Concentration levels of heavy metals in water, agricultural soil, selected vegetables and fruits grown along River Athi in Makueni County, Kenya

Environmental carcinogens in different components of the environment have been associated with different types of cancers globally. Over the past ten years, the number of cancer cases in Kenya had risen steadily. Levels of environmental carcinogens (heavy metals – cadmium, lead, chromium, mercury, and arsenic) in the water from the river Athi, farm soil and in selected vegetables (Amaranthus, kales, spinach, tomatoes) grown along the Athi river in Makueni County were assessed. The water’s physicochemical parameters i.e., electrical conductivity, total dissolved solids, temperature, and pH were also measured. Independent T-test tested the significant difference (p ≤ 0.05) of the samples in the two sampling sites. Pearson correlation was used to determine the relationship between heavy metal concentrations in water, farm soil, selected vegetables and fruits, where chromium and mercury had positive correlation with all samples, As showed an inverse relationship in most samples. Results showed that the mean concentrations (mg/l) of Hg (BDL), Cd (BDL), Pb (0.0073–0.0089), Cr (0.0218–0.0250) and As (0.6920–0.7707) in water all were within the limits except from As. Mean concentration values (mg/kg) of Hg (BDL), Cd (BDL), Pb (3.89–5.395), Cr (39.245–45.895), and As (1.5-1.7169) in soil which exceeded the WHO and FAO limit. Pb (0.8533–1.0433), Cr (16.68-25.3925), As (0.87-1.0133), Hg (0.755-.3935), and Cd (BDL-0.026), the heavy metal concentrations in kales exceeded the recommended limits except Cd and As in Site 1. The heavy metals mean values in spinach were Pb (0.86–1.37), Cr (11.795-13.5025), As (0.765–1.17), Hg (1.18–2.827), and Cd (0.043–0.165) they exceeded the limits except from Cd and As. In amaranthus the heavy metal concentrations means exceeded the set limits Pb (0.8333–1.575), Cr (16.5425–18.845), As (0.71–1.26), Hg (0.705–1.4535) and Cr (BDL-0.015) which was within the recommended levels. Pb (0.9733–1.0233), Cr (27.64-28.025), As (0.8733–1.1667), Hg (0.935–1.1175) in tomatoes the mean concentrations exceeded the limits except Cd (0.025–0.1215) which was within the set limits. In watermelon the heavy metal mean concentrations also exceeded the set limits Pb (1.1715-1.50), Cr (17.5667–30.285), As (0.449–0.942), Hg (0.62–0.818), except from Cd (BDL-0.145) which was within the recommended limits. Presence of heavy metals in the vegetables and fruits signified a health risk hazard from consumption of these vegetables. Evaluation of the uptake of heavy metals efficiency was calculated using the transfer factors of heavy metals from the soil to plants. The highest TF factors were in Cr in tomatoes with site 2 recording 0.704 and in watermelons in site 1 recording 0.660, similarly As recorded a significant uptake with amaranthus in site 2 recording 0.840 and site 2 recording 0.780 in spinach. The lowest levels were recorded in Pb however they still exceeded the safety limits in watermelon and spinach. The measurements on water quality parameters indicated that EC, TDS and pH were within the safety limits, though temperature in site 2 exceeded the set WHO safety limits. Similar contamination sources and exposure levels between the two sites for most parameters were suggested as an independent t-test (p ≤ 0.05) indicated no significance difference. Both the ministries of agriculture and health determined populations at increased risk thus directing the development of preventative strategies.