BACKGROUND: Active cultivation of wetlands without consideration of the quality of the sediment is a common practice in the city of Lagos. Wetlands in several parts of the city have been cleared for growing vegetables and other crops. As a buffer for all surface run-off from the surrounding areas, wetlands are a depository for whatever contaminants are sourced from the catchments; hence the need to ascertain the quality of sediment on which edible crops are grown to determine suitability for agriculture. METHODS: Wetland water samples were tested for pH levels, electrical conductivity, and total dissolved solids. Randomly selected core samples from one of the cultivated wetland areas located in the city center were taken to up to 8 cm in depth, then dried, recovered from the barrel and divided into groups of the following depths: 0-2 cm, 2-4 cm, 4-6 cm, and 6-8 cm. The dried and divided samples were subsequently sieved and analyzed for metal content using inductively coupled plasma mass spectrometry. Results for copper (Cu), lead (Pb), zinc (Zn), nickel (Ni), chromium (Cr) and vanadium (V) were geochemically evaluated. RESULTS: The wetland water samples were found to be acidic, ranging from 5.9-6.4. The electrical conductivity was 430-500 μS/cm, and total dissolved solids, 280-320 mg/L. The metal content results (in mg/kg) for samples from 0-2, 2-4, 4-6 and 6-8 cm depths were: Cu (13-861, 12-752, 10-899 and 11-707); Pb (29-1646, 26-2660, 33-2400 and 25-1818); Zn (112-7237, 76-9908, 63-7517 and 47-6579); Ni (3-219, 3-178, 3-186 and 3-176); Cr (10-147, 9-157, 14-160 and 16-147); and V (14-72, 12-75, 17-77 and 19-77). The evaluated results showed that the selected metal concentrations exceeded various guideline values. Calculated geo-accumulation index, metal ratio, and enrichment factor showed marked enrichment of metals in the wetland sediment samples. DISCUSSION: For the majority of the metals observed, the correlation matrix revealed strong positive relationships. For Cu, Pb, Zn, Ni, and Cr, there was a correlation matrix >0.8. This indicates similar origin and sourcing of the sediments. Vanadium, however, displayed a negative correlation with all the other elements. CONCLUSIONS: The study revealed that most of the cultivated sediment samples contained elevated levels of potentially toxic elements in the form of Pb, Cu, and Zn. The acidic nature of the wetlands water in the sediment samples also make them unsuitable for cultivation as the possibility of metal dissolution in transpirated water and bio-accumulation of potentially toxic elements in the cultivated vegetables is high.
BACKGROUND: Active cultivation of wetlands without consideration of the quality of the sediment is a common practice in the city of Lagos. Wetlands in several parts of the city have been cleared for growing vegetables and other crops. As a buffer for all surface run-off from the surrounding areas, wetlands are a depository for whatever contaminants are sourced from the catchments; hence the need to ascertain the quality of sediment on which edible crops are grown to determine suitability for agriculture. METHODS: Wetland water samples were tested for pH levels, electrical conductivity, and total dissolved solids. Randomly selected core samples from one of the cultivated wetland areas located in the city center were taken to up to 8 cm in depth, then dried, recovered from the barrel and divided into groups of the following depths: 0-2 cm, 2-4 cm, 4-6 cm, and 6-8 cm. The dried and divided samples were subsequently sieved and analyzed for metal content using inductively coupled plasma mass spectrometry. Results for copper (Cu), lead (Pb), zinc (Zn), nickel (Ni), chromium (Cr) and vanadium (V) were geochemically evaluated. RESULTS: The wetland water samples were found to be acidic, ranging from 5.9-6.4. The electrical conductivity was 430-500 μS/cm, and total dissolved solids, 280-320 mg/L. The metal content results (in mg/kg) for samples from 0-2, 2-4, 4-6 and 6-8 cm depths were: Cu (13-861, 12-752, 10-899 and 11-707); Pb (29-1646, 26-2660, 33-2400 and 25-1818); Zn (112-7237, 76-9908, 63-7517 and 47-6579); Ni (3-219, 3-178, 3-186 and 3-176); Cr (10-147, 9-157, 14-160 and 16-147); and V (14-72, 12-75, 17-77 and 19-77). The evaluated results showed that the selected metal concentrations exceeded various guideline values. Calculated geo-accumulation index, metal ratio, and enrichment factor showed marked enrichment of metals in the wetland sediment samples. DISCUSSION: For the majority of the metals observed, the correlation matrix revealed strong positive relationships. For Cu, Pb, Zn, Ni, and Cr, there was a correlation matrix >0.8. This indicates similar origin and sourcing of the sediments. Vanadium, however, displayed a negative correlation with all the other elements. CONCLUSIONS: The study revealed that most of the cultivated sediment samples contained elevated levels of potentially toxic elements in the form of Pb, Cu, and Zn. The acidic nature of the wetlands water in the sediment samples also make them unsuitable for cultivation as the possibility of metal dissolution in transpirated water and bio-accumulation of potentially toxic elements in the cultivated vegetables is high.