Study On The Evolution Of Shallow Groundwater Chemical Characteristics In Tongliao City

With the development of groundwater, The TDS and total hardness of the shallow groundwater exceed the standard near Tongliao City, which threats the safety of domestic and industrial water usage. Therefore, supported by the project of "Assessment of groundwater resource and its environmental problems in Xiliao River Plain", this dissertation focused on the in depth study on the spatial-temporal evolution of shallow groundwater chemical characteristics in Tongliao City, which integrated the theories and methodologies of hydrogeology, hydro-geochemistry, and hydrogeochemical simulation.The study area focus on the Quaternary shallow groundwater aquifer (with thickness of more than 80m), which mainly consists of medium-find sand, fine sand, silty sand, and shaly fine sand. Precipitation and lateral flow are the main recharge of the groundwater. In general, the groundwater quality was relatively good, most of which can be classified into the standard of good or better. The major hydrochemical type of groundwater is HCO₃-Ca and HCO₃-Ca·Mg. Groundwater with relatively poor quality existed in some parts. Close to the city, groundwater exhibited the hydrochemical type of HCO₃·SO₄-Ca'Mg, HCO₃-Ca·Na and HCO₃-Na·Ca. Overall, TDS was less than lg/L and most part of the region possessed the relative hard water. Near the depression cone in the Tongliao City, TDS was higher, which ranged up to 962.99mg/L, with extremely hard water occurred. Based on the analysis of evolution of groundwater chemicals, TDS and Hardness tended to increase, which increased greatly in the suburbs in Tongliao City. By 2003, TDS increased 662.99mg/L, the area covered by hard water reached to 1427.8 km², while the extremely hard water covered an area of over 100 km².Along the flow direction, saturation index of aragonite, calcite and dolomite increased from less than zero to larger than zero, which means they changed from unsaturation to saturation gradually. The hydrogeochemical simulation indicated that the increase in TDS and total hardness resulted from the dissolving of aragonite, calcite, dolomite, halite, gypsum and C02(g) dissolved into shallow groundwater, which caused TDS and hardness increasing, as well as the adsorption of calcium ion and dissociative of sodium ion during the groundwater flow process..The major factors that affected the formation and evolution of shallow groundwater chemical characteristics are water-rock interaction and human activities.â‘ the formation of shallow groundwater chemical characteristics depended on the processes of dissolution-precipitation, cation exchange and absorption, as well as evaporation and concentration;â‘¡over-development of groundwater and decreasing of groundwater table changed the flow path of groundwater, so that the saturated zone became vadose zone, and the reduction environment changed to oxidation. In addition, vertical recharge increased such as precipitation, which enhanced the eluviations of ions such as Ca²⁺, Mg²⁺, Na⁺and C1, resulting directly in the TDS and total hardness of groundwater.â‘¢Human activities such as incorrect fertilization in agricultural irrigation, randomly stacking of domestic wastes and industrial wastes around the city, and the existing of leakage wells and pits directly and severely polluted the groundwater.