Investigation On The Mechanism And Control Of Karst Groundwater Pollution Due To Abandoned Coal Mines

In recent years,with the gradual depletion of shallow resources of many small mines in China,the reform of energy supply structure and the implementation of policies such as eliminating backward production capacity,a large number of mines that do not meet the national security and ecological environment standards were closed,resulting in an increase of closed coal mines in China.However,after mine closure,the acid mine water results in sorts of environmental problems due to groundwater diffusion,especially in the karst region with strong karstification greatly promoting the groundwater pollution migration.Therefore,research on groundwater pollution mechanism and pollution prevention and control measure in karst area is of great importance.The dessitation takes Longdong Spring pollution as research background and with the two key scientific problems of pollution mechanism of coal mine closure in karst area,and prevention and control mechaniam.Hydrogeological investigation,water chemical analysis,field monitoring,theoretical analysis,case verification were used to investigate and reveal the pollution model causes and mechanism of coal mine closure,the pollution trend prediction,and pollution prevention and control measure.The main achievements are as follows:(1)The spatial distribution of water pollution in the Yudong River Basin was revealed.The characteristics of water pollution in the Yudong River Basin were analyzed by sampling and water quality analysis(including rivers,polluted karst spring points and wellhead wastewater discharge points).The investigation results show that the pollution of the Yudong River Basin is caused by local coal mining,and the acid wastewater from coal mines and the discharge of polluted karst water are the main reasons for the river pollution,and out of limite of p H value,sulfate,total iron and manganese.Due to the increase of coal mine wastewater discharge,the pollution degree of the Yudong River Basin becomes more and more serious,and the heavy metal content of downstream water exceeds the standard seriously.(2)The water environment pollution pattern in karst area is defined as the combination of pollution source,pollution channel,pollution mode and polluted water body.It is proposed that groundwater pollution can be divided into four types according to pollution channels: direct discharge from coal mine wellhead,indirect discharge from karst pipeline,fissure infiltration and a combined type.According to the relationship between the pollution point and the discharge point,the groundwater pollution are divided into three types: direct injection type(outflow type),overflow type and cross layer pollution.(3)According to the concept of pollution pattern,in terms of a model of "pollution source + channel + pollution mode + pollution receptor",water environemnt pollution model in karst area can be divided into the following five models:(1)Acid mine water directly infiltrated into the coal mine wellhead and polluted the surface water;(2)Acid mine water directly injected to pollute groundwater;(3)Acid mine water directly injected to pollute surface water through karst pipeline;(4)Acid mine water overflow and polluted surface water through karst pipeline;(5)Acid mine water infiltrate to other layers and pollutd groundwater through fissure.The first pattern is the major pattern of abandoned coal mines in the Yudong River Basin.(4)Revealed the relationship between typical pollution ion content,spring discharge,rainfall and mine water level in the Longdong Spring.In order to further find out the dynamic characteristics of the Longdong Spring pollution,the characteristic pollutant iron was taken as the characteristic factor of spring pollution,and the dynamic characteristics of iron ion and rainfall were studied by field monitoring method.Finally,it was determined that the pollution degree was serious in wet season and decreased slowly in later period,while the pollution degree was light in dry season.The relation between mine water level in goaf and spring water condition is that when the water level rises to 908 m,the spring water begins to be polluted,and when the water level drops below 908 m,the spring water is gradually becoming clear and stable.The mechanism of the Longdong Spring is revealed,and a "U-shaped tube effect" of the mine acid wastewater pollution of the Longdong Spring is put forward.It has laid a foundation for pollution source engineering prevention and control.(5)A method for predicting the trend of typical pollution ion in karst groundwater pollution caused by closed coal mine is established.Considering the short monitoring time of karst area and other factors,the improved gray GM(1,1)model was adopted to forecast the pollution trend of the Longdong Spring with the iron content.The prediction results showed that the pollution trend of Longdong Spring will grow continuously without taking any pollution control measures.The reasons of the increasing pollution are illustrated from the forming reasons,supplementing diameter and discharging conditions of the spring water and mine waste water.The BP neural network is used to predict the pollutant concentration in the Longdong Spring which can be used to predict the pollutant concentration in the future,which provides a basis for pollution source engineering prevention and control.(6)The prevention and control ideas,and key technology of groundwater pollution of abandoned coal mines in karst areas are put forward.The pollution control idea of "source control and key channel blocking" was put forward,and auxiliary measures such as terminal curtain blocking and extraction treatment were adopted.Borehole verification and water quality monitoring results of the Longdong Spring proved that the treatment effect is remarkable,which provides a basis for groundwater pollution treatment.