THE HEAVY METAL CONTENT OF CROPS IRRIGATED WITH UNTREATED WASTEWATER: A CASE STUDY OF NAIROBI, KENYA

Abstract

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.