Environmental quality assessment of reservoirs impacted by Hg from chlor-alkali technologies: case study of a recovery

Mercury (Hg) pollution legacy of chlor-alkali plants will be an important issue in the next decades with the planned phase out of Hg-based electrodes by 2025 within the Minamata convention. In such a context, the present study aimed to examine the extent of Hg contamination in the reservoirs surrounding the Oltchim plant and to evaluate the possible improvement of the environmental quality since the closure of its chlor-alkali unit. This plant is the largest chlor-alkali plant in Romania, which partly switched to Hg-free technology in 1999 and definitely stopped the use of Hg electrolysis in May 2012. Total Hg (THg) and methylmercury (CH₃Hg) concentrations were found to decrease in the surface waters and sediments of the reservoirs receiving the effluents of the chlor-alkali platform since the closure of Hg units. Hence, calculated risk quotients (RQ) indicated no adverse effect of Hg for aquatic organisms from the ambient water exposure. RQ of Hg in sediments were mostly all higher than 1, showing important risks for benthic organisms. However, ecotoxicity testing of water and sediments suggest possible impact of other contaminants and their mixtures. Hg hotspots were found in soils around the platform with RQ values much higher than 1. Finally, THg and CH₃Hg concentrations in fish were below the food safety limit set by the WHO, which contrasts with previous measurements made in 2007 revealing that 92 % of the studied fish were of high risk of consumption. Discontinuing the use of Hg electrodes greatly improved the surrounding environment of chlor-alkali plants within the following years and led to the decrease environmental exposure to Hg through fish consumption. However, sediment and soil still remained highly contaminated and problematic for the river reservoir management. The results of this ecological risk assessment study have important implications for the evaluation of the benefits as well as limits of the Minamata Convention implementation.