Assessment And Prediction Of The Impact Of A Molybdenum Ore On Groundwater Environment In Metamorphic Rock Area

Molybdenum (Mo) is a rare nonferrous metal, is an essential non-renewable mineral resource. Because of the wide application, Mo plays a strategic role in the development of the national economy. In the mining and dressing process of Mo ore mine, large amount of solid waste containing metal sulfide is produced. During weathering, erosion and leaching processes, the heavy metal elements enter the environment, leading to the groundwater seriously polluted, threatening human health.A typical porphyry-type Mo deposit was selected as the research object in this paper. The deposit is located in metamorphic rock mountainous area. And metamorphic rock fissure water is the main groundwater in the region, which is the main source of water supply for local people and livestock. The aquifers will be destroyed by the mine exploitation, accentuating the rift between water supply and demand in local area. Therefore, study of weathering fissure water in the mine and its affected zone and put forward several measures to protect groundwater have important practical significance, which is the key to meet domestic water demand. Based on the analysis of geological and hydrogeological conditions and assessment of the present groundwater environment in study area, the paper used the Visual MODFLOW to simulate the groundwater and contaminant, predicting and evaluating the impacts on groundwater environment during the process of mining, reached the following conclusions: The grade of groundwater environmental impact assessment of the project was determined and the level and quality of groundwater was monitored and assessed, based on collection and collation of the study area’s natural geography, geological and hydrogeological conditions, development and utilization of groundwater etc. The results showed that the annual changing range of groundwater level was small and all indicators of the wells had not been found exceeded the standard in study area.The vertical permeability of unsaturated zone was studied by infiltration test in mining industrial site, beneficiation industrial site, waste dump and tailing dam. The permeability coefficient of aquifer was identified by water pressure test in boreholes. Leaching test was carried out for solid waste in tailing dam and waste dump, we got the main pollution factors (Cd, Cu, Fe, Mn, Mo, Zn, As) and their leaching concentrations.COD and BOD concentration and migration distance were predicted and analyzed by analytical method in the mining and beneficiation industrial sites. Results showed that the wastewater of production and living had simple composition, although the wastewater infiltrated into the unsaturated zone under abnormal operation modes, nor would it result a great impact on shallow groundwater quality.In order to predict the quantity of groundwater impacted by the mine exploitation, a two-dimensional non-steady flow simulation model was built based on Visual MODFLOW. Results showed that the amount of water drained from the pit was6300m3/d under condition of mine exploitation. After100days,1000days and7300days, the open pit, upstream region of tailing dam, upstream region of Nanzhuanghe were dry in simulation zone, and the area of dewatering zone was increasing with time.According to the hydrogeological condition of waste dump and tailing dam, a two-dimensional hydrodynamic advection-dispersion model was built based on non-steady state flow model. Under abnormal operation modes, heavy metals of the leachate in waste dump and tailing dam will penetrate into the aquifer. Thus, the concentration and migration distance of heavy metals were predicted by the solute transport model. Results showed that each heavy metal had a short migration distance, and there’s no sensitive targets within the affected area, such as village and well. Also, the concentration of each heavy metal was less than0.01mg/L, which was below the limit. Therefore, the leachate will have little effect on groundwater quality.Based on the present status of groundwater environment monitoring and evaluation of the study area, predicting and evaluating the impact and damage on groundwater environment during the process of mining, which based on the numerical simulation method. It is effective to provide scientific basis of groundwater environmental management for the project construction and operation.