| A certain trench in Anxian is located in Longmenshan fracture zone withcomplicated geological structure and developed folds and faults, where geologicaldisasters occurred frequently after the Wenchuan Earthquake. A certain unit wishes tobuild a low radioactive waste disposal site in the trench. The studying of recharge,runoff,excretion,chemistry characteristics and evolution of groundwater couldprovide technical support to the construction of extremely low radioactive wastedisposal site.In this paper, we analysed local geology and hydrogeology data of the study area,twice collected water samples, carried out standard chemical analysis and hydrogenand oxygen isotope analysis of the samples, The results as bellow: Ca2+,whoseaverage concentration is79mg/L, is the foremost cation,; SO42-and HCO3-,whoseaverage concentration are134.6mg/L and173.4mg/L, are the main anions. Otherions appear relatively lower concentrations, such as K+、Na+、Cl-、CO32-.In thisstudying area, average value of Total Dissolved Solids （TDS） of the water bodies is338.3mg/L, which belongs to freshwater with low salinity. According to the hydrogenand oxygen isotope analysis, δD mostly concentrated in the range of-66‰～-54‰,and δ18O mostly in the range of-12‰～-8‰. In the δD-δ18O diagram,isotope distribution points located above meteoric water line nearby, this refers thatwater in the studying area is mainly derived from meteoric water. The elevation ofrecharged water is about1400～1500m.The Piper diagram of the major cations and anions shows three main chemicaltypes: SO4-HCO3-Ca、HCO3-SO4-Ca、HCO3-SO4-Ca-Mg, whose gross concentrationreaches about80%. Leaching and oxidation are the main geochemical actions ofchemical composition of water bodies. The foremost geochemical action of karsticfissure water is leaching in Shuijing Fm, the leaching of calcite、dolomite and gypsumresult in water with low salinity, whose types are HCO3-SO4-Ca andHCO3-SO4-Ca-Mg. The main geochemical actions of bedrock fissure water areoxidation and leaching in Qiujiagou Fm and Youfanggou Fm, the oxidation of pyriteand dissolution of calcite and dolomite result in water with low salinity,whose types are SO4-HCO3-Ca and HCO3-SO4-Ca.In order to search the evolution law of groundwater in alluvial fan, we arrange aprofile along the direction of groundwater flow according to local hydrogeologicalconditions, and choose the groundwater of drill hole ZK16as initial solution andseeping spring S4of the leading edge of the alluvial fan as final solution. theSaturation Index（SI） of mineral phases in the initial solution and final solution showsthat aragonite、calcite and dolomite reach saturation point，in addition, the SIs ofminerals in initial solution are higher than in final solution without exception. Basedon possible geochemical reactions and mineral phases which could be observed andsurveyed in the field （including calcite、dolomite、gypsum、CO2（g）、pyrite, et. al.），we established a model of water-rock interaction, and carried out inverse geochemicalmodeling of evolution of the chemical composition of groundwater in alluvial fanwith PHREEQCI. The results are bellow: While groundwater in alluvial fan wasflowing to S4Spring from Drill hole ZK16,4.07×10-6mol/L rock salt and7.30×10-5mol/L gypsum were dissolved,1.33×10-4mol/L oxygen（O2） was consumed and3.56×10-5mol/L pyrite was produced as well. Oxidation of pyrite has decreased pHvalue of groundwater, accelerating dissolution of dolomite, which increased theconcentration of CO32-in the solution. Finally the dissolution of dolomite reachedsaturation, causing the fact that calcite not being dissolved in subsequent groundwaterrunoff. Na-Ca cation exchanges, which was effected by groundwater, led3.02×10-5mol/L Na+in the water was desorbed and1.51×10-5mol/L Ca2+was absorbed by rockand soil. |
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