Geochemical Processes Of Acid Mine Drainage Formation And Associated Antimony Contamination At Xikuangshan Mine,China

China is a leading producer of antimony (Sb) in the world and has the largest Sb mine called Xikuangshan (XKS) mine, which is fondly known as the Sb World Capital. The XKS mine has been mined for over 200 years, and mining activities have caused various environmental problems. The XKS mine is located 13 km north of Lengshuijiang City, in Hunan province. Research groups that have worked at this mine have focused on the environmental impacts of mining on soil and plant contaminations, water quality and human health. However, there was lack of documentation that provided a conceptual assessment of mine water to better understand the dominant geochemical processes to determine production and buffering of acid mine drainage (AMD); as well as those controlling spatial variation in chemical composition of the mine water. The main purpose of this research was to find out the acid mine drainage production potential and associated geochemical characteristics influencing mine water quality at XKS mine as an environmental monitoring initiative. This thesis articulately outlined a sequence of mine water and waste studies that aimed at achieving the set objectives, with each specific objective addressing a particular gap in knowledge.The specific objectives of this research were (1) ascertain whether the mine water was coming into contact with an oxidizing environment in the mine; and which carbonate minerals were neutralizing the acid producing species; (2) identify chemical signatures and their spatial variations at XKS mine by using multivariate analysis; (3) investigate the hydrogeochemical characteristics influencing AMD production/neutralizing and its contribution to water quality; (4) explore the potential of waste rocks, slag and tailings to generate acid, and the subsequent neutralizing capacity of carbonates as a determinant of AMD formation; and (5) assess the distribution of Sb and arsenic (As) in surface water and groundwater, in relation to mine waste. These research objectives for the mining area focused on identifying strategies to control potential risks through geochemical assessments, which included water and mine waste management. The research findings in this thesis are summarized as follows:Hydrogeochemical Characteristics of Groundwater:The management of AMD is central to environmental sustainability hence developing an understanding of hydrogeochemical processes that influence groundwater chemistry and its quality is very important. The chapter investigated the hydrogeochemical characteristics influenced by AMD and its contribution to groundwater quality at the XKS mine. The sampled groundwater’s (24) from the study site were analyzed in the laboratory for major and trace elements while temperature, pH, Eh and total dissolved solids (TDS) were recorded in situ. Piper diagram illustrated the mine groundwater was Ca-HCO3-SO4 type with 34 percent of sample points being Ca-SO4-HCO3 type waters evolving from AMD buffering by calcite and dolomite minerals. Modeling results using PHREEQC suggested that SO42-, Ca2+ and Mg2+ evolved from the dolomite and calcite AMD buffering process reactions. The study identified that Sb and arsenic (As) concentrations averaged at 3.31mg/L and 0.31 mg/L respectively and exceeded the acceptable limits for drinking (0.01 mg/L).Determination of carbonate minerals, using thermodynamic chemical equilibrium model:This chapter identified and characterized minerals responsible for mine water quality at the XKS mine by using a computer-assisted thermodynamic chemical equilibrium model. The Eh-pH diagrams identified that Fe2O3 was the dominant iron species while SO42- was the dominant sulfide species, which indicated acid production. Calculated saturation index (SI) for acid producing minerals specified that the major minerals undergoing oxidation were pyrite, pyrrhotite, arsenopyrite and siderite. Other secondary sulfate minerals that contributed to SO42-concentration in the mine water were identified and included gypsum and epsomite. The SI also recognized calcite and dolomite as the main buffering carbonate minerals. However, other carbonate minerals in the mine included those containing calcium (huntite, portlandite and calcium arsenate), magnesium (magnesite, nesquehonite and artinite) and sodium (natron and thermonatrite). Identification of the specific acid producing and consuming minerals present in the mine area was critical for determination of an effective water management plan.Using Factor Analysis to Assess the Source of Minerals Influencing Water Quality:Factor analysis was used to identify chemical signatures and spatial variations of chemicals relative to their sources at the Xikuangshan antimony (Sb) mine in China. A total of 29 samples were analyzed in the laboratory while pH, Eh, and total dissolved solids (TDS) were recorded in situ. The factor analysis resulted in a four-factor model that explained approximately 83.4% of the variation in mine water quality. Factor 1 (K+, Na+, Ca+, Mg2+ Cl-, F-, SO42-, NO3- and TDS) explained 46.1% that indicated that the group had the most influential geochemical processes in the mine area. The presence of Ca2+ and Mg2+ together with SO42- indicated that FeS2 oxidation was being buffered by calcite and dolomite; and that Gypsum dissolution was occurring. The presence of TDS together with Ca2+, Mg2+, and SO42-specified that these were the major elements in the area. Factor 2 explained 18.5% of the total variance with a negative strong absolute loading of HCO,; while Sb, and As had moderate loadings. The last two factors (3 and 4) explained 18.7% of the total variance that had strong positive loading value for Fe and pH; moderate negative loading values for HCO3-. The model results showed a great need to properly address As and Sb contamination in the mine and the area around.Mine waste acidic potential and distribution of antimony and arsenic in water:This chapter estimated the potential acid generation (PAG) by using paste pH, acid base accounting (ABA) and net acid generation (NAG) geochemical tests. Distribution of Sb and As in surface and groundwater in relation to mine waste was also presented. Water samples and representative samples of three mine wastes from different periods (fresh,10 and 50 years) were collected for this study:waste rock, smelting slag and tailings. Static geochemical test results showed that waste rock and smelting slag of 10 and 50 years were PAG; while the fresh rock waste and tailings were non PAG with paste pH (>7) indicating the presence of reactive carbonates. Hence AMD generation may have occurred after dissolution of carbonates. Water analysis found Sb with higher concentration than As with means of 3.74 mg/L and 0.19 mg/L respectively. Highest Sb concentrations (39.16 mg/L) was identified in the North mines, around the area draining from waste rock heaps; Mine water also had high Sb concentration (14.25 mg/L). Surface water also had high Sb concentrations (South mine = 4.56 mg/L; and North mine= 10.28 mg/L) confirming the influence on mine wastes on water. Proper mine waste management and collection and treatment of outflow from the waste rock heaps and tailing ponds were promising mitigation options.This thesis was innovative in the following two ways:(1) for the first time, this research used geochemical tests to monitor AMD generation potential for both the closed North mine and the operational South mine to verify the effectiveness of waste rock storage methods; (2) this research was a regional study that contributed valid information on mine water management and enhanced the state of knowledge of some of the hydrogeochemical processes that occurred in this particular mining environment.