Stochastic analysis of the groundwater velocity field and implications for contaminant transport within the Ga East and Adentan municipalities, Ghana

The hydrogeological systems of the Ga East and Adentan Districts of the Greater Accra Region, Ghana were studied using a numerical model. Historical hydrogeological and groundwater monitoring data on twenty boreholes were used in conceptualising the hydrogeological system. The objective was to estimate the aquifer hydraulic conductivity and recharge, and also simulate the velocity field to assess the impact of contaminant transport within the study area. A steady-state model was developed depicting the groundwater flow conditions of the terrain. A single aquifer system (quartzite-schist formation) has been identified. The aquifer hydraulic conductivity estimates indicate that over 90% of the terrain is lower than 15.0 m day(-1). The observed outliers are attributable to the fractured and jointed quartzites. The effective aquifer recharge through precipitation for this study ranges between 2.70 x 10(-5) m day(-1) and 8.10 x 10(-5) m day(-1), representing 1.2%-3.6% of the average rainfall records per year in the area. Cases of local and intermediate flow systems, and potential recharge areas were identified. The estimated velocity field ranges from 0.002 to 11.2 m day(-1), with over 90% of the area having velocities below 0.85 m day(-1). The spatial distribution of the velocity field ties in well with the hydraulic conductivity field observed for the terrain. The estimated velocity field makes contamination transport significant in the domain. For instance, contaminants leached into the aquifer zone through recharge by rainfall from the landfill site at Pantang will travel at approximately 0.85 m day(-1) towards Oyarifa, Teiman and Ayimensa communities along an identified flowpath. Seven principal groundwater flowpaths have been identified using the particle tracking technique. The travel times along the flowpaths from recharge to discharge locations ranges from 7 years to 833 years. Stochastic simulations carried out on the calibrated model indicate that the model is unique with respect to the aquifer recharge, hydraulic conductivity and hydraulic head.