Mercury in the River Nura and its floodplain, Central Kazakhstan: I. Riversediments and water

The River Nura in Central Kazakhstan has been heavily polluted by mercury o riginating from an acetaldehyde plant. Mercury in the riverbed is mainly as sociated with power station fly ash, forming a new type of technogenic depo sit. A systematic survey of the bed was carried out to establish the locati on, extent and nature of the contaminated sediments, and to evaluate the po tential for sediment transport. The bed sediments were found to contain ver y high concentrations of mercury, particularly in the first 15 km downstrea m of the source of the pollution. Average total mercury concentrations in t his section of the river are typically between 150 and 240 mg/kg, falling r apidly with increasing distance downstream. The estimated total volume of s ilts in the riverbed between Temirtau, the origin of the pollution, and Int umak Reservoir, located 75 km downstream, has been calculated as 463 500 m( 3), containing an estimated 9.4 tonnes mercury. Forty-six percent of the to tal volume of contaminated silts containing almost 95% of the mercury are l ocated in the upper 25 km of the river, however. The data clearly support t he hypothesis that large quantities of polluted sediment are not transporte d long distances downstream but are removed from the aquatic environment in times of flood and deposited on the low-lying lands adjacent: to the river . This process, however, does not stop mercury moving further downstream in the water column. (C) 2000 Elsevier Science B.V. All rights reserved.