Assessment of heavy metal pollution and scope definition in the Indigenous Zinc Smelting agglomeration areas of northwest Guizhou Province of China

Heavy metal pollution has been a serious risk in the Indigenous Zinc Smelting in Northwest Guizhou, China. Long-term open-air stacking of waste slag has also led to a great increase in the heavy metal reactivity under suitable environmental conditions after leaching, thus continuously migrating to the surrounding groundwater, surface water, and soil. A substantial amount of mining slag has been left behind, due to outdated technology and extensive production. Therefore, it is very urgent to clarify the contamination status of heavy metals in the soil, water bodies, and sediments in the areas, particularly with the concentrated distribution of Indigenous Zinc Smelting slag and their potential ecological risks, in order to protect the local land and water resources for the food safety and public health. This study focused on an 8 km² area with a relatively wide distribution of Indigenous Zinc Smelting slag from traditional smelting areas. Human pollution sources were avoided to collect the surface soil samples within a grid of 1/9 km². The background levels of heavy metals in surface soils were established over the region. Some samples were collected from the smaller watersheds, including surface soil, sediment from water systems, leachate, mine slag, and water bodies, according to the hydrogeological and topographical conditions. Additionally, the samples of deep soil profile were gathered from the site, where the largest amount of mine slag was stored for the longest time. The content of heavy metals and pH value were measured in various samples using the single-factor pollution index, Nemerow comprehensive pollution index, and potential ecological risk index. The spatial distribution and pollution status of heavy metals were analyzed in different media. The results show that the surface soil in the study area was mainly acidic, where the average contents of As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn were 23.54, 3.20, 208.78, 193.02, 0.07, 91.41, 315.80, and 479.13 mg/kg, respectively. Except for Hg and pH, the contents of the rest elements were higher than the background value of surface soil in the Bijie region. The contents of As, Cd, Cr, Cu, Ni, Pb, and Zn exceeded the national environmental background values of surface soil. The overall slag was neutral with the average contents of As, Cd, Cu, Hg, Pb, and Zn higher than the average value of surface soil in the study area. Especially Pb, the raw materials used in local traditional zinc smelting were imported the lead-zinc ores. The soil profile samples show that the heavy metals were mainly concentrated in the surface soil of 0-20 cm, whereas, the heavy metals in deep soil were mainly controlled by geological background and primary soil. The highest contents of As, Cd, Cu, Pb, and Zn in small watershed sediments were 274.00, 11.90, 488.00, 2 340.00, and 2 580.00 mg/kg, respectively, all of which were located in the central area of traditional zinc smelting slag. The highest contents of Cr, Cu, Ni, Pb, and Zn in the surface water of a small watershed were distributed downstream of the slag concentrated distribution area, which were 43.60, 64.90, 15.70, 226.00, and 223.00 mg/L, respectively. Only Ni was slightly higher than the content of slag leachate in groundwater. The contents of the rest elements were lower than those in the leachate. Single-factor evaluation was performed on the pollution of heavy metals in the surface soil. The Cd and Cu shared the highest proportion of pollution, with the Cd in the widest range of pollution. The Nemerow comprehensive pollution index for heavy metals in small watershed sediments was heavily polluted by As, Cd, Cu, Pb, and Zn. The very strong potential ecological risk was only found in the central part of the traditional Indigenous Zinc Smelting slag distribution area. The Nemerow comprehensive pollution index of heavy metals in small watershed water bodies was severely polluted; The medium to very strong potential ecological risk areas were concentrated in the traditional zinc smelting slag distribution areas. There was an extremely uneven distribution of As, Cd, Pb, and Zn in the surface soil with the coefficients of variation exceeding 1.5, indicating the strong human factors. About 12.50% of the study area shared a strong ecological risk, which was mainly distributed around the waste dump site of the Indigenous Zinc Smelting plant. As such, the Indigenous Zinc Smelting slag posed a significant impact on the surrounding ecology. The pollution range of heavy metals was concentrated mainly within 2 500 m in the water sediments in the small watershed; The impact of slag on groundwater was less than that on surface water, due to the restriction from the hydrogeological conditions. Overall, the pollution areas of the zinc smelting slag in the study area were mainly distributed around the centralized storage area of the slag, with a relatively limited range of impacts.