| The new airport of Chengdu is located in the wide red siltstones hilly area of the central Sichuan basin, and its west side is the lingering Longquan Mountains and east side is the Tuojiang River. Special hydrogeological conditions lead to different water quality of shallow and deep groundwater, the groundwater quality presents obvious vertical zoning phenomenon. At present, the residents' production and living water mainly comes from Shallow freshwater, and have found that individual Shallow groundwater salinization through the hydrogeology survey in the field. And the foundation pit excavation and the pumping of groundwater, is bound to be an influence of groundwater flow field.If the shallow depth and the pressure of salt water was revealed, the mixing action of salt water and fresh water will pollute the fresh water resource, even lead to soil salinization or cause corrosion to the airport foundation concrete and steel structure.In this paper, according to the results of water quality analysis in the study area, by means of the theory of hydrogeology and hydrogeochemistry method, analyze statistical characteristics and spatial variation of the groundwater chemical composition. And trying to explore the groundwater supply source, circulation law and the formation mechanism of chemical composition of groundwater through the advanced methods such as hydrogeochemical simulation. And Providing a theoretical basis for the rational and scientific exploitation of groundwater resources. Study achieved the following knowledge and conclusions:(1) The coefficient of variation of K++Na+,Mg2+,HCO3-,Cl- and TDS is greater in the shallow groundwater than deep. Groundwater chemical composition is more easily influenced by man-made and natural factors such as topography, weather, hydrology, aquifer lithology.(2) Comparing the groundwater TDS contour map of shallow strata and middle-deep strata, found that the salinity of middle-deep strata is higher than the shallow strata, and groundwater salinity present obvious vertical zoning phenomenon. Comparing the main ion components contour map of shallow strata groundwater and middle-deep strata groundwater, found that the content of main ions in middle-deep strata is higher than that in shallow strata.(3) It is concluded that the correlation between the TDS and HCO3- are osculating in shallow strata groundwater, and the correlation between the TDS and SO42- are osculating in middle-deep strata groundwater, through the correlation analysis and partial correlation analysis of groundwater of shallow strata and middle-deep strata. They reveal that the SO42- is main factors to control its higher TDS in middle-deep strata groundwater, there may be a SO42- supply source.(4) Through the three line graph, it can be seen that groundwater hydrochemistry type is changed from HCO3-Ca to HCO3×Cl×SO4-Ca×Mg.(5) From the groundwater runoff section, it can be seen that the groundwater salinity increased along the groundwater runoff. It can be seen that the water alternating positive and hydrodynamic condition of shallow groundwater is better because the gCl-/gCa2+ is lower The adequate water- rock interaction make gCl-/gCa2+ high in middle-deep groundwater. The gCl-/gCa2+ of individual sample points is very high, high concentrations of chloride may come from the mixing action of deep brine.(6) Gibbs model explains that water-rock interaction is the main factors that affect the formation of groundwater chemical composition in the study area. By analyzing the ratio relationship of the other major ions and Cl- in groundwater, the following conclusions are drawn:Water chemistry is not only influenced by rock salt dissolution, also by sodium feldspar and glauber's salt dissolve or human activity.By the relationship between SO42- and Cl-, it can be seen that shallow groundwater has the following characteristics: shallow groundwater levels, better hydrodynamic conditions, a positive water circulation. Therefore, it occur fully dissolution reaction with gypsum and glauberite in the runoff process. In middle-deep groundwater, low concentration of SO42-, reflects the deeper buried depth of groundwater and the slower circulation of groundwater, and sulfate reduction reaction may have happened. High concentrations of SO42- may come from the mixing action of deep brine.(7) In the process of rainwater converted into shallow groundwater, The water-rock interaction occurred between groundwater and mineral in the aquifer, such as halite, gypsum, kaolinite, illite, calcite, potash feldspar, etc. Formation water chemical composition of shallow groundwater is mainly controlled by the solution of rock salt, gypsum, illite, calcite, CO2 and the precipitation of kaolinite, feldspar.(8) Through the mixing action simulation of shallow groundwater and middle-deep groundwater, it is conclued that the conformation 1kg of deep groundwater S5, need a mixing between 0.917 kg of shallow groundwater J30 with 0.083 kg of deep groundwater W1, at the same time will produce certain material transfer between different reactants. |
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