An evaluation of groundwater vulnerability assessment methods in a rapidly urbanizing city: evidence from Dakar, Senegal

Abstract

In rapidly growing cities in the tropics, unregulated urban development presents a major risk to groundwater quality. Here, we assess the vulnerability of an unconfined aquifer of Quaternary sands in the Thiaroye area of Dakar (Senegal) to contamination using four GIS-based indices (DRASTIC, DRASTIC_N, SINTACS, SI). Our correlation of assessed vulnerability to observed impact is semi-quantitative, relating observed groundwater quality, based on nitrate concentrations and tryptophan-like fluorescence to vulnerability degrees (i.e. coincidence rates). We show that considerably more of the Thiaroye area has a “very high vulnerability” according to SI (36%) relative to DRASTIC (5%) and SINTACS (9%); “high vulnerability” is estimated using DRASTIC_N (100%), DRASTIC (66%) and SINTACS (69%). Single-parameter sensitivity tests show that groundwater depth, soil, topography, land use and redox parameters strongly influence assessments of groundwater vulnerability. Correlation with observed nitrate concentrations reveals aquifer vulnerability is better represented by SI (coincidence rates of 56%) relative to DRASTIC_N (43%), SINTACS (38%) and DRASTIC (34%). The underestimation of groundwater vulnerability in Dakar using DRASTIC, DRASTIC_N and SINTACS is attributed to their reliance on an assumed capacity of the unsaturated zone to attenuate surface or near-surface contaminant loading, which in the low-income (Thiaroye) area of Dakar is thin and affords limited protection. The inclusion of a land-use parameter in SI improves the characterization of groundwater vulnerability in this low-income, rapidly urbanizing area of Dakar.

Lessons Learnt from the Influencing Factors of Forested Areas’ Vulnerability under Climatic Change and Human Pressure in Arid Areas: A Case Study of the Thiès Region, Senegal

“…Dakar, the capital of Senegal, is a rapidly expanding metropolitan city in sub-Saharan Africa [1]. Consequently, the rapid urban sprawl of Dakar has resulted in the expansion of the Thiès region, situated 70 km away from Dakar.…”

“…In recent years, concerns regarding the decline in forest resources have been investigated. According to the literature, unregulated urban development poses a significant risk to groundwater quality [1] and exerts critical pressure on natural resources. Moreover, changes in the functioning of the Earth's ecological systems present a significant threat to human survival [2].…”

Groundwater Geochemistry and Saltwater Intrusion in the Dakar Coastal Area, Senegal

“…In the Thiaroye aquifer, measured nitrate concentrations rapidly increased over the last decades mainly caused by the expansion of unplanned urbanization and the infiltration of domestic wastewater into the groundwater due the poor sanitation system in the peri-urban areas. Nitrate values in Thiaroye aquifer varies between 5 to 34 mg·L −1 between 1966 and 1972 and presently reach an average of 400 to 450 mg·L −1 (Diédhiou et al, 2011;Diouf et al, 2012;Diaw et al, 2020;Pouye et al, 2022).…”

Application of geological and geo-electric methods in the assessment of corrosivity, competence, and vulnerability of soils around Southeastern Nigeria

“…Many approaches have been developed to evaluate aquifer vulnerability. They include process-based methods, statistical methods, and overlay and index methods (Pouye et al, 2022;Kirlas et al, 2022;Eynakware et al 2022c and d;Kalinski et al (1993) and Eke (2017) opined that the basis of index-based integrated electrical conductivity is that the vertical travel time of water through a set of geological layers can be related to the resistivity properties of these layers, which implies that, the method (which is based on the principle that the distribution of electrical potential in the subsurface around current-carrying electrodes) depends on the electrical resistivity. Oli et al (2022) demonstrated that the protection degree of an aquifer or vulnerability may be considered directly proportional to the longitudinal conductance (LC) of the overburden materials.…”