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Botswana International University of Science & Technology (BIUST) 2017

Impact Assessment of Long Term Treated Wastewater Discharge on Heavy Metal Concentration in Soils from Palapye East, Botswana

Kebonye, Ndiye Michael.

Titre : Impact Assessment of Long Term Treated Wastewater Discharge on Heavy Metal Concentration in Soils from Palapye East, Botswana

Auteur : Kebonye, Ndiye Michael.

Université de soutenance  : Botswana International University of Science & Technology (BIUST)

Grade : Master of Science (MS) 2017

Grade  : MSc-Environmental Sciences 2017

Résumé
Issues on environmental soil quality continue to attract much global attention. Domestic wastewater is a potential source of contaminants capable of degrading the soil environment. This study reports the accumulative effects of 21 years (1995 – 2016) of treated wastewater discharge on heavy metal concentration levels in the proximal environment of Palapye wastewater treatment Plant (PWTP), Central Botswana. Soil samples were collected from eight geo-referenced pedons : four situated along the treated wastewater channel and four on an adjacent well-drained channel (control). Selected physico-chemical properties of the soils were determined using routine laboratory procedures and heavy metal concentrations with portable XRF (Olympus Delta Premium Analyser SN : 550255, USA). Results of the study show that heavy metal (Fe, Cu, Zn, Mn, Pb and U) concentrations in the two drainage classes (poorly drained and somewhat excessively drained) did not vary significantly (p > 0.05). There was strong correlation between organic matter (OM) and Fe (R2 = 0.896, p < 0.01), OM and Cu (R2 = 0.908, p < 0.01), OM and Zn (R2 = 0.956, p < 0.01) and OM and Mn (R2 = 0.954, p < 0.01) in control soils, while treated wastewater affected soils showed strong correlation for OM and Fe (R2 = 0.765, p < 0.01), OM and Zn (R2 = 0.770, p < 0.01) and OM and Mn (R2 = 0.802, p < 0.01). Source apportionment of heavy metals using PCA shows one component in the control soils accounted for 77% of the total variance, while two components accounted for 97% of the total variance in treated wastewater affected soils. Geochemical mass balance plots for selected profiles showed fluctuations for Fe and Mn at various depths while Zn maintained an almost constant trend. The geoaccumulation (Igeo) and pollution load (PLI) indices both show all soils to be unpolluted. Conclusion is made that two decades of treated wastewater discharge has not led to the degradation of environmental soil quality in Palapye. However, it is strongly recommended that heavy metal levels be checked intermittently as a routine environmental monitoring exercise. Manganese which showed severe enrichment in two profiles 7 and 8 might indicates possible soil pollution, but their speciation and bio-accessibility, rather than simply total concentrations, have to be established.

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Page publiée le 29 mai 2021, mise à jour le 6 juin 2021