The heavy metal content of crops irrigated with untreated wastewater: a case study of Nairobi, Kenya

Use of untreated wastewater for irrigation could have devastating effects on crop quality. A study was conducted to determine the content of lead, cadmium and chromium in food crops irrigated with untreated wastewater at Kibera and Maili Saba, in Nairobi, Kenya. Crop samples were collected from farms irrigated with untreated wastewater during the dry and wet seasons. While the safe limits of lead and cadmium in food crops are 0.3 and 0.2 ppm, the concentration of lead and cadmium, at Maili Saba downstream of the industrial area, in the edible crops during the dry season was 48.4 and 26.5 ppm, respectively. Enrichment factor (EF) was used as a measure of the risk and hazard sustained when crops are irrigated with water that is contaminated heavy metals. Plots planted with black nightshade (BNS) and Kales at Maili Saba, downstream of Nairobi’s industrial area, had the highest EF values. Lead in BNS at Maili Saba during the dry season had the highest EF value of about 2200, suggesting that irrigating BNS with contaminated sewage could be hazardous. This study has confirmed that irrigation of food crops increases the concentration of heavy metals such as lead and cadmium to unsafe levels.Key words: Heavy metals, wastewater, vegetables, health hazard, enrichment factor (EF

Towards a national policy on wastewater reuse in Kenya

Kenya is a water-scarce country with the capital city, Nairobi, receiving less than 100 l/capita/day. Potable water for irrigation and industrial use is generally unavailable, and this calls for alternative water sources. Despite use of wastewater being illegal in Kenya, it is used to irrigate over 720 ha in Nairobi. In order to justify the formulation of a national policy to support wastewater reuse, secondary data which included the authors’ previous  work was reviewed. In a study done between 2006 and 2007, the levels of nitrates (100 mg/l) and TDS (630 mg/l) in the wastewater were found to be within the acceptable NEMA standards. The concentration of lead was 0.1 mg/lwhile cadmium and chromium were non-detectable. However, levels of BOD and Coliform bacteria were higher than NEMA limits. This implied that Nairobi sewage needed to be treated for the removal of BOD, turbidity and microbial contamination. In order to allow for safe use of wastewater in Kenya, there is need to formulate a national wastewater reuse policy which provides guidelines for maximum allowable levels of pesticides, herbicides, and heavy metals in wastewater reuse. Such a policy should also indicate the required water quality monitoring frequency for faecal indicators (Escherichia coli, faecal coliforms, enterococci), and suggest the maximumallowable concentration of nutrients (nitrogen and phosphorus) which may are usually abundant in wastewater

Using GIS for spatial exploratory analysis of borehole data: a firsthand approach towards groundwater development

Groundwater is an inimitable resource that provides water to communities especially in arid and semi-arid regions. However, the spatial variability of the resource as well as the heterogeneity and complex nature of aquifer systems that store groundwater presents difficulties for groundwater development. Thus, understanding the spatial structure of aquifer characteristics could be used as a resourceful tool and as a first point of call towards groundwater development. The study investigated aquifer characteristics particularly on transmissivity and specific capacity to determine how aquifer characteristics could be used in making predictions for new sites that need to be identified for potential groundwater development. Prediction maps were developed for transmissivity and specific capacity using ordinary kriging J-Bessel and exponential semi variograms techniques. The study revealed that 28% of the study area had high potential for groundwater development with transmissivity ranges of 34.40 – 46.52 m2/d. Equally, viable areas of 16% and 20% were identified with transmissivities of 27.99 – 34.39 m2/d and 13.07 – 19.47 m2/d respectively. Based on specific capacity, 21% of the study area was identified to have groundwater development potential with corresponding specific capacity of 13.54 – 20.7 m2/d. Similarly, 24% and 34% of the study area were also identified to be good for the exploitation of groundwater with corresponding specific capacities of 8.74 – 13.55 m2/d and 5.52 – 8.74 m2/d respectively. In terms of spatial structure, transmissivity was found to be variable compared to specific capacity. The application of Geographic Information Systems (GIS) is thus considered to be an immeasurable tool that could be used in mapping aquifer characteristics that can serve as firsthand information for groundwater development.Keywords: Spatial, Groundwater, Transmissivity, Specific capacity, Kriging, Semivariogram        

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