Mongolia signed the Minamata Convention on mercury on 10 October 2013, with the overall aim to protect the human health and the environment from the adverse effects of mercury. Towards this objective, United Nations Industrial Development Organization in association with the Ministry of Environment and Green Development of Mongolia implemented a GEF funded project entitled “Reduction of exposure of mercury to human health and the environment by promoting sound chemical management in Mongolia”.
The project focused on strengthening national and local capacity to effectively manage and reduce mercury emissions. In this context UNIDO published an international tender in March 2015 for deploying pilot scale demonstration of remediation technologies at Boroo River mercury contaminated hot spot.
Boroo site is located 150km north-west from the capital city Ulaanbaatar. The site used to be exploited by a gold extraction company. A large crack in an amalgamation tank in 1956 released a big amount of mercury into the river basin. Large areas around the site were contaminated and mercury spread over the river basin and penetrated deeper in soil and in river sediments.
The project for the pilot remediation tests was awarded to the Consortium of Polyeco, Mayasa and Emgrisa. The ultimate goals of this project were a) to design and establish a monitoring program at Boroo site; b) to present pilot mercury stabilization and solidification remediation methods; c) to deliver theoretical and practical training on site monitoring and remediation technologies to increase national capacity.
A monitoring network was set up at the site in October 2015 and a monitoring plan was developed to acquire additional data to set up a detailed conceptual site model. Twelve monitoring wells were established upstream, downstream and within the contaminated area. Several surface soil, surface water and groundwater samples were collected and analyzed for heavy metals. At the same time several influencing parameters, such as conductivity, pH, temperature, redox potential, oxygen demand and groundwater level were measured to evaluate site conditions. The established monitoring network allows continuous monitoring of the site in the future.
Results of the monitoring survey showed that a widespread contamination of arsenic and mercury was present on surface soil. The total polluted area is estimated to have a surface of around 100.000 square meters. Arsenic and mercury traces were also found on surface water and groundwater samples. An arsenic plume was spread north-northwest of the site in the direction of groundwater flow.
Pilot tests were conducted in September 2016.
The first pilot test was a chemical stabilization test. Polluted soil was removed and placed on plastic liners in piles of maximum 40cm in height. Soil was sieved to disaggregate fine fractions and a selected reagent was added and mixed with the soil with the use of a rotovator. The treated soil was sampled and analyzed for mercury, arsenic and cadmium in the solid matter and leachate.
The second pilot test was an immobilization test. Polluted soil was removed and prepared in a similar way as in the stabilization test. A slurry of additives was prepared with the mix of water. The slurry was sprinkled on the polluted soil while the soil was being mixed.
Concentrations of leachable mercury, cadmium and arsenic were compared before and after treatment for both pilot tests. A drastic reduction in concentrations was observed. Chemical stabilization test proved to be more efficient compared to immobilization test, but both remediation techniques tested in the surface soil proved to be technically feasible to be applied at the site for the reduction of leaching potential of heavy metals.
It has to be highlighted that also laboratory tests have been conducted by Consortium partners EMGRISA/MAYASA for the evaluation of other treatment methods efficiency such as phytoremediation, thermal desorption and soil washing. Due to technical and financial contingencies, chemical stabilization and immobilization were selected for pilot tests.