Legacy Contamination from Mercury Mining in the Fergana Valley Region of Central Asia

The toxic metal mercury (Hg) has been mined, processed, and used throughout the Fergana Valley region of post-Soviet Central Asia for millennia. Although most historical Hg mining activities have ceased throughout the Fergana Valley region, Hg is still mined, processed, and exported globally from the Khaidarkan kombinat in southwestern Kyrgyzstan. Despite the rich history of Hg mining and use throughout the Fergana Valley region, the legacy effects of these activities on environmental Hg contamination remain undescribed. Mercury concentrations were analyzed in topsoil, terrestrial vegetation, earthworms, riverine sediments, and fish collected from sites with varied histories of Hg mining within the Fergana Valley region. Environmental and biological Hg concentrations were greatest at contemporary mining sites where Hg has been mined after 1940, intermediate at ancient mining sites where all historical Hg mining activities ceased before 1300 AD, and lowest at reference sites without known Hg mining history. For all environmental media and biota, Hg concentrations were 1-2 orders of magnitude greater at contemporary mining sites than at reference sites. Elevated Hg concentrations at contemporary mining sites are attributed to the recency and intensity of Hg mining and showcase the detrimental effects of Hg mining on diverse environmental media and biota. Elevated Hg concentrations at ancient mining sites are attributed to a combination of (1) legacy Hg contamination in soils and sediments introduced by historical mining and processing activities over 700 years ago and (2) the presence of naturally Hg-rich geologic belts upon which ancient mines were constructed.