DYNAMICS OF SOIL NEMATODES AND EARTHWORMS IN URBAN VEGETABLE IRRIGATED WITH WASTEWATER IN THE NAIROBI RIVER BASIN, KENYA

The effects of heavy metals lead (Pb), Cadmium (Cd) and Cromium (Cr) on nematode communities and earthworm density and biomass were studied in the wastewater irrigated farms of the Nairobi river basin. The levels of Cr and Pb in the wastewater were below the threshold values considered to be toxic while those of Cd exceeded the permissible limit. Heavy metal accumulation in soils in the Kibera and Maili Saba farms  were Cd (14.3 mg kg-1), Cr (9.7 mg kg-1) and Pb (1.7 mg kg-1) and 98.7 mg Cd kg-1, 4.0 mg Cr kg-1 and 74.3 mg Pb kg-1, respectively. High heavy metal concentrations as well as soil organic matter were negatively correlated with plant feeding nematodes in the genera Criconema, Meloidogyne, Paratylenchus, Pratylenchus and Scutellonema. Bacterial feeding nematodes genera Rhabditis, Plectus, Cephalobus and Acrobeles were predominant in the gardens treated with wastewater. An average density of 198 m-2 earthworms and a biomass of 68 g m-2 and 102 earthworms m-2 with 33g m-2 biomass were recorded in Kibera and Maili Saba, respectively. The earthworms isolated from both sites all epigeic with the metal content in Maili Saba suppressing their populations. This study has demonstrated that the use of untreated urban wastewater for irrigation has an adverse effect on nematodes and earthworms diversity and abundance

Concentrations of Heavy Metals and Pesticide Residues in Leafy Vegetables and Implications for Peri-urban Farming in Nairobi, Kenya

The primary objective of this study was to assess the levels of heavy metals, nitrate, and pesticide residues in kale (Brassicaolaracea var. olecephala), a leafy vegetable. The study was conducted in three peri-urban sites in Nairobi, Kenya, where the use of wastewater irrigation is pervasive. The results indicated elevated levels of boron (B), cobalt (Co), chromium (Cr), copper (Cu), lead (Pb), zinc (Zn) and nickel (Ni) in vegetables irrigated with wastewater in two sites. High loadings of the heavy metals Ni, arsenic (As), Pb, Cr, Cu, and Zn into agricultural soils were demonstrated to be from chicken manure. Nitrate levels were within acceptable daily intakes. Residues of diazinon, cypermethrin, biternol, and profenofos pesticides exceeded permissible maximum residue limits in kale at one site. Study findings suggest that consumers of vegetables grown in wastewater-irrigated soils may ingest significant amounts of heavy metals and pesticides. Considering these findings, reducing heavy metal deposits into soils should be a strategic aim of soil protection policies in Kenya

ASSESSMENT OF ENVIRONMENTAL AND PUBLIC HEALTH HAZARDS IN WASTEWATER USED FOR URBAN AGRICULTURE IN NAIROBI, KENYA

Thirty percent of residents in Nairobi practise urban agriculture (UA) with a majority of the farmers using untreated sewage to irrigate crop and fodder. Due to the environmental and health risks associated with wastewater irrigation, a study was carried out in partnership with farmers in Kibera and Maili Saba which are informal settlements along the Ngong River, a tributary of the Nairobi River Basin. Soil, water, crops and human faecal samples from the farming and non-farming households were analysed to elucidate sources, types and level of heavy metal pollutants in the wastewater and the pathogen loads in humans and vegetable crops.  Heavy metal accumulation in soils collected from Kibera and Maili Saba were Cd (14.3 mg kg-1), Cr (9.7 mg kg-1) and Pb (1.7 mg kg-1) and Cd (98.7 mg kg-1),  Cr (4.0 mg kg-1) and Pb (74.3 mg kg-1), respectively.  This led to high phytoaccumulation of Cd, Cr and Pb in the crops that exceeded the maximum permissible limits. No parasitic eggs were detected in the vegetables but coliform count in the wastewater was 4.8 x108±2.2 x1011/100ml. Soils irrigated with this water had parasitic eggs and non-parasitic larvae counts of 54.62 and 27.5/kg respectively. Faecal coliform and parasitic eggs of common intestinal parasites increased in leafy vegetable sampled from the informal markets along the value chain