Study On Groundwater Environmental Effect Of Line Projects On Turfy Soil Wetland In Seasonal Frozen Region

The new structure,which interphase distribution of mountains,basins and valleys,of the Changbai Mountain region was formed after the Yanshan movement and quaternary NNE fracture zone.In these intermountain basins,the"box type"landforms are highly developed and the"partition"effects are remarkable because of the abundant precipitation and low lying land,and because of the long seasonal frozen climate in this region,the remains of the marsh vegetation can not be completely decomposed.After thousands of years,a special soil called turfy soil was formed in the intermountain basins,and these basins are called the seasonal frozen turfy soil wetland.For a long time,due to the continuous expansion of the human line projects,lots of line projects pass through turfy wetland inevitably,which caused a series of environmental problems.Such as the construction of the line projects resulted in the fragmentation of the wetland and the reduction of the wetland area.The water isolation treatments of the subgrade had caused serious disturbance to the water system connectedness of the turfy soil wetland on both sides of the line projects,and more seriously the one side of the wetland had dried up.And the water quality of the wetland had been influenced by the emission of various pollutants during the construction and operation period.However,current researches about turfy soil in seasonal frozen wetland mostly focused on the engineering characteristics and subgrade treatment measures while only a few focused on its environmental effect and even fewer were focusing on its water environment.But the impacts that line projects exert on water environment of turfy soil wetland are quite serious and cannot be ignored.So it is necessary to conduct deep study on the environmental problems caused by line projects to realize the harmonious coexistence of engineering and wetland ecological environment.Based on the above reasons,this article selected four typical turfy wetlands affected by line projects,combined with the Natural Science Foundation of China?NSFC?Study on Environmental Effect of Line Projects on Turfy Soil Wetland in Seasonal Frozen Region,systematically studied water environment effect in turfy soil wetland under the influences of line projects.Water samples were collected at each selecting sites according to different distances at both sides of highways and then conducted relevant experiments,mainly includes:hydrogeology survey,water system connectivity,water-chemical analysis,test analysis of heavy metals,groundwater simulation,etc.The conclusions are as follows:1.The main types of groundwater in the turfy wetland are:base-free submergence,micro-confined water,basalt fissure water and granite weathering fissure water.The main supply sources are:direct infiltration from atmospheric precipitation,infiltration from lateral mountain and water seepage from surface water;The main drainage channels are:artificial exploitation,leakage flow,evaporation and spring discharge.2.The different roadbed treatment measures of line engineering have significant different influences on the water level of the wetland.Through monitoring water levels and evaluating water drainage connectivity of both sides of highways,the influences on the water level are determined as:gravel filling>composite geotextile+sand pile>geogrid+EPS board.3.The groundwater chemical types of turfy soil wetland are HCO₃-Ca and HCO₃-Ca+Mg,which are mainly caused by rock weathering.The influence on the groundwater chemical types caused by line projects is quite less but much greater on heavy metals'concentrations.The concentrations of heavy metals are as the following sequence:Zn>Cu>Pb>Ni>As>Cr>Cd>Hg,among which the content of Zn is between70 to 130?g·L^-1,and 0.05 to 2.8?g·L^-1 for Hg.The content of Pb has exceeded the?-level limit value of Standard for Groundwater Quality GB/T 14848-2017,but not exceeded the IV-level limit value,while other heavy metals do not exceeded the?-level limit value.The contents of Cu,Pb,Cd,Cr,Hg,Ni and As are negatively correlated with the distance from the line projects,significantly for Cu and Pb.Undoubtedly,it is line projects that lead to an increase in heavy metals content near the road in the wetland.There is a significant positive correlation between Cr and As,Hg and Ni,and it is believed that the sources of these heavy metals are the same.4.The turfy soil wetland groundwater was simulated by PHREEQC.Albite,Anorthite,Calcite,Chlorite?14A?,Dolomite,Fluorite,Gypsum,Hausmannite,Manganite,Melanterite and Talc all these saturation indexes are far less than 0,and it shows that these minerals have a tendency to continue to dissolve.K-feldspar,Quartz,Rhodochrosite and Siderite these saturation indexes are between-0.8 and 0.6,and it means that these minerals are in the state of basic reaction equilibrium.The saturation index of oxygen is between-46.6 and-47.9,and it shows that the groundwater environment is severe anaerobic,which is relate to the mass decomposition of organic matter in turfy soil wetland groundwater.This decomposition also lead to the reduction of oxidation-reduction potential and pH,which is also lead to iron and manganese rich in groundwater,and mainly in the forms of Fe²⁺and Mn²⁺.