Nitrate contamination of urban groundwater and heavy rainfall: Observations from Dakar, Senegal

Abstract

In low-income urban areas of major cities in Africa, sanitation provision derives primarily from onsite systems often comprising septic tanks and pit latrines. Such systems rely upon the ability of the surrounding soil and substratum to attenuate contaminants like nitrate and pathogenic microorganisms in wastewater. Here, we assess soil–water and solute dynamics in Quaternary aeolian sands underlying a densely populated suburb (Keur Massar) of Dakar (Senegal) using high-frequency monitoring and vadose zone modeling (Hydrus-1D). Observations of rainfall intensity, soil moisture content, and shallow groundwater-level fluctuations and nitrate concentrations were carried out at an experimental site adjacent to a septic tank supplied by toilets used by a primary school. Rapid rises in soil moisture content and episodic recharge contributions observed in groundwater levels caused by heavy (>10 mm h−1) and extreme (>20 mm h−1) rainfall are well modeled (R2 =.79–.83; RMSE = 0.012–0.019) by pore-matrix flow in the unsaturated zone by the Darcy–Richards equation. Spot sampling around the most intense rainfall of 2020 (45 mm h−1) reveals a fivefold rise and fall in the concentration of nitrate in soil moisture (∼500 to ∼2,500 mg L−1). These measurements provide new insight into the hydrological dynamics by which shallow groundwater is grossly contaminated (>500 mg L−1) by nitrate through episodic flushing by heavy rainfall of wastewater from a vast estimated network of over 250,000 septic tanks underlying this suburb of Dakar.