Probability of Health Risk, Bioaccumulation, and Geochemical Fractions of Toxic Elements in Soils and Vegetables Impacted by Manures in Nigeria

Abstract

The intensive application of manures for the improvement of plant yield is a global practice by farmers notwithstanding the associated negative effects. The probability of health risk, bioaccumulation, and speciation of arsenic (As), cadmium (Cd), nickel (Ni), and lead (Pb) in soils and vegetables (Talinum triangulare and Telfairia occidentalis) impacted by untreated wastes from animal farms and inorganic fertilizers used as manures in the oil producing section of Nigeria were investigated. The total metal was determined using spectrophotometric methods while its geochemical fractions were identified following the modified Community Bureau of Reference (BCR) procedures of sequential extraction. The results obtained indicated the following mean concentrations (mg kg^-1): 1.64 ± 0.70, 3.17 ± 1.47, 62.22 ± 8.83, and 187 ± 29.73 for As, Cd, Ni, and Pb, respectively. The results revealed that the mean concentrations of all the metals in the studied locations, except As, were above their recommended limits by the Food and Agricultural Organization/World Health Organization (FAO/WHO). The mean values of As and Pb in T. triangulare
and T. occidentalis were also above the acceptable limits. Relatively higher levels of the metals were recorded at the studied locations as compared to the control site. The transfer factors of metals determined were generally lower than one; hence, the human exposure rate to these metals may not be high. The sequential extractions of metal indicated that As and Pb existed in the reducible fraction, Cd in the acid extractable fraction, and Ni in the residual fraction. The contamination factor of the metals revealed that As and Ni belong to the low contamination class, while Cd and Pb fall within the very high contamination category. The study showed that all the studied locations were heavily contaminated, with Uyo being the most polluted site. The principal component analysis identified anthropogenic factor as the main contributor to the presence of these metals in the investigated area. Daily intake rates for these metals via vegetable consumption by both the elderly and young populations were generally lower than their recommended oral reference doses. The non-carcinogenic risks obtained for both populations were less than one. However, the consumers of T. triangulare and T. occidentalis from the studied locations were exposed to Pb toxicity, with children being more vulnerable. This study has shown that the application of untreated animal wastes and inorganic fertilizers as manures has the potential to affect human health, bioavailability of metals, and the geochemical fractions of metals in soil over time.