Hydrochemical Formation And Evolution Of The Karstic Groundwater In Liulin Spring Catchment

The unique nature resource endowment of Liulin spring catchment makes a series of regional industries related to coal industry growing vigorously. As the fast development of local economic inevitably leads to an increasing demand for water, coupled with the limited surface water supply, the high-quality groundwater serves as the main water-supply source in Liulin spring catchment. Generally, groundwater exploitation can gradually modify hydrochemistry field, makes irreversible deterioration on water quality, and finally threatens the regional water security. This study spatially and temporally analyzes evolution characteristics of karstic groundwater in Liulin spring catchment from the perspective of hydrogeochemistry in order to provide a scientific basis for the future spring water protection, improvement of water quality and ecological restoration.Starting from the thorough review of the predecessors’ research achievements, we analyzed the hydrochemistry field and its spatial variation regularity in aspects of temperature, acidity and alkalinity, oxidation reduction environment, hydrochemical type, TDS and cation-anion concentration. Besides, we discussed the formation and evolution rules by using ion ratio coefficient method and Piper triangular diagram method and simulated the dissolving-sedimentation balance, mass balance, reaction path and mixing simulations by PHREEQC. Our study on the spatial variation regularity in hydrochemistry field has shown that, along the groundwater flow direction from the east spring catchment to the west, the temperature of karst groundwater raises, pH value decreases, TDS increases gradually, and the karst hydrochemical environment transits from the oxidation environment to the deoxidation environment. From recharge area to stagnant area, hydrochemical type changes in order from HCO3-Ca·Mg, HCO3-Ca to SO4·HCO3-Ca, HCO3·SO4-Ca-Mg, SO4·Cl-Ca·na and finally to Cl-SO4-Na·Ca, Cl·SO4-Na.The main chemical interactions in recharge area are the dissolution of minerals, combined with cation exchanges, by which the Na+dissolves in groundwater and displace Ca2+、Mg2+in the water. Additionally, the incongruent dissolution of dolomite may also appear in the runoff area. Groundwater in the discharge area drains in the form of springs. However, the chemical characteristics between the north and south sides of Sanchuan river is significantly different, namely "salty north and fresh south". The difference of the buried conditions and hydrodynamic conditions is the main cause of this phenomenon, and the strong evaporation in north side is another explanation. The chemical interactions in stagnant area are similar to those in the north riverside, but with more intensive evaporation and concentration. Because of the change in redox conditions, desulphidation happens in the region.The dissolving-sedimentation equilibrium simulation indicates that, from recharge area to stagnant area, calcite, dolomite and gypsum have experienced four geochemical dynamic stages:all are dissolved�'only calcite is saturated�'calcite and dolomite are saturated with gypsum is dissolved at the same time�'all kinds of minerals are saturated. This process demonstrates the deterioration on characteristics of water dynamic conditions.By the mass balance simulation and inversed reaction path simulation, we obtained the hydrogeochemical evolution path of the entire spring catchment. Firstly, calcite, dolomite, gypsum and halite are all dissolved and the ions go into the groundwater. As the various minerals are gradually saturated, calcite and dolomite begin to transfer from groundwater to mineral surface. Then gypsum is precipitated, and at the same time, halite is saturated in stagnant area. Cation exchange interaction here exhibits as the active Na+constantly displacing the stable Ca2+and Mg2+in the water. Together with evaporation and concentration, the groundwater in stagnant area becomes high salinity brine with high Na+concentration.Mixing simulation of karstic groundwater in Liulin spring catchment has shown that, the groundwater in stagnant area is recharged with tje low salinity water coming from the east catchment, which makes up10～20%of the total water. Moreover, the ratio is increasing year by year along with the emissions of artesian well.