Geochemical Evolution And Characteristics Of Rare Earth Elements In Groundwaters In The North China Plain

Groundwater is the main water resources in the North China Plain (NCP), whichaccounts for more than seventy percent of the total water-use. Study of evolution forgroundwater chemistry can not only provide the scientific data for sustainable usage ofgroundwater resources of NCP, but also helps us in better understanding ofhydrogeochemical processes in aquifers. Based on zonation of the groundwater in the plain,characteristics of major components and rare earth elements (REE) in shallow and deepgroundwaters of different zones were studied. Studies showed that:(1) Concentrations of major components in shallow groundwater were higher thanthose in deep one. The average concentrations of Ca2+and Mg2+in shallow groundwaterwere104~121mg/L and60.2~98.0mg/L, respectively, while the maximum concentrationsin deep groundwater were54.1mg/L and28.9mg/L, respectively.(2) Horizontal distributions of Na+, Cl-, SO42-and HCO3-in shallow groundwater weremuch more patchy than those in deep one.(3) Temporal evolution showed that major components in shallow groundwaters hadsignificantly increased since1970s. Concentration of HCO3-in shallow groundwater frompiedmont alluvial fan (Zone I) had a maximum increase rate of7.8(mg/L)/yr during1995and2000. In deep groundwater of Zone I and coast plain-discharge zone (Zone III), theincrease rates were basically less than0.7(mg/L)/yr. The main components in deepgroundwater from central alluvial plain-intermediate zone (Zone II) showed great increasesduring1985and1995, being9.5(mg/L)/y of Cl-and1.8(mg/L)/y of Mg2+.(4) Pollutant introduced by human activities, over-exploitation of deep groundwaterwhich caused groundwater levels decline sharply and changes of the dynamic and chemicalconditions may attributed to the increases in major components in NCP groundwaters.(5) Concentration of total REEs had poor correlation with redox conditions, HCO3-and pH, which might be affected by mineral composition of aquifer sediments along thegroundwater path.(6) Distribution patterns of REEs were similar in shallow groundwater and deepgroundwater in Zone I, while different between in Zone II and III. Distribution patterns ofREEs were similar in different zones of deep groundwater, while those in shallow groundwater were different. Results showed that there was remarkable hydraulicrelationship between shallow groundwater and deep groundwater in Zone I, while not inZone II and III. It indicated that shallow groundwaters of different zones were not in thesame groundwater system, while deep groundwaters were.