Abstract
Regional three-dimensional groundwater-flow and saltwater transport models were built to analyse saltwater intrusion in the Great Maputo area, southern Mozambique. Increased water demand has led to many private groundwater abstractions, as the local public water supply network has already reached maximum capacity. Pushing for new strategies to tackle the water-supply shortages exposes the aquifer system to saltwater intrusion from entrapped fossil saline groundwater and seawater. Previous attempts at modelling have been frustrated by data limitations. This study compiled all the available data to build the models, which were subsequently calibrated with observed heads, discharges and salt concentrations. The transport models were used to test hypotheses of potential sources of saltwater resulting in the current salinity distribution. Furthermore, scenarios were simulated to assess the impacts of sea-level rise and projected groundwater abstractions. Results show that saline groundwater is widely distributed in the aquifer’s western sector, where it is a limiting factor for groundwater development, and seawater intrusion is a risk along the coastline. Newly constructed wells (46) along the Infulene River can be operated with some impacts of saltwater upconing and must be closely monitored. Although current groundwater abstractions (60,340 m3/day) are still small compared with groundwater recharge (980,823 m3/day), larger volumes of abstraction are feasible only when using a high number of production wells further away from the city with relatively low yields to avoid saltwater upconing. Capture of fresh groundwater upstream of discharge areas by wells for water supply is possible while maintaining groundwater discharges for groundwater dependent ecosystems.
Highlights
Coastal groundwater resources are often the main freshwater source for densely populated areas, and at the same time vulnerable to climatic variations and anthropogenic pressures (e.g. Nelson 2013; Unsal et al 2014; Michael et al 2017; Post and Werner 2017)
This study aims to develop such numerical tools to: (1) analyse the current saline groundwater distribution originating from different hypotheses of saltwater sources and (2) assess the potential of groundwater resources development under saltwater intrusion threats
Regional groundwater flow and saltwater transport models were constructed based on limited hydrogeological data and preliminary calibrations were performed in this study
Summary
Coastal groundwater resources are often the main freshwater source for densely populated areas, and at the same time vulnerable to climatic variations and anthropogenic pressures (e.g. Nelson 2013; Unsal et al 2014; Michael et al 2017; Post and Werner 2017). Coastal groundwater resources are often the main freshwater source for densely populated areas, and at the same time vulnerable to climatic variations and anthropogenic pressures Several studies applied numerical saltwater transport models for the simulation of saltwater intrusion and assessment of climatic and human impacts, including those presented for the Pioneer Valley in Australia (Werner and Gallagher 2006), for the coastal area in the Danish-German border adjacent to the Wadden Sea (Meyer et al 2019), for the Neogene aquifer in Flanders, Belgium (Coetsiers et al 2004), for Kiribati (Lal and Datta 2019) and Kish (Ataie-Ashtiani et al 2013) Islands in the Pacific, and for Paleo-modelling of coastal saltwater, the Netherlands (Delsman et al 2014). Variable density groundwater flow modelling can be used to assess the current state of the freshwater/saltwater distributions and to predict effects of future management decisions (Oude Essink et al 2010)
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