Application of environmental isotopes in sustainability assessment of the groundwater resources of Lagos Coastal Basin (LCB), south-west, Nigeria

Abstract

Owing to the fact that groundwater is the main source of water supply for both domestic and commercial activities by the inhabitants of the Lagos Coastal Basin (LCB) in Lagos, Nigeria, detailed study with respect to recharge and Mean Residence Time (MRT) of deep groundwater samples across the area was carried out to enable proper management of this resource. The study used environmental isotopes of water particularly carbon isotope and tritium (3H) contents in the groundwater to determine the mean residence time, and stable isotope of 2H and 18O to reveal origin of the recharging water. The stable Isotope of δ18O and δ2H plot around and along the local meteoric water line indicating meteoric origin, while the tritium results range between non-detectable and 0.3 TU in the confined aquifer connote old recharged water. This agrees with the old recharged water indicated by low 14C activities that resulted in MRT range from 7400 ± 10 to 12,030 ± 10 years. Comparison of the integrated results of the shallow unconfined aquifer with the deep confined aquifers showed that the shallow aquifer was younger and recharged in Holocene to present (modern climatic condition), whereas the confined deep groundwater was older and it recharge took place during Late Pleistocene/early Holocene (more humid condition than present). Consequently, the unconfined shallow aquifer may be considered renewable while the confined deep groundwater is being mined under non-steady state condition. Therefore, increasing water demand by various economic sectors require planned intervention in groundwater resource management of the LCB to achieve sustainable development.