Vulnerability to the faecal contamination in the Thiaroye shallow aquifer, Dakar, Senegal

Abstract

In rapidly urbanizing low-income cities where the use of on-site sanitation facilities (septic tanks or latrines) is high, shallow groundwater is often vulnerable to faecal contamination. Here, we develop a multi-criteria modelling of the vulnerability to chemical and bacteriological faecal contamination of an unconfined aquifer of Quaternary sands in the Thiaroye area of Dakar (Senegal) using a model coupling GIS with Analytic Hierarchy Process (AHP). The approach includes the following factors (criteria): density of sanitation facilities, depth of water table, hydraulic conductivity, wetlands, potential recharge areas, population density and wealth inequality. Results show 11% of the study area with very high vulnerability, 20% of the area with high vulnerability and 69% of the area with low vulnerability.The vulnerability of the shallow aquifer was validated with monitoring of faecal contamination indictors (nitrate, E. coli) in groundwater carried out over a three-year period (2017-2019) using a network of 48 wells including dug-wells, piezometers, hand tubewells and boreholes. This validation showed a significant association (R2=0.7) between faecal contamination and the degree of vulnerability. Analysis of the results reveals that failures of on-site sanitation facilities in the area plays an important role in determining the quality of shallow aquifers in urban areas. It is, therefore, necessary to focus more on on-site sanitation facilities that significantly attenuate faecal contamination of shallow aquifers.