Impact of increasing abstraction on groundwater sustainability within the Ga East and Adentan Municipalities, Ghana.

Abstract

The hydrogeological systems of the Ga East and Adentan municipalities 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 recharge and hydraulic conductivity, as well as forecast likely effects of different groundwater recharge and abstraction scenarios on the sustainability of groundwater resources in the area. A calibrated steady-state groundwater flow model was developed for the terrain. A single aquifer system (quartzite-schist formation) was identified. The aquifer hydraulic conductivity estimates for about 90% of the terrain are lower than 15.0 m day-1. The observed outliers are attributable to the fractured and jointed quartzites. The effective aquifer recharge through precipitation ranges from 2.70×10^-5 m day-1 to 8.10×10^-5 m day^-1, representing 1.2% to 3.6% of the average annual rainfall in the area. Cases of local and intermediate flow systems, and potential recharge areas were identified. The calibrated model suggests that the current groundwater recharge rates estimates can sustain groundwater abstraction up to 200% without any substantial geometrical change and drawdown in the hydraulic heads. This implies the system can support demands from groundwater usage for a period of 80 years, using the current population growth rate of 2.5% per annum. However, a reduction of 50% in groundwater recharge within the same period may result in considerable drawdowns throughout the terrain if the current abstraction rates are to be sustained solely by groundwater resource. An increase in groundwater abstractions by up to three times with a 10% reduction in the current recharge rates for the same 80-year period will result in considerable drawdown.