Effects of Soil Texture, PH and Distance on Levels of Heavy Metals in Plants and Soils along a Section of Nairobi-Nakuru Highway
Muli, Maurice Muteti
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Increasing population in Kenya has resulted into scarcity of land for farming. As a result, many people are opting to do farming even in the small land spaces available along the highways. However, heavy metal pollution along the highways is an issue of concern despite the fact that leaded fuel is no longer in use in Kenya because this exposes man and animals to heavy metals. The increase in transportation services is becoming a major source of heavy metals pollution particularly along the busy urban highways. The increased roadside farming is further enhanced due to unemployment and availability of fertile soils along the road reserves. Soils and plants on road reserves and the adjacent land usually accumulate heavy metals and the levels depend on some soil characteristics and the distance of farms from the highway. There is therefore need to continue monitoring the heavy metal levels in the environment. The purpose of the study was to determine the effects of clay content, pH and distance on levels of selected heavy metals in some vegetables grown along section of Nairobi–Nakuru highway, Kenya. Kale (Brassica oleracia), spinach (Spinacia oleracia), irish potatoes tuber (Solanum tuberosum) and irish potatoes leaves commonly grown on the highway and the soils were analysed for Pb, Cu, Zn and Cd. Samples were collected from the three selected sites along the highway during rainy and dry seasons in the months of April and August respectively. Each site was divided into four portions, which were 100 m apart. At each portion soil and vegetable samples were collected at a distance of 15 m, 30 m, 45 m and 60 m from the edge of the road. The soils were air-dried, crushed, sieved,weighed digested and analysed for pH, texture and total levels of heavy metals while vegetable samples were dried, crushed, weighed, digested and analysed for levels of the selected heavy metals using atomic absorption spectroscopy (AAS). ANOVA and t-test were used for data analysis. The results indicated that the soil pH ranged from 6.82±0.10 to 7.33±0.07 during the dry season and 6.55±0.17 to 7.07±0.05 during the wet season along the three sites. The pH differed significantly (p<0.05) along the three sites. The levels of Cd, Zn , Cu and Pb in the soil samples ranged from 0.02±0.01 to 0.13±0.03, 1.32±0.08 to 3.84±0.02, 0.03±0.01 to 1.12±0.07 and 0.05±0.00 to 0.80±0.02 μg/g respectively during the wet and dry season. The levels of Cd in the plant samples ranged from 0.01±0.00 to 0.19±0.00 μg/g. The levels of Zn in the plant samples were the highest and ranged from 0.02±0.00 to 5.22±0.06 μg/g . The levels of Cu and Pb in the plant samples ranged from 0.01±0.01 to 1.00±0.01 and 0.03±0.02 to 0.75±0.04 μg/g respectively. There was decrease in soil metal levels with increase in perpendicular distance (15-60 M) from the highway and the levels differed significantly (p<0.05). Despite higher levels of metals in the soils than plants, plants grown in such soils did not record corresponding high levels. Cu showed the highest correlations between the levels in the soils and plants. The levels of the metals in all the plant samples increased in the order Zn>Cu>Pb>Cd. The heavy metals concentrations varied among the tested crop plants which reflects their difference in their uptake capabilities and surface deposition. Among the samples analysed Spinacia oleracia was the highest accumulant of the metals therefore they would be recommended to be grown at distances of atleast 30 m off the highways. Plants grown in regions with lower soil pH had higher levels of metals. Soils with highest clay content had higher levels of metals. It would also be recommended that the soil texture, soil pH and vegetables type to be considered when selecting a farming land in areas prone to heavy metal pollution.