Modelling Seawater Intrusion into Multi-layered Aquifer System of Lagos Area, Nigeria
Investigation as indicated that in the immediate vicinity of Lagos metropolitan, four separate aquifers are tapped. Over-exploitation of these aquifers through boreholes for both domestic and industrial water supply has resulted in salt water intrusion occurring in the confined aquifers. Monitoring and management of salt water intrusion is not done in Nigeria hence there is a need to study and ascertain the extent of sea water intrusion along the coastal region of Nigeria.
Data were collected from the boreholes within the studied area to model a two-dimensional advection-dispersion problem. The basic differential equation was solved for two-dimensional transport in an incompressible porous medium and transport in the absence of sources. The velocity flow was solved as a separate problem, independent of the solution for the concentration. A computer program was developed to solve the two dimensional dispersion problems by the finite element method. The model was calibrated in two steps, with the transient runs modelling Chloride concentration for the three aquifers. The runs simulated the 1982 and 1996 chloride concentration and predicted what condition it will be in 2010.The result indicated that the area of chloride concentration increased from 13km2 in 1996 to 38km2for upper coastal plains sands while the concentration increases from 68mg/L to 83mg/L. Comparing the simulated chloride for all the three aquifers Abeokuta formation has the lowest value of chloride concentration. This confirms that Abeokuta formation is freshwater aquifer. Generally the results obtained for chloride concentration agrees with report of Coode Blizard Ltd.
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