Human Health Risk Assessment of Heavy Metals in Vegetables from Farmlands Irrigated with Palm Oil Mill Effluents in Umuaka, Njaba, Imo State

Abstract

This study evaluated human health risk assessment of heavy metals in vegetables from farmlands irrigated with palm oil mill effluents in Umuaka, Njaba, Imo State. Drinking water from boreholes, soil samples and Vegetables (Amaranthus hybridus, Gongronema latifolium, Occidentalis telfairia, Venonia amygdlina, Ocimum gratissimum and Talinum triangulare) from palm oil mill effluents irrigated farmlands in various communities (Ezeala, Eziisu, Nnerim, Olori, Amurie and Obeakpu) were used for this study. Sample digestion, heavy metals analyses, toxicological characterization and risk assessements were carried out using standard methods and models. The farmlands irrigated with palm oil mill effluents had significantly higher concentrations of heavy metals relative to their respective control soils. In Ezeala, lead and manganese (0.04 ± 0.01 and 1.03 ± 0.04 ppm) in drinking water, arsenic in drinking water of Amurie (0.02 ± 0.00 ppm) were higher than their respective permissible limits in drinking water. Heavy metal concentrations in vegetables varies with species, types of metals and the level of palm oil mill effluents used for irrigating the farmlands. The plant concentration factors of heavy metals indicate that V. amygdalina from Ezeala and Amurie farmlands accumulate cobalt, zinc and nickel respectively posing serious adverse health concern to consumers. T. triangulare from Obeakpu farmland was the only vegetable capable of bioaccumulating iron, thus accounting for high risk of iron toxicity. The enrichment factors show that only zinc has natural source across the palm oil mill effluents irrigated farmlands while Hg and As have anthropogenic source with high enrichment across the farmlands. However, Cd, Cr, Pb, Mn, Ni, Cu, and Co show both natural and anthropogenic sources across the irrigated farmlands. The pollution load index (PLI) of Zn and Fe were below 50 in all the farmlands irrigated with palm oil mill effluents, while Cr, Pb, Hg and As had PLI ≥50 to ≥100 across the farmlands. However, Cd, Mn, Ni, Cu and Co have PLI < 50 to ≥ 100 in most of the farmlands. Moderate daily intake of Cd, Cr, Zn, Pb, Cu, Co, Fe and high daily intake of Mn, Ni, Hg and As were detected in vegetables. The target hazard quotients of the heavy metals indicate Cd, Cr, Zn, Pb, Cu and Co without adverse health effects while Mn, Ni, Hg and As exemplified adverse health risks in all the vegetables. High hazardous index (HI > 1) was observed in all the vegetables. The lifetime cancer risks for Cr, Ni, and As were greater than the minimum predictable range of lifetime risks for carcinogens (> 10-4). The Pb and Cd cancer risks were within the predicted lifetime cancer risks of 10-6-10-4 for carcinogenic substances. The total cancer risks of carcinogenic metals of vegetables were above the predicted range of lifetime cancer risks. The findings of this study suggest that palm oil mill effluents are not suitable for irrigation of farmlands as it contains high levels of heavy metals which contaminate soils. The uptake of these heavy metals from soils by vegetables have adverse effects on the quality of the vegetables which could negatively impair health of the consumers. The high levels of Mn, Ni, Hg and As in these vegetables pose the highest risks of non-carcinogenic effects to the consumers. Additionally, Ni, Cd, Pb, As and Cr with high cancer risks and total cancer risks within the acceptable lifetime predicted cancer risks indicate that the consumption of these vegetables could elicit harmful effects and palm oil mill effluents should not be used for irrigation purposes in order to safeguard human health.