Investigation of hydrological processes and the impacts of Acacia mearnsii plantations on groundwater in secondary aquifers: Case study at the two-stream research catchment, South Africa

Study region: Two-Streams catchment, KwaZulu-Natal Midlands, South Africa. Study focus: The impacts of Acacia mearnsii plantations on hydrological processes including groundwater is investigated using the Two-Streams experimental catchment as a case study site. Integrated hydrological, hydrogeological, hydrochemical and environmental isotope methods were adopted. Groundwater recharge is estimated using the chloride mass balance (CMB) method, and baseflow is separated using digital filters and delta O-18 isotope approaches. The direct and indirect impacts of the tree stands on groundwater including on recharge, groundwater level and baseflow are assessed. New hydrological insights for the region: Systematic analyses of all the hydrological data show that direct groundwater uptake by tree roots at the Two-Streams catchment would not be possible due to limiting root depths. However, there is clear evidence on the impacts of the tree stands on groundwater levels and baseflow at the study site. Thus, in instances where the regional groundwater is not available for direct abstraction by tree roots, trees can have considerable impacts on groundwater by extracting water from the unsaturated zone, reducing recharge to aquifers, without having direct access to the groundwater proper. The impact of Acacia mearnsii plantations on groundwater which are observed in terms of reductions in groundwater recharge and consequent reduction in groundwater levels and baseflow starts to be observed on average six years after planting.