Abstract
The quaternary sandy sediments which cover most part of the Cap Vert peninsula bear considerable groundwater resources. The aquifer lying beneath a densely populated suburb zone is encountered with major issues such as induced recharge from anthropogenic surface derived pollution and rising water table to ground surface. The present study was aimed at investigating the recharge in the unconfined aquifer of Thiaroye zone using both water table fluctuation (WTF), chloride mass balance (CMB) methods and environmental isotopes. Seasonal fluctuations of groundwater in response to precipitation are monitored during time period (2010 to 2011) using “Thalimede Orpheus mini” recorders in two piezometers (P3-1 and PSQ1) as well as long term record. Chemical and isotopic characterization of groundwater, rainfall and the unsaturated zone were also carried out using a network of 48 points consisting of 8 rainfall stations, 10 unsaturated zone profiles and 30 dug wells, boreholes and piezometers. The concentrations of chloride in rainwater are between 3.2 and 53.4 mg/L. These unsaturated zone profiles range from 65 and 572 mg/L. The recharge obtained by WTF method ranged between 18 and 144 mm during the rainy season (June to October), whereas the recharge given by CMB method ranged ​​between 8.7 and 73 mm/year. The Thiaroye aquifer recharge obtained from these different methods also showed relatively similar range values. In this study, the WTF method applied computes both infiltration from rainwater and domestic waste water, while the CMB method estimates potential recharge from rainwater. Therefore, in the urban area, the CMB method cannot be applied due to the chloride input from waste water infiltration. Key words: Thiaroye basin, recharge, isotopes, chloride mass balance (CMB), quaternary sandy aquifer (Dakar)
Highlights
The Quaternary sandy aquifer located in the region of Dakar plays a major role in supplying drinking and irrigation water in rural areas
Where Sy is the specific yield, ∆h is the seasonal water level variation and t is time. This method based on the premise that rises in groundwater levels in unconfined aquifers are due to the recharge induced by rainfall infiltration with regard to seasons is suitable for shallow groundwater displaying rise and decline (Healy and Cook, 2002; Scanlon et al, 2002)
The daily groundwater levels records (Figure 4) in response to rainfall were obtained in two piezometers (P3-1 and PSQ1) to estimate recharge
Summary
The Quaternary sandy aquifer located in the region of Dakar plays a major role in supplying drinking and irrigation water in rural areas. Use of this resource began in the 50s with periods of higher and lower exploitation ranging from 15 000 to 1 300 m3/day. The combined effects of the reduced pumping and induced recharge in addition to natural recharge cause rise of groundwater table and flooding in the suburban area. This present paper aims to investigate recharge processes using various techniques such as water table.
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