Heavy metals and methylmercury in a tropical coastal estuary and a mangrove in Brazil

Concentrations of mercury, lead, zinc, iron, chromium, cadmium and copper were determined in sediment profiles from Jequiá mangrove, a polluted region of Guanabara Bay in Brazil. Methylmercury concentrations were also determined, together with the ratios of methylmercury to total mercury (% MeHg). The mangrove is influenced by a polluted creek and estuarine waters. In general, 87% of the concentration of lead, iron, chromium, cadmium and copper was determined not to be bioavailable due to the elevated organic carbon and sulphide content in sediments. For most of the contaminants studied, the sediment enrichment factor for metals was high (cadmium, 650; lead, 450; copper, 250; chromium, 170; zinc, 24; mercury, 9). Sediment samples did not contain high concentrations of methylmercury (range 3.11–9.74 ng MeHg g−1 dry wt) or total mercury (0.52–2.38 mg THg kg−1 dry wt). Total mercury (THg) concentrations in sediment were enriched in the upper layers and decreased gradually with depth. THg concentrations in sediment with little coastal influence contained greater concentrations in the superficial layers (mean: 2.16±0.22 mg kg−1 dry wt) than ones with intensive estuarine waters influence (mean: 1.31±0.07 mg kg−1 dry wt). The percentage of total mercury to MeHg decreased with distance from the river mouth to the estuary, and was greater in locations with the least influence from estuarine waters (6.34%) than in ones close to estuarine waters (1.51%).