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What happened?
In the region of the Romanian Nagybánya (Baia Mare) AURUL Co., a Romanian-Australian joint venture is engaged in reprocessing refuse deposits accumulating at neighbouring non-ferrous metal mines. The aim of the processing is to extract precious metal (gold, silver) content of the (otherwise considerable) refuse remains by cyanide dissolving method. This ore refining procedure has a large water demand, thereofre washing waters containing cyanide go back into the technology after sedimentation. The enbankment of the sedimentation pond of the company bursted in 30. January 2000. at about 22 hours on a 25-30 m section, so some 100-120 thousand m3 slop water extremely contaminated with cyanide and heavy metals got into rivers Zazar and Lápos (Lãpuº), from where through the Szamos (Someº) getting into the Tisza, it caused the most serious water pollution registered in Hungary till now. By the standpoint of the international examining commission, the pollution has been caused by human negligence!
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It should be mentioned, that there was no damage averting plan to be use in case of disorder, so when the disaster ensued no attempt have been made to localise the pollution, i. e. to prevent the expansion of the pollution. Though subsequently attempts have been made to lessen poisoning by adding sodium-hypoclorite, but this has been proved ineffective then. Cessation of the polluting source has happened on 31. January.
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In the slop water of the clarifying plant concentration of cyanide compounds were 400 mg/l. It is characteristic to the measure of the pollution, that cyanide concentration values in the Szamos entering Hungary showed about 30 mg/l. In the Tisza near the Szamos mouth 13.5 mg/l, and after covering more than 500 km distance, in spite if the dilution effect of the tributaries, within the southernmost Hungarian section at Tiszasziget, it could be measured still 1.49 mg/l concentration. The pollution has further devastated on the Danube - which it has reached on 13. February - by the report of the Serbian minister of environment protection, with some 0.5 mg/l value. Fish death could be observed even below Beograd. The concentration of the pollution entering Romania again at the Iron Gate was 0.045 mg/l, but values (0,139 mg/l) above the limit have been measured even on the Bulgarian section.
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The ore leaching technology applied by the AURUL Co. is known since 1890. In the course of the process the pulverized ore containing refuse is first treated with sodium-cyanide (NaCN) solution, as an effect of which the gold content gets dissolved in the form of gold-cyano-complex: Na[Au(CN)2]. From this solution solid gold is separated with adding zinc powder (Zn)
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1. 4Au + 8NaCN + O2 + 2H2O => 4Na[Au(CN)2] + 4NaHO 2. 2Na[Au(CN)2] + Zn => 2Au + Na2 [Zn(CN4)]
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On the basis of these the slop water of the clarifying plant contained primarily sodium-cyanide (NaCN), and different kinds of metal-cyano-complexes, - by the results of the later measurings - in the largest amount copper [Cu(CN)2] - [Cu(CN)3], in lesser amount zinc and other heavy metals. NaCN is present in the water practically in dissociated form, this makes the so-called free cyanide, which is able to leave the system by evaporation. Metal-cyano-complexes are compounds dissolving well in water, and are stable within chemical conditions generally characteristic to surface waters, that is they remain within the water unchanged for a longer period.
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The secound considerable pollurion happened in Romania one month after flowing down of cyanide pollution. Remin Mine Company operationg neas Borsabánya (Baia Borºa) is engaged in exploitation heavy and non-ferrous metal and preparing them for smelting. The dam of the slop water clarifying plant of the company, in consequence of considerable rise in water level ensued by sudden snow melting, has bursted in the morning of 10. March 2000., and according to the estimations about 20 thousand m3 mud contaminated with heavy metals has got into the valley below the reservoir. As an effect of the rains this mud has continuously washed into the local brook named Novac, from where the pollution got into the Tisza in more waves through the river Visó (Viºeu). This later pollution has come upon the section of the Tisza above Szamos uninjured by the cyanide pollution too. Since the dirt wave has arrived by floods in all cases, that time the forebank of the Tisza was under water. In consequence of this slop water and the deposited mud has threatened the whole forebank plant and animal life, including backwaters and navvy pits of extra nature conservation value too.
The outflow of the contamination from the reservoir could be reduced in 11. March 11:22, but ceasing the dam burst and percolation ended only days afterwards.
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 The first wave of the heavy metal pollution has reached the Hungarian course of the Tisza at Tiszabecs in 11. March in the evening hours, the secound in 15. March at dawn, while the third on the same day in the morning. Pollution appeared in March in gradually lower peaks and the last wave has left the country in the beginning of April.
On the basis of the examinations it could be stated, that within the contaminated mud lead, copper and zinc were present in the largest amount, mostly linked to floating material. Highest heavy metal concentrations at Tiszabecs in case of lead (2.9 mg/l), copper (0.86 mg/l) and zinc (2.9 mg/l) too, significantly (values of lead and zinc of one order) exceeded the limit given for the 5th class, "heavily polluted" surface waters. Heavy metals linked to floating material are typically not dissolving among chemical circumstances of surface waters, but with the decrease of the stream speed they are deposited according to their different density. Their further life is determined by chemical relations of the sediment (pH, oxygen content) and biological parameters. |