HEAVY METAL CONCENTRATIONS AND SYSTEMIC TOXICITY IN Rattus norvegicus EXPOSED IN SITU TO GROUNDWATER AT OLUSOSUN LANDFILL, LAGOS, NIGERIA

Abstract

Landfilling remains the major method of municipal solid waste disposal in Nigeria. However, the inadequacy of personal protective equipment and proximity of residential quarters to waste dumpsites predispose workers and residents to landfill toxic heavy metals, which could pose public health risk. There is a plethora of ex situ studies on systemic toxicity of landfill soil, groundwater and leachates in prokaryotic and eukaryotic systems. However, paucity of information exist on body-burden of heavy metals and systemic effects on exposed populations in situ. This study was carried out to assess heavy metal accumulation in tissues, and systemic toxicity in rats (Rattus norvegicus) exposed in situ to groundwater at Olusosun landfill in Lagos, Nigeria. Olusosun landfill and a residential environment (17.3 km southwest of the landfill) were chosen as the study and control sites, respectively. Male rats (weight = 69.2±4.1g; 8-9 weeks old; n=30 per location) were exposed daily to groundwater via drinking at three locations on the landfill, and the control for 4, 8, 12, 16, 20 and 24 weeks. Post-exposure, blood and organs were collected (5 rats per location per periods); lead, cadmium and chromium were analysed in blood, liver and kidney using standard procedures. The DNA damage was assessed using bone marrow micronucleus assay. Gastric physiopathological indices (Parietal Cell Count, PCC; Mucus Cell Count, MCC; and Gastric Mucous Secretion, GMS), serum hepatic and renal function parameters (Aspartate Aminotransferase, AST; Alanine Aminotransferase, ALT; urea; and creatinine) were assessed using standard procedures. Tissues (liver, kidney, lungs, spleen and stomach) histology and hepatic oxidative stress biomarkers (Catalase, CAT; Superoxide Dismutase, SOD; Reduced Glutathione, GSH; and Malondialdehyde, MDA) were also evaluated using standard procedures. Data were analysed using descriptive statistics, ANOVA and Pearson’s correlation (r) at α0.05. The concentrations (mg/L) of selected metals were higher in blood (Pb:0.04-0.61, Cd:0.03-0.49 and Cr:0.05-0.27), liver (Pb:0.22-0.56, Cd:0.03-0.50 and Cr:0.10-0.31) and kidney (Pb:0.38-0.69, Cd:0.32-0.45 and Cr:0.10-0.23) of exposed rats compared with control (blood-Pb:0.03-0.41, Cd:0.02-0.15 and Cr:0.02-0.08; liver-Pb:0.05-0.22, Cd:0.01-0.16 and Cr:0.05-0.13; kidney-Pb:0.10-0.31, Cd:0.07-0.14 and Cr:0.04-0.18) at all exposure periods. There was significant time-dependent (r=0.5) increased in micronuclei induced in bone marrow cells of exposed rats (1.9-6.0 folds) suggesting that the accumulated heavy metals were genotoxic in the organ-system of the exposed rats. Compared with control, PCC significantly increased (1.1-1.5 folds), while MCC and GMS decreased (1.1-2.2 and 1.0-1.3 folds, respectively). Also, there was time-dependent significant decrease in PCC (r=-0.7), MCC (r=-0.3) and GMS (r=-0.4). There were significant increases in activities of AST (2.0-4.9 folds), ALT (1.1-3.1 folds), levels of urea (1.1-1.7 folds) and creatinine (1.3-2.2 folds). Tissue-lesions observed in exposed rats included hepatic necrosis and steatosis; tubular fibrosis and glomerular degeneration; alveolar hyperplasia; fusion of white pulps and haemosiderin deposition; and gastric mucosa desquamation. The CAT activities, levels of GSH and MDA increased significantly (1.2-3.5, 1.3-1.5 and 1.1-2.8 folds, respectively); while SOD decreased (1.3-2.4 folds) suggesting free radical generation in exposed rats. Exposed Rattus norvegicus at Olusosun landfill had heavy metal accumulation in organs which induced genetic, histologic and gastric-physiological damage.