Groundwater has often been described as an invisible resource, yet it is important to recognise that almost 98% of the world’s available freshwater resources are in the form of groundwater. Integrated water resources management (IWMI) acknowledges the important role of groundwater within frameworks that can sometimes be unduly surface water focused. The importance of IWRM is recognised in Sustainable Development Goal 6 ‘to ensure availability and sustainable management of water and sanitation for all’ (United Nations, 2017). Groundwater development is central to meeting this goal. A pressing need though IWRM is to recognise that many aquifers identified for resource use may cross national borders (i.e. through transboundary aquifers). It is then critical that transboundary cooperation comes into play to allow sustainable groundwater of use of these aquifers by those involved.
The current challenge faced by many countries is to accurately define the movement of groundwater through transboundary aquifers and translate this knowledge into management policies (Fraser et al, 2018). Mixing cell modelling can be used to determine groundwater flow through an aquifer system. Based on mass balance between cells, mixing cell modelling is a concept that is easy to understand and communicate. Until now, mixing cell models have not been applied to a transboundary aquifer context however it is believed that they could bridge the gap between science and policy as they can translate hydrogeological science in an accessible format to inform policy.
Malawi is a low-income developing country bordered by Mozambique, Tanzania and Zambia where transboundary aquifer knowledge is limited. The Shire River Basin is shared between Malawi and Mozambique. The Shire River within this Basin is transboundary however to date there is no scientific data to support the conceptual assumption that groundwater also flows across the international border shared between the two countries. This limits management of the resource and thus sustainable use of the aquifers within the Basin.
A transboundary aquifer within the Shire River Basin has been analysed using geochemical and stable isotope data through a mixing cell model. Results indicate that there is indeed transboundary groundwater flow from Malawi to Mozambique and that groundwater interact directly with surface water within the region. Verification of transboundary groundwater flow across the international border will now allow both counties to move forward to cooperatively manage the aquifer and ensure its sustainable development.