Quaternary sand dunes and underlying Pliocene-Oligocene materials host the key groundwater resources that provide the only source of drinking water for a large proportion of Maputo district (Mozambique). This resource is at risk due to potential over exploitation, pollution and salinization. Few hydrogeochemical studies have been conducted in the region, with generally poor quality assurance and availability of datasets. This study provides the first isotopic results to calculate groundwater residence times in the region. Analysis in monitoring bores included major ions, selected minor ions and trace elements, water stable isotopes (δ18O, δ2H), carbon stable isotopes in dissolved inorganic and organic carbon (δ13CDIC, δ13CDOC), strontium isotopes (87Sr/86Sr), radiocarbon (14CDIC) and tritium (3H).

Elevated sand dunes are parallel to the coast and constitute recharge “windows” into a phreatic (unconfined) aquifer. The presence of confining clay-rich layers below sand dunes is not continuous, favouring groundwater flow and potential transport of pollutants into a deeper semi-confined aquifer locally. Recharged water evolves from a Na-Cl-type to Na-HCO3-type groundwater with depth. This change in water chemistry is triggered by interaction with carbonate cements in underlying calcarenites and ion exchange processes. Recharge is provided by major rainfall events with smaller rains showing strong evaporation effects on water stable isotopes and limited or no influence on recharge. Groundwater residence times within Quaternary sand dunes are of a few decades as supported by measurable tritium. Deeper portions of the semi-confined aquifer (e.g. 23PZ) have no tritium and could have residence times of up to 1000 yr. Groundwater recharged through Quaternary sand dunes north of Marracuene show low major ion concentrations, low dissolved organic carbon and low nitrate concentrations suggesting a fairly healthy groundwater system. However, closer to the city centre the combined effect of large pollution sources and thinner or absent confining layers have exposed sections of the semi-confined aquifer to nitrate levels above international guidelines.