Impact assessment of long term treated wastewater discharge on heavy metal concentration in soils from Palapye east, Botswana

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.