Uptake characteristics of trace elements: lead, zinc, cadmium and mercury by selected food crops grown along Nairobi River

Abstract

The relationship between the trace elements content of lead (Pb), ZInC (Zn), cadmium (Cd) and mercury (Hg) in vegetable food crops samples and agricultural soil samples on which they are grown along the Nairobi River catchment were investigated in this study. Various biological plant samples which included 12 sugarcane (saccharum ofJinaram); 15 arrow roots (Colocasia esculenta); 10 kale (Brassica olevacea). In addition 12 soil and 19-river water samples were sampled and analysed from sampling sites. Energy Dispersive X-ray Fluorescence (EDXRF) analytical technique was used to determine lead (Pb) and zinc (Zn) levels in sugarcane and arrow roots samples, while atomic absorption spectroscopy (AAS) analytical technique was used to determine cadmium (Cd) and mercury (Hg) in all sample types and also for lead and zinc levels in kale samples. The concentration of most elements was observed to be at concentration levels normally observed in vegetables in uncontaminated soils. However, elevated concentration levels of lead and zinc were observed, particularly in arrow roots and kale leaf samples. The dominant pathway for most trace elements in arrow roots was the soil on which these crops are grown. Arrow roots were found to accumulate more zinc than the other trace elements studied. The levels of mercury and cadmium in river water samples were found to be below detection limits «0.2 ug/ml and 0.0006) ug/ml, respectively). However, lead (Pb) and zinc (Zn) levels in the raw water used for domestic purposes exceeded stipulated WHO limits, (> 0.05 ug/litre and 0.5 ug/litre respectively). Mercury level in soil samples were below detection limit (0.2 ug/g), but the levels of cadmium observed in soil samples were within the concentration for uncontaminated soils «2 ug/g). Lead and zinc levels in soil samples are as high as 228 ug/g and 400 ug/g, respectively. The 'available' levels of mercury and cadmium in the soil samples for plant absorption are below detection limits (0.2 and 0.0006 ug/g respectively), while the highest 'available' levels of lead and zinc account for less than 1% of the total soil content. The level of mercury in all food crop samples is below detection limit «0.2 ug/g). Sugarcane samples were found to readily absorb lead and zinc from the soil, but slowly absorb soil cadmium. It accumulates these elements more in the roots, than in leaves and the stem. The levels of lead, cadmium and zinc in the stems samples were observed to be within WHO recommended limits «232 ug/day, 46 ug/g and 45 mg/day respectively), for human consumption. Arrow roots readily absorb soil zinc, but slowly absorb lead and cadmium from the soil. Lead is accumulated more in the tubers than, leaves, while cadmium and zinc are accumulated more in leaves than tubers. The levels of lead, cadmium and zinc found in the tubers in these studies are within WHO recommended limits for human consumption, the limits are <232 ug/day, 65 ug/day and 45 mg/day respectively. Kale accumulate lead and zinc in the roots more than in the leaves and stems in levels which are within WHO limits «232 ug/day, 46 ug/g and 45 mg/day respectively), for human consumption.