Study On The Geochemical Evolution And Numerical Simulation Of Deep Limestone Groundwater In North China Coalfield

As the continuous exploitation of the deep coal resource in the east of North China Coalfield,the mining depth in key mines has reached about 600 m and keeps increasing at the rate of about 12 m/a.The coal seams of most coalfields in North China directly deposit over an extremely thick and water-rich Ordovician carbonate aquifer,and the mining disturbance may result in the activity of faults or fractures to form groundwater channels which may give rise to the connex between Ordovician Limestone Groundwater(OLG)and Carboniferous Taiyuan Group Limestone Groundwater(CTGLG)and become the soures of inrushing water.All these gradually make the deep mining to face much higher risk from OLG and CTGLG.On the other hand,many coalfields in North China are short of water resource,and the evacuating water,reducing the piezometric head and the draining water for long time make the water shortage more and more severe.So,study on the hydrogeochemistry and environmental isotope characteristics,the hydrogeochemical evolution and the hydrodynamic field is a very important problem for the safety of deep coal mining and the protecting water resource to a largest degree.Taking Dongtan,Baodian and Xinglong mines in Yanzhou Coalfield for example,firstly,the hydrogeochemical and isotope characteristics of OLG and CTGLG,and their indicators for groundwater circulation were studied.Secondly,the main water-rock interactions that caused the hydrogeochemical evolution of OLG were deduced from the relationship among different ions,saturation index and TDS.Thirdly,the inverse geochemical model was built to quantificationally study the water-rock interactions in Ordovician aquifer.Lastly,the groundwater flow modeling was employed to study the OLG.The main results got from these studies were showed as follows:(1)The main water types of OLG and CTGLG are respectively Ca-Mg-SO4 and Na-HCO3.The calcite and dolomite in them are oversaturated,and the gypsum in them is unsaturated.The concentration of Ca2+,Mg2+ and SO42-in OLG is obviously higher than that in CTGLG,but the concentration of K++Na+ and HCO3-is on the contrary.Otherwise,there is no CO32-in OLG.On the basis of these analyses,the OLG could be directly identified.(2)The representative meteoric water line of the study area is?D=7.80 ?18O+6.28.There is close relationship between OLG and atmospheric precipitation,also between CTGLG and atmospheric precipitation.And the values of?18O and ?D in OLG and CTGLG indicate they got recharge under colder environment,but not from local precipitation.That the average recharge elevation of them is respectively 378.9 m and 408.6 m could also prove they did not get recharge from local precipitation.(3)The OLG containing less than 1 TU tritium was recharged before 1960.The OLG and CTGLG containing 18.4?23.7 TU tritium were partly recharged after 1960.The OLG and CTGLG containing 2.2?14.5 TU tritium were recharged by mix of the water recharged before 1960 and after 1986.(4)The main water-rock interacitions during OLG hydrogeochemical evolution are the dissolution of gypsum,the precipitation of calcite,the dissolution or precipitation of dolomite,the dissolution of halite and cation exchange.The dissolution of gypsum resulted in the increase of Ca2+and SO42,and cation exchange leaded to the higher concentration of SO42-than that of Ca2+,and the FeS2 may be a source of SO42-,but not a main source.Halite is the main source of Na+and Cl-,and cation exchange give rise to the higher concentration of Na+ than that of Cl-.Mg2+may result from the dissolution of dolomite,because the dissolution of gypsum may cause the precipitation of calcite and the dissolution of dolomite along the flow path.(5)The results of inverse hydrogeochemical modeling indicate the precipitation of calcite,the dissolution of gypsum and dolomite,the dissolution of halite,cation exchange and the dissolution or emission of CO2 are the main water-rock interactions,which quantificationally proved the upper analyses.(6)The GMS was used to simulate the OLG flow,the rational initial hydraulic head field was got from the steady flow modeling.Then,the initial hydraulic head field was used in the transient flow model to simulate the distribution of OLG flow field and get the rational coefficient of permeability(K)and storage coefficient(?*).