Numerical Simulation And Hydrochemical Characteristics Of Groundwater In Middle Section Of Sunit Paleochannel,Inner Mongolia

Affected by frequent human activities,groundwater resource depletion and groundwater pollution have become the main problems faced by groundwater in arid and semi-arid areas.Groundwater in the Sunit paleochannel is the only groundwater source with significance for water supply along the paleochannel.It plays a leading role in the social and economic development of the region,and it is necessary to rationally allocate and protect groundwater resource.At present,the study of groundwater environment in this area is rarely reported.In order to realize the sustainable utilization of groundwater resources,the primary task is to have a comprehensive understanding of its groundwater environment characteristics.Based on this,this paper took the groundwater in the middle section of Sunit paleochannel as the research object to study its groundwater flow field and groundwater chemical characteristics.Firstly,based on the theory of groundwater flow system and on the premise of collecting stratigraphic data,aquifer characteristics,source and sink terms,hydrogeological parameters and other data,the groundwater numerical model was built by using GMS,which is applied to the simulation and prediction of groundwater flow field in the next 5～15 years.At the same time,based on the established numerical model,MT3D module was used to simulate the solute transport of NO₃^-in groundwater.Then,the traditional hydrogeochemical methods such as mathematical statistics,schukarev classification,SI and ion proportion coefficient method were used to study the hydro-chemical features and evolution law of groundwater in the study area.The main conclusions are as follows:（1）Under the current hydrogeological and mining conditions,the groundwater flow field will change in the next 5～15 years.It is mainly reflected in the slight decline of groundwater level with a drop depth of about 2～4 m.Generally speaking,the groundwater flow field is in a relatively stable state.The decline of groundwater level in areas with large amount of artificial mining is larger than that in other areas.Artificial mining is an important factor affecting groundwater level.（2）The movement of NO₃^-in groundwater takes the pollution source as the center and diffuses in a circular shape.The influence range and exceeding standard range of pollution plume are expanding with time.By the 15th year,the horizontal diffusion distance of pollutant influence range has expanded by about 2.5 km and the vertical diffusion distance has expanded by 2.3 km;The exceeding standard range has been expanded by 2 km horizontally and 2.4 km vertically.The maximum concentration of pollutants is also gradually increasing,from 62 mg/L to 230 mg/L.The solute transport rate and the rate of concentration increase are gradually decreasing.Affected by the groundwater flow field,the vertical diffusion of pollutants is faster than the horizontal diffusion.（3）The groundwater is weakly alkaline（the average p H=7.86）,TDS is high,and it is brackish water.Na⁺and Cl^-is the dominant cation and anion.In terms of spatial distribution characteristics,p H is evenly distributed,TDS is relatively stable,and the spatial distribution characteristics of Na⁺and SO₄^2-,Ca²⁺and Mg²⁺are similar.The hydrochemical type of groundwater is mainly HCO₃·SO₄·Cl-Na type.（4）The main natural mechanisms controlling groundwater in the study area are rock weathering and evaporation concentration.The main ions in groundwater mainly come from the dissolution of various rocks and minerals.Na⁺mainly comes from salt rock dissolution and cation exchange and adsorption;Ca²⁺and Mg²⁺are from the weathering of aluminosilicate and sulfate minerals;Cl^-mainly comes from salt rock;SO₄^2-mainly comes from the dissolution of gypsum and anhydrite minerals;The content of HCO₃^-is low,which mainly comes from the dissolution of a small amount of carbonate rocks such as dolomite and calcite.（5）NO₃^-,F^-and As exceed the standard in groundwater,which are the main pollutants in groundwater in the study area.NO₃^-is closely related to human activities and mainly distributed in areas with frequent human activities.It comes from domestic and breeding wastewater、human and livestock manure.Fluorite minerals are dissolved in groundwater and are the main source of F^-in groundwater.Low Ca²⁺content and weak alkaline hydrochemical environment are the main enrichment factors of fluoride.As comes from the inland lacustrine sediments in the aquifer.The main factors affecting its enrichment are the competitive adsorption between As and HCO₃^-and the weak alkaline hydrochemical environment.In addition,under the influence of topography and groundwater flow field,F^-and As in groundwater are enriched in lake basin depression areas with low terrain and poor groundwater runoff conditions.