Assessment of the Impact of Climate Change on Groundwater Resources of the Greater Banjul Areas (Gba) in The Gambia

Abstract

Groundwater is an important natural water resource, and it serves as an alternative source of water supply for surface water in the Greater Banjul Areas (GBA), which is saline. Thus resources regardless of their importance have been increasingly threatened by pollution due to saline intrusion and infiltration of chemicals from agricultural activities, urbanization, industrial development, seasonal climate variability, and climate change. Subsurface storage and groundwater recharge are affected by climate variability and change through changes in precipitation pattern and intensity. In coastal areas of the GBA, the aquifers are affected by marine water intrusion sea level rise. In the past decades, reduced precipitation has been persistent in the country which contributes to a drop in water table level and hence shortage in water supply in many areas. This research aims to assess the impact of climate change on groundwater quality and quantity of the GBA, southwest of The Gambia. The hydrodynamics and hydrochemistry of the GBA basin were assessed for 114 wells and 52 boreholes. The Piper diagram was used to assess the groundwater type of the study area. The analysis of the Piper diagram identified eight major water-types based on the major ion chemistry of groundwater: i) bicarbonate (m-HCO3, Ca-HCO3, and Na-HCO3) facies, ii) chloride (Na-Cl and m-Cl) facies, iii) and iv) mixed (m-m and Na-m) facies. The order of dominance of the anions in groundwater samples is HOC3- > NO3 > Cl-> SO4 ->Br, and the cations Na+> Ca2+ > Mg2+ >K+. Mean and median values show that most groundwater samples do not exceed the WHO standard. 23% of samples have NO3 content above the WHO guideline (50mg/L) with a maximum of 545 mg/L located in the urbanized areas. The high nitrate concentration suggests organic pollution through one-site sanitation and domestic wastewater. The Thornthwaite model was used to estimate groundwater recharge trends for RCP4.5 and RCP8.5 scenarios. The output of the Thornthwaite model (recharge) was used as input into the three-dimensional finite difference groundwater flow model MODFLOW to simulate groundwater dynamics and the impacts of present and future climate change on groundwater heads after calibration. Three different scenarios (Abstraction scenario: A1=2500m3/day; A2=4000m3/day, Climate change scenario and Abstraction and climate change scenario) are used to evaluate the groundwater head trend over the 2006-2050 period. The results obtained show a decreasing groundwater head trend for both the RCP4.5 and RCP8.5 scenarios with the Thornthwaite model. In RCP4.5, the annual groundwater head decreases by 5% (-5%) during the period 2006-2020 and by 8% (-8%) during the period 2030-2060 while it increases between these two periods (2006-2020 and 2030-2060) by 5.5% (+5.5%). Generally, groundwater head for the two scenarios is decreasing and this is more significant in RCP8.5 than in RCP4.5. The decreasing head over time with RCP8.5 has a growth rate estimated at -14%. Climate change adaptation policies and a sustainable management strategy for the basin should be in place, including legislation to regulate the indiscriminate drilling of boreholes and dumping of solid and liquid waste, and regulate the abstraction of groundwater in an unsustainable manner.