Groundwater Modelling, An Effective Tool in the Assessment of Groundwater Contamination: A Case Study of Umunwanwa in Umuahia-South, Abia State, Nigeria

To determine the environmental suitability (impacts on groundwater) of an excavated pit located within the eastern boundary of Umunwanwa for any form of waste discharge (sewage or solid waste), modelling approach was adopted. A thorough geophysical / hydrogeological investigation carried out in the area revealed the existence of 7 layers and in the vicinity of the site an unsaturated semi-permeable zone with clay lenses. Site specific hydraulic conductivity of 9m/d was determined through grain size analysis. The groundwater flow direction of the area is from south-east to north-west and the flow system consists of two aquifers separated by a 6.1m thick confining layer. The simulation was carried out for short and long term assessment which gave rise to two sample problems.
Sample Problem 1: (Upper aquifer) which is the “Modelling of Groundwater Mound Resulting from Effluent Infiltration,” where analytical and numerical models for calculating the groundwater mound were applied. Ten analytical simulations with recharge rates varying from 0.003125m/d to 0.75m/d were carried out. After 100days there was no significant rise in the groundwater table with recharge rates less than 0.0375m/d, while recharge rates above 0.0375m/d gave significant groundwater mound. A further numerical simulation using a recharge rate of 0.75m/d showed that the results of analytical and numerical simulations are found to be in good agreement by predicting that at the end of 50 days a 12m groundwater mound beneath the centre of excavated pit is expected.
Sample Problem 2: (Lower aquifer) is the “Modelling of Solute Transport Resulting from Effluent (Leachate) Infiltration”, where a particle tracking technique introduced uses a semi-analytical particle tracking scheme that allows an analytical expression of the particle’s flow path to be obtained within each finite-difference grid cell. Particle paths are computed by tracking particles from one cell to the next until the particle reaches a boundary, an internal sink/source, or satisfies some other termination criterion while keeping track of the time of travel for particles moving through the system. After 15 years, the contaminant plume was able to reach the well head. This has shown the effectiveness of groundwater modelling in the assessment of groundwater contamination thus revealing the unsuitability of the site for any form of waste discharge.