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

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 km2) 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 km2) 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.