Groundwater sustainability in African Metropolises: Case study from Calabar, Nigeria

Study region: Calabar, Cross River State, NigeriaStudy focus: Urban groundwater is a strategic water resource in Africa that offers a buffer against climate variability. Increasing population, extensive pollution, and climate change threaten the contribution of urban groundwater in compensating increasing freshwater demand. Modern urban areas are often established on productive alluvial aquifers because of the proximity between shallow water bodies and historical settlements. Groundwater dynamics in urban aquifers are complex. Impervious surfaces impede water infiltration, and leakage from water infrastructure contributes to groundwater recharge. In this study, groundwater dynamics of an African metropolis are assessed, and insights into aquifer protection are provided through detailed hydrogeological analysis and numerical models.New hydrogeological insights for the region: The Calabar metropolis (area of 166 km2 and 570,000 inhabitants) is characterized by a tropical climate, with annual rainfall of over 3000 mm/y and large groundwater resources enabling supply for 69% of the population. A simplified threedimensional hydrogeological model of the metropolis was proposed using USGS-MODFLOW to assess groundwater resources and recharge rates. Recharge decreased to 46% from water-rich rural areas to disadvantaged and highly urbanized communities. Aquifer recharge driven by huge rainfall dilutes the nitrogen content and pathogens, contributing to an acceptable drinking water quality during the prolonged wet season.