Abstract:

The Ordovician limestone aquifer, which is tapped by the Ninghebei wellfield, occurs as a NE-trending syncline and is overlain by the CarboniferousPermian sandstone and shale and the Neogene sediments.Only in the northeastern part of the syncline does the limestone aquifer hydraulically connect with the overlying Quaternary aquifer. Groundwater in the limestone aquifer receives recharge mainly through leakage from the shallow Quaternary aquifer. With hydrogeochemical modeling methods, this paper examines water-rock reactions occurring in the flow path from the Quaternary aquifer to the Ordovician aquifer, including dissolution or precipitation of fluorite, calcite and dolomite. For CO²，it may dissolve or overflow, leading to a decrease in hardness total dissolved solids of groundwater. The reason for this is that the dissolution ratio of minerals which containCa²⁺、Mg²⁺ and HCO^-₃ is less than the overflow ratio, reflecting that from the Quaternary aquifer to the Ordivician aquifer, the concentrations of Ca²⁺ and HCO^-₃ decrease in a great deal.Through simulation, water-rock interactions and mass exchange along the groundwater path can be quantitatively determined, which is helpful in describing the evolution mechanism of geochemical environment in groundwater.

Key words: groundwater, hydrogeochemistry, simulation, water-rock reaction, Tianjin