Lead, Cadmium and Zinc Speciation in Garage Soils, Their Levels in Kales and Water Along Katothyani Stream, Machakos Town, Kenya

Garage waste is a major source of environmental pollution since it contains heavy metals. These heavy metals end up being swept by rainwater into rivers. When the water is used for drinking or irrigation the heavy metals end up in animal and human tissues causing adverse health effects. In soil, heavy metals exist in different forms which influence their mobility and bioavailability. These forms include those soluble in water, those exchangeable with other metal ions, the reducible forms (carbonate and oxides), and the oxidisable forms (organic and residual). The water soluble and the exchangeable forms are the most mobile and bioavailable while the oxidisable forms are the least mobile. In Machakos town Kenya, there are many garages situated next to each other and at close intervals. The waste from these garages is swept by rainwater to the recipient katothyani stream which is only 50 metres away. Water from this stream is used for irrigation and drinking hence posing as health risk to the community. The objective of the present study was to determine forms of Pb, Zn and Cd in garage soils of Machakos town and levels in the nearby Katothyani stream water and kales by atomic absorption spectrometry. Nine composite soil samples each made of three replicates from depth intervals of 0-15, 15-30 and 30-60 cm of Kaunda, Industrial area and Mwangi sites were collected by stratified random sampling and extracted sequentially to determine forms. Forms of Pb and Zn were mainly oxides and organic type. The water soluble and exchangeable fractions were associated with the garage soils at relatively low concentrations with Pb and Zn in these forms contributing less than 5% and 2.6% respectively of the total of each metal extracted. Total heavy metal concentrations (ppm) in soils ranged between 1.61±0.02 to 69.19±0.14 for Pb, 1.61±0.02 to 11.51±0.01 for Zn and Cd was below detection limit. The mobility factors of lead in the topsoil profiles ranged between 4.20% and 10.69% while those of zinc were between 9.20 % and 15.71%. Levels of the metals in Katothyani stream obtained by random sampling and irrigated kales obtained purposively were also determined. In kales the highest mean concentrations of Pb, Cd and Zn were: 0.03±0.04, 0.01±0.01 and 1.62±0.02 ppm respectively. The mean Pb concentrations for water samples ranged between 0.01±0.01 and 0.14±0.04 ppm while Zn mean concentrations were between 0.00±0.01 and 0.92±0.01 ppm with Cd being below detection limit. ANOVA, SNK test showed significant differences in mean concentrations of Pb and Zn in Industrial area and Mwangi sites with respect to depth. This meant that activities at garage sites caused change in concentrations with depth. In Kaunda garage, the mean concentrations for lead were not significantly different while the mean concentrations for zinc were significantly different for the three soil profiles fractionated (p<0.05, Anova, SNK test). Pearson product moment correlation of water soluble and exchangeable fractions with the total metals showed negative correlation. This indicates that high concentration of heavy metals in garage soils does not necessarily infer their mobility and therefore their bioavailability. The stream and garage soils are slightly contaminated with respect to lead while the kales contained all metals within tolerable levels. Due to variation of mean concentrations of Pb and Zn With depth, as an evidence of surface pollution, there is not only need to relocate garages from the town centre but also to plant indigenous plant species on affected soils to mop up the bioavailable heavy metal forms. (author)