Characterization of the Dynamics of Leachate Contamination Plumes in the Surroundings of the Hulene-B Waste Dump in Maputo, Mozambique

Abstract

The contamination of areas around solid urban waste dumps is a global challenge for the maintenance of environmental quality in large urban centres in developing countries. This study applied a geophysical method (electrical resistivity) to identify leachate contamination plumes in the subsoil and groundwater, as well as to describe their temporal dynamics (2020 and 2021) in the surroundings of the Hulene-B waste dump, Maputo, Mozambique. Eight 400 m electrical resistivity profiles were performed, four profiles in January 2020 and four profiles in May 2021 overlapped, and the data were inverted with RES2D software. The electrical resistivity models predominantly indicate an E-W movement of large contamination plumes that are successively diluted with saturated media and groundwater, creating zones of less resistive anomalies (<4.2–< 8.5 Ω·m) possibly contaminated at the two analysed seasons, between 2020–2021. The thickness of the contamination plumes was higher in summer (2020) for profiles 1 and 2, and we associate it with the production and migration mechanisms of leachate that are intense in the hot and rainy season. Southwest of the dump, profile 4b showed the propagation of anomalous areas on the surface and at depth, which are associated with the production of leachate resulting from the continuous decomposition of waste that is continuously deposited in a new area southwest of the dump, thus generating a slow and continuous migration of leachate at depth, mainly in winter (2021). The spatial distribution of contamination plumes during both seasons was reduced significantly farther away from the waste deposit, revealing the attenuating effect of groundwater and lithological substrate (Profile 3).

Integration of Electrical Resistivity and Modified DRASTIC Model to Assess Groundwater Vulnerability in the Surrounding Area of Hulene-B Waste Dump, Maputo, Mozambique

“…Waste deposition in the Hulene-B dump is mostly located in QMa, spreading to the East (TPv). Geologically, the Hulene-B waste dump is in the Mesocenozoic sedimentary basin, in southern Mozambique [36] in a contact zone of two lithologies (Ponta Vermelha TPv, and Malhazine QMa Formations) [27] (Figure 2). The Ponta Vermelha Formation dates from the upper Pliocene to the lower Pleistocene, being composed in the upper part of ferruginous sandstones and red silty sands, which gradually change to yellow and whitish sands [37].…”

“…Momade et al [38] performed drillings L16 and L125 (Figure 2a) in two geological formations around the dump, showing that TPv formation underlain QMa, and TSa (Miocene-Pliocene) is composed of clayey, calcareous sandstone with Ostrea cullata in its upper part. Geologically, the Hulene-B waste dump is in the Mesocenozoic sedimentary bas in southern Mozambique [36] in a contact zone of two lithologies (Ponta Vermelha TP and Malhazine QMa Formations) [27] (Figure 2). The Ponta Vermelha Formation dat from the upper Pliocene to the lower Pleistocene, being composed in the upper part ferruginous sandstones and red silty sands, which gradually change to yellow an whitish sands [37].…”

“…The water level on local wells varies between 1.5 and 9.3 m in depth, with an average of 3.8 m [38]. Bernardo et al [27] used electrical resistivity profiles in 2020 and 2021, suggesting that groundwater in the western boundary of the Hulene-B dump was at variable depths and with a potential risk of being contaminated by leachate plumes resulting from vertical and horizontal migration, which, in the subsurface environment, were assigned d values 4.26 to 8.5 Ω.m. The hydraulic conductivity was estimated to be 1 to 5 m/d on the surroundings of the Hulene-B dump [28].…”

“…Previous studies on groundwater in Mozambique, particularly in Maputo city, considered it vulnerable to contamination due to the hydrogeological context characterized by its shallow level and high urban growth without adequate planning and sanitation systems [25,26]. Hulene-B, the largest open-air dump in Mozambique (~17 ha) and its influence on soil and groundwater contamination due to horizontal and vertical leachate migration has been studied [27,28]. This study aims to integrate electrical resistivity and the modified DRASTIC model to identify anomalous leachate migration and to estimate groundwater vulnerability in the surrounding area of the Hulene-B waste dump, Maputo, Mozambique.…”