Analysis of lead zinc and copper levels of roadside soil and plants across Thika-Nairobi highway, Kenya

Abstract

The importance of studying highway pollution has increased dramatically in recent years due to the rapid growth in the transportation sector which has resulted in environmental degradation. Increase of registered motor vehicles has been on the rise with 70 % being petrol driven while 30 % having diesel propelled engines. In the year 2000, the number of registered vehicles increased by 20,262. But due to improvement in economic growth and reforms in transportation sector that facilitated the reinforcement of traffic rules and regulations by Kenya government on January 2004, a major influx of motor vehicles on Kenyan roads increased by 42,634 which rose further by 85,324 in the year 2007. This rapid increase in transportation services is now a major source of heavy metals pollution, particularly along the busy urban highways. The exposure of man to these heavy metals has caused great concern due to their effect on human health. Therefore, there is need to continue monitoring the heavy metal inputs and levels in the environment. This study was undertaken to determine the levels of Pb, Zn and Cu in the soil and plants across the Thika-Nairobi highway, which is one of the busiest road connecting Nairobi City and other provinces. The samples were collected from five different sites situated along the highway and their levels determined by X-Ray Fluorescence (XRF) technique. The soil and plants samples were investigated as a function of traffic density on the highway, distance from highway, and depth in soil profile. The results showed significant difference (P<0.05) for Pb, Zn and Cu to increase with increase in traffic volume. There was gradual decrease in soil metal levels with increase in distance (5-80 M) from the highway. The mean average levels of Pb distribution on the top soil (up to 10 cm below surface) in the five sampling sites along the Thika-Nairobi highway ranged from 83.2 to 208.1 µg/g during the dry season and 52.5 to 151.8 µg/g during the wet season. Similarly, Zn values ranged from 94.4 to 267.2 pg/g for the dry season and 57.5 to 212.6 µg/g during the wet season. In addition, Cu recorded between 10.4 to 25.3 µg/g during the dry season and 5.6 to 14.7 µg/g during the wet season. The study also showed higher accumulation of Pb, Zn and Cu in the soil depth, 0-10 cm than in the soil depths 10-20 and 20-30 cm. The mean Pb levels on 0-10 cm ranged from 14.5 to 343.5 pg/g and rapidly decreased with depth from ND to 167.6 µg/g at 20-30 cm while Zn registered 21.5 to 376.9 µg/g (0-10 cm) and ND to 194.3 µg/g (20-30 cm) in addition to Cu recording ND to 56.8 µg/g (0-10 cm) and ND to 19.2 µg/g (20-30 cm) across the highway during dry and wet seasons. The levels of Pb, Zn and Cu in pasture grass (Typha elephantina) were higher than in napier grass (Pennisetum purpureum). This indicated that pasture grass posed greater hazard potential than napier grass since it is widely spread along the highway and is oftenly harvested by farmers to make hay to feed animals. The study findings would enable planning and decision making by the concerned stakeholders in resource management geared towards attainment of vision 2030 and thus making Kenyan highways pollution free and therefore creating a habitable ecosystem for all