Sustainable development of groundwater resources under salinity and climate change impacts in Great Maputo, Mozambique

The area of Great Maputo in Mozambique, located on the south-eastern coast of Africa is facing a severe water crisis. Due to growing demand and a problem of groundwater salinity, freshwater resources are under extreme pressure. The ongoing drought has further worsened the water supply situation, and in future, the climate variability is imposing an additional threat. Therefore, a study is carried out to understand the impact of seasonal and inter-annual fluctuations in the rainfall on the groundwater resources. The main focus is to address the potential consequence of future climatic variations on groundwater development, with a possibility of enhancing aquifer recharge. To address the objectives, a transient groundwater flow and chloride transport models are developed for a baseline period of 2010-2018. Transient groundwater recharge is computed by soil-water balance method to analyse the seasonal fluctuations. The baseline models are tested for three scenarios 1) climate change for a period of 2019-2100 by incorporating Regional Climate Model (RCM) projected data; 2) groundwater development by modelling an optimized distribution of abstraction wells; 3) managed aquifer recharge (MAR) by changing vegetation cover in the study area. Results of the baseline model indicated a rapid depletion of groundwater storage after the start of severe drought in 2015, with a drop in water levels that is more pronounced in the urban areas. The results highlight that western and northern regions contain brackish to saline groundwater, due to seepage of entrapped fossil saline water from the semi-permeable aquitard units, also discharging as baseflow into brackish surface waters and wetlands. The eastern part of Maputo City also presents a high groundwater salinity associated with the mixing with seawater in the coastal zone. Climate change is foreseen to affect the groundwater levels and storage in a negative manner in the longterm, but it becomes more threatening when an enhanced groundwater development scenario is coupled with it. The groundwater levels show a drop in all the zones but more noticeably below the Maputo City due to high well density and pumping rates. The integration of newly planned wells by the Water Utility showed a continuous decline in the water levels and increasing salinity over the coming decades. Application of both optimized well distribution and MAR strategies resulted in the inversion of the falling water levels and storage improved significantly. The importance of these strategies becomes more critical for the sustainability of the groundwater resources in the coming decades due to longer periods of drought. To continue groundwater abstraction and avoid excessive drawdown and groundwater salinization, both optimized well distribution and MAR need to be discussed with the local water resource users, planners and authorities.