| Groundwater is closely related to human life.Once groundwater is polluted,it will bring great harm to the natural environment.Karst water resources is an important part of groundwater resources and plays an important role in urban water supply in China.Shanxi Province is the northern province with the widest distribution of karst groundwater in China.However,in recent years,the water environment in many karst springs in Shanxi Province has changed greatly,and various problems have become increasingly prominent.Longzici spring is one of the 19 karst springs in Shanxi Province and an important surface water source in Linfen City.In recent decades,the water quality in the spring area has been declining,which has greatly affected the residents’ life and local economic development in the spring area to a certain extent.Therefore,it is very important to understand the overall situation of water pollution in the spring area,analyze its pollution sources,clarify the migration law of major pollutants,and put forward water pollution control plans based on this.Taking Longzici spring area in Shanxi Province as the research object,the fuzzy comprehensive index method is used to evaluate the water quality in the study area,and the overall groundwater quality grade distribution and pollution situation in the spring area are clarified;Based on this result,and through the comparative analysis with the historical data of the same region,the main pollution sources in the study area are determined.Then,the groundwater flow and solute transport model in the study area is built through the Visual MODFLOW software platform.Two typical coal mines in the study area are selected as pollution sources.The MT3 DMS module is used to simulate the migration and diffusion of the main pollutants in the groundwater when the main pollutants caused by coal mining are discharged and infiltrated in the surrounding mine water.On this basis,the control effects of different human intervention measures on the migration of pollutants in groundwater are studied.Through the above work,the following conclusions are drawn:(1)The overall water quality of Longzici spring area is in a relatively bad situation,and the situation is not optimistic.According to the groundwater quality standard,nearly 85% of the groundwater sampling points have water quality grades of class IV and class V,and only about 15% of the sampling points have water quality grades of no more than class III.compared with the water quality in the previous spring area,the current water quality shows a further deterioration trend.On the basis of groundwater quality standards and with appropriate reference to surface water standards,the water quality evaluation results show that class IV and class V water samples account for 7.7% and 84.6% of the water samples at all groundwater sampling points respectively;The sampling points whose water quality does not exceed the class III standard for groundwater only have one remaining spring boundary,accounting for 7.7% of the total sampling points.The water quality evaluation results of surface water sampling points show that the water quality of all sampling points is class V,and the water quality of the whole surface water system has been polluted to a certain extent.,It is most likely that coal mining is the main man-made pollution source of the overall water quality in the spring area.(2)Without human intervention,pollutants migrate and diffuse in a large range in underground water.After 20 years of migration,the distance between sulfate diffusion edge and coal mine can be more than 110 m,and the distance between ammonia nitrogen diffusion edge and coal mine can be more than 240 m.Its migration and diffusion direction is mainly affected by the direction of groundwater flow,and it spreads from the higher altitude areas to the eastern and southern low altitude areas;And the closer to the pollution source,the greater the pollutant concentration,causing obvious pollution to the surrounding groundwater to a certain extent.(3)When the anti-seepage wall is set in the main diffusion direction of pollutants 30 m away from the coal mine pollution source,the migration of pollutants can be restrained to a certain extent.After 20 years’ migration of pollutants,the distance between the sulfate diffusion edge and the coal mine can be contained within 100 m,and the distance between the ammonia nitrogen diffusion edge and the coal mine can be contained within 180 m.However,this measure also makes the concentration of sulfate and ammonia nitrogen near the pollution source show a further rising trend compared with the previous ones.Therefore,the overall pollution control effect is not ideal.In the case of setting up hydraulic forced pumping wells in the main diffusion direction of pollutants 30 m away from the coal mine pollution source,the model simulation results show that,compared with setting anti-seepage walls,the migration and diffusion range of pollutants near the two coal mines in groundwater is greatly shortened in the same time;The distance between the sulfate diffusion edge and the coal mine can be contained within 30 m,and the distance between the ammonia nitrogen diffusion edge and the coal mine can be contained within 160 m.The pollutant concentration is also significantly reduced,and the pollution control effect is obvious.On the basis of setting up hydraulic forced pumping wells,the addition of hydraulic interception wells in the main diffusion direction of pollutants 50 meters away from the pollution source,acting together with hydraulic forced pumping wells,can further and more effectively curb the migration and diffusion of pollutants.The distance between sulfate diffusion edge and coal mine can be restrained to about 20 m,and the distance between ammonia nitrogen diffusion edge and coal mine can be restrained to less than 90 m.Without considering the impact on the whole groundwater system in the spring area caused by long-term mass pumping,this scheme is the best scheme to curb the migration and diffusion of pollutants in the simulation area. |
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