Effects of rain and sewage irrigation on heavy metal concentrations of spinach, kales and soils in Maili Saba village, Nairobi, Kenya
Mahungu, Richard Muriithi
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The study set out to quantify and document the relationships between sewage irrigation and heavy metal content of soils and leafy vegetables during the dry season; and compare it with heavy metal content of rain fed leafy vegetables and soils of the same previously sewage irrigated field with a view to ascertaining if rain water "flushes out" toxic metals from soils and thus make appropriate recommendations to maintain safe levels of sewage irrigation practice. A Random Complete Block Design (RCBD) was set in Maili Saba village, an informal settlement in the peri urban area of Nairobi province, Kenya, located 20 km east of city centre. The area hosts about 2000 farmers. Farm sizes ranges from 0.25 to 1 acre, and raw sewage irrigation has been practiced since 1975. The area supplies one tenth of spinach and kales consumed in the city annually. A control plot was set in Githurai, an area with no sewage irrigation, 10 km north east of Maili Saba. The study area was stratified into three blocks of different soil textures, each with three replicate plots of the treatments. Spinach (spinach oleraceae) and kales (kale oleraceae) were planted and irrigated with raw sewage (dry season) for three months in the respective plots. During the wet season, spinach and kales were planted and allowed to grow using rain fed cultivation. Samples of irrigation waters (sewage, rain, and tap), soils and plant tissues of spinach and kale for both watering regimes were processed and analyzed for metals by use of AAS. Soil and vegetable samples at the control plots, watered with municipal treated tap (dry season) and rain water(s) (wet season) were also analyzed for the same metals. Sewage water samples recorded mean values of 0.603 ppm for Zn and undetectable levels for both Cd and Pb. Rain and municipal treated tap water samples recorded similar mean values of 0.26, 0.010 and nil ppm for Zn, Cd and Pb respectively. Sewage irrigated and rain fed soils recorded mean values of 131.06, 1.13, 32.93 and 130.21, 1.20, 33.96 ppm for Zn, Cd and Pb respectively. Control soil samples recorded mean values of 114.58, 1.00, and 23.93 for Zn, Cd and Pb respectively. Sewage irrigated spinach and kales recorded mean values of 125.428, 1.323, 9.953 and 144.25, 1.1.242, 7.357 ppm for Zn, Cd and Pb respectively. Control spinach and kale samples recorded 61.5, 0.933, 6.7 and 43.5, 0.9, 5.7 for Zn, Cd and Pb respectively. The data obtained was subjected to ANOVA and the means separated to determine if there are significant differences in mean metal concentrations between sewage irrigated, rain fed and control samples. The results showed that `flushing out' effect by rain occurred for Zn while Cd and Pb recorded an increase in concentrations. The ANOVA (p>0.001) showed no significant differences in mean values between sewage irrigated and rain fed spinach and kales. In all cases, heavy metals uptake increased with plant age. However, the ANOVA (p<0.001) for mean heavy metal concentrations showed significant difference between sewage/rain fed spinach and kales, and control. The study showed there are significant contributions to heavy metal loads of soils, spinach and kales from use of raw sewage for irrigation in Maili Saba village and, if uncontrolled may reach extremely toxic levels in future.