Challenges in groundwater resource management in coastal aquifers of East Africa: Investigations and lessons learnt in the Comoros Islands, Kenya and Tanzania

•Groundwater is a key resource for development in coastal East Africa.•Resource availability is driven by physical, environmental and societal factors.•Natural saline intrusion and governance failures affect water quality in the region.•Demographic pressure and environmental change are limiting groundwater availability.•Understanding of the resource and community engagement are key to sustainability.