Gaza Strip Libya MODFLOW/SEAWAT Nile Delta aquifer Nile Delta governorates Radon SGD SGD model Tongatapu Tripoli aquifer arid and semi-arid regions artificial neural network atoll atoll island cation exchange climate change coastal aquifer direct prediction extraction fish ponds flooding fresh groundwater volume freshwater lens freshwater resilience freshwater-saltwater interface geophysics groundwater groundwater resources groundwater storage