Using urban nitrate-rich groundwater for peri-urban irrigation in Dakar, Senegal: an evaluation of water quality concerns

Résumé

Contamination of urban groundwater from faecal effluent in sewered and unsewered areas is a widespread constraint to its use for water-supply provision. Here, we present the first multi-annual (2018–2024) assessment of the hydrogeochemistry and irrigation suitability of urban groundwater from the Thiaroye Aquifer. Groundwater samples ( n = 225) were collected from 25 sites comprising boreholes, dug wells, hand pumps, and piezometers prior to, and following the monsoon over a seven-year period (2018–2024). Dominant facies groups are Na-K-SO4-Cl and Ca-Mg-SO4-Cl; chloride largely derives from anthropogenic sources including wastewater and on-site sanitation. Application of standard indices of water quality for irrigation (RSC, SAR, and %Na) reveal marked spatial heterogeneity, with 57% of groundwater samples classified as suitable for irrigation. Several wells (PD1, Pts 58, NP3, PAWS, F30), located in densely populated peri-urban areas with agricultural activity, exhibited elevated sodium concentrations limiting their suitability for use in irrigation. Recharge from monsoonal precipitation does not consistently result in the dilution of solute concentrations in shallow groundwater due, in places, to high pollutant loads from the surface and shallow subsurface, and impeded, lateral groundwater flow. Critically, nitrate concentrations reach up to 813 mg/L, significantly surpassing the agronomic thresholds for nitrate-sensitive crops like tomatoes, carrots, and onions cultivated in the Niayes. Coupled with the well-documented faecal contamination of the Thiaroye aquifer, this study suggests that pre-treatment methods such as biological denitrification or reverse osmosis are necessary before this urban groundwater can be safely utilized for irrigation at the most contaminated sites. These findings highlight the spatial heterogeneity of groundwater quality within the Thiaroye aquifer and suggest that targeted management strategies, combining controlled pumping with appropriate treatment and land-use planning, are essential to enhance irrigation potential while mitigating urban groundwater flooding in Dakar.

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