| dc.description.abstract | Urban and peri-urban agriculture (UPA) is considered an important source of food for cities and for vulnerable households. It is usually characterized by high input intensity and use of wastewater for irrigation. The inappropriate intensive use of agrochemicals and poorly treated wastewater in UPA poses a real danger with regard to agricultural produce safety and potential to pollute the environment. Little quantitative knowledge is available on the dynamics of heavy metal movement their bioavailability and the health risks associated to consumers in Botswana. To close this gap, this MSc research work is aimed at investigating the distribution, bioavailability and mobility of heavy metals, as well as their transfer from soil to plant and health risk associated with consumption of heavy metal contaminated crops in UPA land use. The study was conducted at two farms in Glen Valley (NAMPAADD and Glen Valley 1), one farm in Oodi, one farm in Matebele, one farm in Mmankgodi and one farm in Thamaga, thus a total of six farms. At each of the farms, three soil profiles were described on cultivated soil and one on the adjacent virgin land that had not been cultivated. Thus experiment consisted of six treatments (farms at different locations) with four replication (cultivated area 1, cultivated area 2, cultivated area 3 and uncultivated area). The soil profiles were dug up to lm depth or up to the parent material, whichever came first.The elemental analysis of metals in soils and vegetables were determined using Inductively Coupled Plasma (ICP), following digestion with varies chemicals, for example total concentration was digested using aqua regia, sequential extraction using different reagent starting with weak ones to strong ones. Inorder to evaluate potential health risks arising from ingestion of the metal contaminated vegetables, the consumption of vegetables was calculated as the ratio of estimated exposure of test crop and oral reference dose.The results revealed evidence of accumulation of total heavy metal in UPA soils, but the concentrations were below international critical threshold levels for agricultural soils. They further showed that locations had a significant difference in heavy metal content in soils at p<0.001. Cadmium and arsenic were below detectable levels (<0.01 mg/kg). Concentration values for total heavy metal in soils ranged from 43.16 - 70.90 mg kg'1 for Cr, 3.95 - 21.75 mg kg'1 for Cu, 4.95 - 15.35 mg kg'1 for Pb, 44.37 - 74.45 mg kg'1 for Zn and 6.65 - 47. 65 mg kg'1 for Ni. Interestingly concentration levels of metals in vegetables were higher than the recommended maximum permissible level set by FAO/WHO. For example cabbage had concentration of 0.45 mg kg'1 Cd and 3.84 mg kg'1 Pb against safe limit set by FAO/WHO of 0.2 mg kg'1 and 0.3 mg kg'1 respectively. The concentration of heavy metals in irrigation water was below detectable level.The transfer factors (TF) values on vegetables ranged from 3.38 - 35.06 for Cr, 0.35 - 4.13 for Cu and 0.00 - 8.00 for Pb and were very high and statistically significant, while on the other hand Zn had TF range of 1.19 - 5.50 and Ni 1.08 - 5.86 which were not statistically significant but high. A health risk assessment showed that consuption of vegetables from UPA posed risk from heavy metal toxicities since health risk index (HRI) for different vegetables were greater than 1. For example Pb had HRI greater 1 on tomato across all locations it was grown. Cd had HRI greater 1 on all vegetables except on green pepper and tomato at Mmankgodi and Oodi. | en_US |
