Title: Assessment of Selected Heavy Metals in Soil and Untreated Wastewater Used For Irrigation in Urban and Peri-Urban Farms in Eastern Nairobi City County, Kenya
Authors: Scholastica Gatwiri Mathenge, Wachuka Gathigia Njoroge, Onesmus Muia Mutuku
DOI: https://dx.doi.org/10.18535/jmscr/v6i8.124
Abstract
Introduction: Untreated wastewater contains heavy metals harmful to human health and the environment. People get access to untreated water through broken sewer pipes, blocked manholes and direct disposal into rivers. One such river is the Ngong River which traverses Nairobi City County and out through the Njiru and Ruai suburbs. Residents who dwell in the slums along the river use untreated wastewater for irrigating their adjacent farms and washing cars as a means to their livelihood. Soils and vegetables grown or irrigated with untreated wastewater may contain high levels of heavy metals that are detrimental to health. The heavy metals may emanate from the waste water discharged by the small scale industries that are based in the study site. However, reports on levels of heavy metals in untreated waste water and soils in Kenya are very scarce. Therefore this study aimed to determine the concentration of Cd, Pb, Zn, Mn, Fe and Cu in the soil and untreated waste water in Ruai and Njiru.
Methodology: Untreated wastewater and soil were collected during the dry season from various sites in Ruai and Njiru from small scale farms along Ngong River. The samples were also obtained from a control plot at the Kenyatta University. The samples underwent wet digestion pre-analysis procedures. The heavy metals were determined by atomic absorption spectroscopy.
Results: The concentrations (mg/L) of the heavy metals in untreated wastewater ranged from 3.09 to 3.54 for Mn, 0.01 to 0.03 for Zn, 0.21 to 0.28 for Pb, 4.79 to 8.07 for Fe, 0.17 to 0.22 for Cu and 0.42 to 0.47 for Cd.All the target heavy metals were found to be present in the soil in all the sites. The concentration ranged from 3194.30 to 3603.26 mg/kg for Mn, 13.56 to 15.05 mg/kg for Zn, 70.40 to 73.29 mg/kg for Pb, 7500.75 to 7534.77 mg/kg for Fe, 40.44 to 41.01 mg/kg for Cu and 22.20 to 23.34 mg/kg for Cd.
Conclusion: From the study, the level of heavy metals in the untreated wastewater affects the concentration of heavy metals in soil. When the untreated wastewater has a high concentration of heavy metals, the soil adsorbs the heavy metals and thus high concentrations in the soils.
Keywords: Heavy metals, Untreated wastewater, Soil, Adsorption.
References
- Drechsel, Pay, and Alexandra EV Evans. "Wastewater use in irrigated agriculture." Irrigation and Drainage Systems 24.1-2 (2010): 1-3.
- Qadir, Mansoor, et al. "Non-conventional water resources and opportunities for water augmentation to achieve food security in water scarce countries." Agricultural water management1 (2007): 2-22.
- Cornish, G. A., P. Lawrence, and H. R. Wallingford. "Informal Irrigation in periurban areas: A summary of findings and recommendations." Informal irrigation in periurban areas: a summary of findings and recommendations. DFID/HR Wallingford, 2001.
- Laˇcˇtuşu, Radu, et al. "Soil-plant-man relationships in heavy metal polluted areas in Romania." Applied Geochemistry 11.1-2 (1996): 105-107.
- Okoronkwo, N. E., J. C. Igwe, and E. C. Onwuchekwa. "Risk and health implications of polluted soils for crop production." African Journal of Biotechnology 4.13 (2005).
- Cornish, G. A., and N. C. Kielen. "Wastewater irrigation—hazard or lifeline? Empirical results from Nairobi, Kenya and Kumasi, Ghana." Wastewater use in irrigated agriculture (2004): 69-79.
- Karanja, N., et al. "Assessment of environmental and public health hazards in wastewater used for urban agriculture in Nairobi, Kenya." Tropical and Subtropical Agroecosystems 12.1 (2010).
- Nabulo, G., et al. "Does consumption of leafy vegetables grown in peri-urban agriculture pose a risk to human health?." Environmental pollution 162 (2012): 389-398.
- Gweyi-Onyango, Joseph P., and Mildred Osei-Kwarteng. "Safe vegetable production with wastewater in developing countries: demystifying the negative notions." African Journal of Horticultural Science 5 (2011).
- Sharma, R. K., M. Agrawal, and F. Marshall. "Heavy metal contamination in vegetables grown in wastewater irrigated areas of Varanasi, India." Bulletin of environmental contamination and toxicology 77.2 (2006): 312-318.
- Environmental Management and Co-ordination (Water Quality) regulations, 2006 Arrangement of Regulations. Kenya Gazette Supplement No 68. Legal Notice No. 120. 29th September, 2006.
- Mosime, L., O. Areola, and O. Dikinya. "Comparative effects of secondary treated waste water irrigation on soil quality parameters under different crop types." (2011).
- Pescod, M. B. "Wastewater treatment and use in agriculture." (1992).
- Singh, Anita, et al. "Risk assessment of heavy metal toxicity through contaminated vegetables from waste water irrigated area of Varanasi, India." Tropical Ecology 51.2 (2010): 375-387.
- Mapanda, F., et al. "The effect of long-term irrigation using wastewater on heavy metal contents of soils under vegetables in Harare, Zimbabwe." Agriculture, Ecosystems & Environment 107.2-3 (2005): 151-165.
- Nan, Zhongren, et al. "Cadmium and zinc interactions and their transfer in soil-crop system under actual field conditions." Science of the Total Environment 285.1-3 (2002): 187-195.