi-arid to arid regions.…”