Isotopic tracing and numerical modelling of saline groundwater discharge into Matola Wetlands, Mozambique

Population pressure, repeated natural disasters and sea-level rise associated with projected climate changes increase pressure on secure freshwater resources, notably on groundwater of the coastal areas such as the Great Maputo Area in Mozambique. The Matola River, located in the west of Maputo, is a perennial river that carries mostly brackish/saltwater originating from groundwater seepage and salinity makes the river water unusable. The main origin of the saline groundwater is assumed to be fossil seawater, entrapped in the silty marl and clay dominated aquitards for thousands of years. However, detailed studies about the salinity problem of coastal Maputo city, particularly of Matola River are very limited. This research focuses on the integration of regional hydrogeochemistry, isotopic analysis and groundwater flow models to trace the source and evolution of saltwater in Matola wetlands. The Piper plot, Stiff map and bivariate diagrams of major ions reveal salinization through mixing with seawater followed by ion-exchange as the prominent hydrochemical processes of the study area west of the Matola River. On the other hand, east of the Matola River the dominant process seems to be freshening, and this distribution is related to land cover connected recharge rates of the area and the hydraulic properties of the aquitards. Water stable isotopes (δ2H, δ18O) and 18O/Cl further confirm the mixing process between fresh and connate saline groundwater in conjunction with evaporation of shallow groundwater and surface water being the major sources of salinity. The advective transport model was developed in PMWIN based on the existing steady-state model to examine the residence time and flow paths of the groundwater, particularly for the area of Matola River and wetlands. The existing steady-state model shows the highest sensitivity to effective porosity, followed by hydraulic conductivities and model geometry. The range of travel times obtained through particle tracking was frequently large and highly dependent on the number of particles. Using the result of 13C/14C (DIC) isotopes, the evaluation of the existing groundwater flow model was undertaken. Despite several uncertainties in the tracer age calculations as well as model-simulated residence time, both studies agree the residence time of groundwater to be approximately 6000 years at maximum, with an average range of 3000-4000 years. Development of two salt transport models (SEAWAT) further aided to assess the mentioned hypotheses. The “paleo” SEAWAT model partially validates the result of 14C tracer and PMPATH simulated residence time. On the other hand, the “modern” SEAWAT model predicts fresher aquifer system within next 1000-1500 years approximately. Based on the overall findings in combination with the recent transgressional history of the area, this research infers that the Holocene connate seawater cannot be the only source of salinity in Matola wetlands; other sources such as evaporite dissolution and slow diffusion of formation water (older than Holocene period) might also stimulate the intensification of the salinity problem of Matola wetlands. The results obtained in this research can contribute to target adaptation and mitigation measures for high groundwater salinity and assist the policymakers to take feasible solutions to sustainable groundwater management for Maputo region