Progress in soil mercury transport and transformation studies in karst regions

Karst ecosystems with a high mercury background are an important component of the global mercury biogeochemical cycle. China’s southwest karst area is located on the global mercury mineralization belt, is the richest area of mercury resources in China, is also part of China’s “southwest large-scale low-temperature mineralization area”, Guizhou Province is China’s most important mercury resource base. In addition, mercury is an important associate element of large exposed low-temperature hydrothermal deposits, and the mining and smelting of mercury and other mineral resources releases large amounts of mercury to the surface environment. Due to the geochemical background of higher mercury, the mercury content of surface soil and deep soil in the karst region of southwest China is 3-5 times higher than that in other regions. The high-mercury geological background drives the geochemical cycle of high-intensity mercury. China’s southwest karst region is one of the world’s largest karst concentrated contiguous areas. Guizhou Province is located in the heart of the southwest karst. Surface ecosystems are fragile, underground karst structures are highly developed, and soil mercury migration and transformation may affect groundwater quality in high-mercury background karst areas.

In this paper, the vertical transport and transformation of soil mercury in two karst watersheds, Hui Long and Chenqi, the former located near the mercury mining area, representing the high mercury background area, and the latter representing the regional low mercury background area. The results showed that the soil mercury reservoir in the Huilong small watershed was as high as 44.4±4.2 g m-2, while the soil mercury reservoir was 0.17±0.02 g m-2 in the Chen Qi watershed. The results of X-ray absorption proximal structure showed that mercury sulfide, as the main mineral of mercury ore, gradually changed to other mineral types during soil formation, and the slow transformation of mercury’s primary mineral in soil may have an important impact on groundwater pollution. As the soil depth increases, the proportion of mercury in the organic bound state decreases and the proportion of ionic mercury increases. The proportion of organic mercury in the soil organic bound form of farmland in the Huilongxia Basin decreased from 44.0% to 20.3% with soil depth (10-160 cm), the proportion of organic bound mercury in forest soil decreased from 39.3% to 34.5%, the proportion of ionic mercury increased with soil depth, the proportion of ionic mercury in farmland soil increased from 4.2% to 10.7%, and the proportion of ionic mercury in forestland soil increased from 6.7% to 11.6%. The trend of ionized mercury increasing with soil depth indicates an increased risk of soil mercury entering groundwater in karst areas with a high mercury background. The results of mercury isotope source resolution showed that more than 80% of the mercury in groundwater in the small watershed of the study area came from soil mercury input. Therefore, the treatment and remediation of soil mercury pollution in high-mercury background areas should not only focus on the soil surface, but also carry out necessary monitoring and corresponding countermeasures for the possible migration of soil mercury to groundwater.

Figure 1: Transport and transformation of mercury in soil profiles in karst watersheds

The findings were published in Water Research, a journal of environmental science and water research. The first author of the paper is Dr. Xia Jicheng, Institute of Geochemistry, Chinese Academy of Sciences, and the corresponding author is Feng Xinbin, researcher of the Institute of Geochemistry, Chinese Academy of Sciences. This research was supported by the National Natural Science Foundation of China (41921004, U1612442 and 42107497) and the Strategic Key Research Program of the Chinese Academy of Sciences (XDB40000000). (Source: Institute of Geochemistry, Chinese Academy of Sciences)

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