Mercury in the River Nura and its floodplain, Central Kazakhstan: II. Floodplain soils and riverbank silt deposits

A unique and serious case of mercury pollution has occurred in the River Nu ra and its floodplain in Central Kazakhstan, where mercury-rich wastewater from an acetaldehyde plant was discharged largely without treatment for sev eral decades. In the river, the mercury became associated with millions of tonnes of power station fly ash, forming a new type of deposit known as 'te chnogenic silt'. During spring floods these highly contaminated silts are t ransported downstream and are dispersed over the floodplain, leading to wid espread contamination of the land. A detailed survey of the floodplain was carried out to investigate the extent of pollution and to assess the need f or remediation. Total mercury concentrations in the topsoils of the floodpl ain ranged from near background levels to over 100 mg/kg. Mercury concentra tions in river bank deposits were found to range from a mean of 73.3 mg/kg Hg in the most contaminated section of the river to a mean of 13.4 mg/kg Hg at a distance of 70 km downstream. Concentrations were lower than correspo nding concentrations in the riverbed within the first 25 km from the source of the pollution, but thereafter they were significantly higher. The resul ts show that over the past 30-40 years a large proportion of the contaminat ed sediments from the river was deposited on the 70 km of banks and in the floodplain below the pollution source. Topsoils of the floodplain and silt deposits located on or close to the river banks contain an estimated 53 t a nd 65 t of mercury respectively, with an additional 62 t in a small natural swamp which was formerly used as a waste disposal area. The contamination is serious but relatively localized, with > 70% of the total amount of merc ury in topsoils and > 90% of mercury in river bank deposits located within 25 km from the source. (C) 2000 Elsevier Science B.V. All rights reserved.