Variable Density Groundwater Flow Modelling Of The Keta Strip, Southeastern Ghana – Evaluating The Impacts Of Salinity Variations On The Velocity Field

Abstract

The impact of density variations on hydraulic head and flow geometry on the shallow unconfined aquifer part of the Keta Strip was investigated using finite element numerical modelling techniques (FEMWATER). A calibrated transient state numerical model shows groundwater flow pattern in the area to be northwest southeast and northeast southwest with some few cases of south to north flow directions. The calibrated transient state model was used to simulate various scenarios of groundwater abstraction in the area. The results suggest that the spatial variations of groundwater velocities range between 0.93 m/day to 8.37 m/day with an average of 4.05 m/day. The spatial variations of groundwater velocities in the area appear to take the trend of variations in the nature of materials in space and density of the water. Locations of high nodal velocities appear to present low TDS contents and vice versa. The TDS concentration in the area ranges between 180 mg/l and 1800 mg/l with an average of 990 mg/l with significant spatial and temporal variability. The temporal variability in TDS suggests the impact of seasonal variations on groundwater salinity variations in the domain. This may be due to increasing salinity arising from evaporative enrichment of residual groundwater as the shallow aquifer system is prone to evaporation especially in the dry season. This is reflected in the variations in the predicted groundwater velocity field and the general groundwater flow pattern in the domain. The predicted flow field and potential distribution appear to be dictated by groundwater density variations and the nature of the material in space. Groundwater Scenario analysis conducted also suggested that, increasing the TDS concentration via surface activities in the area have no direct impact on the hydraulic head except for subtle changes along the boundaries of the study domain. An increased in TDS however reduced the velocity of groundwater in the aquifer drastically.