Heavy metal levels and mercury speciation in water, soil and sediments in a highly contaminated mining area in Khaidarkan, Kyrgyzstan

The world second largest mercury (Hg) mine and smelter, the Khaidarkan Mercury Plant (KMP) is located in Khaidarkan, a small city in a valley in Batken oblast, Kyrgyzstan. The smelter roasts locally mined cinnabar and captures the mercury by condensation. At the plant there are several slag deposits with over 500 millions tonnes of exhausted ores as well as quarry material from the mines,containing traces of Hg and associated contaminants, such as arsenic, antimony, cadmium, lead and zinc. A large tailing pond containing wastes from the smelter has also been generated. Slag heaps and tailing pond are open and exposed, potentially leading to the spread of pollutants and regional contamination of soils and waters. Lack of fences allows animals to access contaminated sites for drinking and grazing. Seepage water from the tailing drains into a tributary of the Syr Darya River, the Shaktnaya River. Soil samples were collected along an elevation gradient on the main valley slope, and soil, sediments and water were sampled downstream from the Hg tailing area. Background samples were collected from a monitoring site located more than 10 km upstream (south) of the plant. In this study the levels of Hg in water, sediments and soil has been measured with a direct mercury analyser (DMA-80). A sequential extraction procedure was conducted in order to measure the main pools of Hg in the soil and thereby assess main mechanisms governing Hg mobility and transport. Water samples were collected in streams in the area. The water samples and extracts from the sequential extraction were analysed for concentration of Hg with a Millennium Merlin Hg analyser. In addition, the levels of heavy metals in the soil, sediment and water samples were measured with ICP-AES. The level of Hg in soil and sediment in Khaidarkan is high, ranging from 0.4 to 8 795 μg g-1. The highest value, which is somewhat extreme, was found in sediments in a small stream draining the slag heaps. Up the valley slope, values from 0.4 to 53 μg g-1 were found, while downstream from the tailing pond the level of Hg ranged from 3.5 to 217 μg g-1. The concentration of Hg in the tailing pond sediments was 353 μg g-1. This concentration of Hg is quite high and exemplifies the poor efficiency of the waste management at the facility. The levels of other associated heavy metals are generally high, especially for antimony, cadmium and arsenic. Enhanced levels were found also for lead and zinc. It is hypothesised that the contamination of these heavy metals in the area is a result of the anthropogenic activity of the KMP. Interpretation of statistical analysis of the data(correlation matrixes, cluster analysis and Principal Component Analysis (PCA) supports this postulation. The sequential extraction procedure revealed that Hg is mainly present as residual Hg (as HgS or bound in silicates) in the soil and sediments. The soil samples up-valley from the slag heaps contain in addition a significant amount of strongly bound Hg (i.e. acid soluble). As the levels of Hg decrease as a function of distance from the source up-valley from the burners and slag heaps it is apparent that atmospheric emission and transport of Hg is significant. Even though a few of the downstream sediment samples contain some elemental Hg, it appears as Hg is mainly mobilised and transported from the slag heaps and tailing pond as residual Hg, in the form of colloids or particles. High level of Hg in a flooding area indicates that Hg is mobilised and transported in periodic peak flow events. The high level of dissolved Hg that was found in the drainage waters indicate that solved Hg species are also mobilised from the waste areas to the surrounding environment