Study On Water-rock Interaction Between Karst Fractured Water And Roofwater During Recharge Process

The urban impervious area gradually expanded with rapid urbanization in Jinan, whichdisrupted the natural hydrological cycle process of rainwater seriously. On the one hand,rainwater runoff was discharged as soon as possible for flood control and resulted in the wasteof water resources. On the other hand, groundwater was over-exploited increasingly for watershortage, which generated many problems, such as land subsidence, karst collapse, seawaterintrusion, underground depression cone etc. In addition, the urban area is prone to floods assoon as being caught in a rainstorm for the low standard of drainage. Therefore, technologiesof rainwater comprehensive utilization were urgent needed, which can improve the utilizationefficiency of water resources, reduce the pressure of drainage pipe networks, conservegroundwater, and increase the quantity of available water resources.Due to its low cost and risk, roofwater become the focus in the utilization of urbanrainwater, as well as the important water resources of recharging aquifer. However, themixture of different water resources and water-rock-gas interactions among carbonate aquiferswould affect groundwater’s physical and chemical properties, supply, runoff and drainageconditions, and aquifer porosities evolution inevitably. According to the above problems, thewater quality changing regulations during the processing of roofwater recharge system werestudied, and the water-rock interactions in fracture-karst aquifer were also simulated inlaboratory. Furthermore, the mechanism and factors of water-rock interaction under practicalcondition were deeply discussed. All of which were aimed at providing scientific basis andguidance for artificial recharge of fracture-karst aquifer. The conclusions are showed asfollows:（1） When roofwater flowed through the regulating tank and filter tank, the concentrationsof Cl^-, SO₄^2-, Ca²⁺, Mg²⁺and HCO₃^-remain unchanged mainly. There is a high removal rateof NH₃-N, which reached up to92.9%. After treated, NH₃-N could meet the groundwaterquality standard III. The removal rate of turbidity was46.4%and still exceeded the standardafter processing. Roofwater recharge increased the temperature and pH of groundwaterslightly, while decreased the concentrations of other ions generally for the effect of dilution. The water quality is still good, which suggested that roofwater recharge did not cause seriousnegative impacts on groundwater.（2） Water-rock interaction simulation experiment of groundwater and roofwater withvarious mixing proportions indicated that: the pressure of CO₂in mixture gradually declinedwith oscillation because of temperature rise and de-carbonated effect. Ca²⁺concentrationslightly decreased at the mixing proportions of9:1,7:3and5:5, and other ions’ concentrationshowed no obvious changes. Saturation indexes of calcite, dolomite and gypsum werecontinuously decreased with the extension of reaction between roofwater and karst system,which indicated that roofwater can strengthen the dissolution of calcite, dolomite and even thewhole aquifer. And the larger amount of roofwater recharged, the higher dissolution intensityand the more obvious impact presented. PHREEQC’s MIX simulation showed that corrosionamount caused by roofwater recharge was so limited, which could not have serious effect onaquifer stability around recharge well.（3） The reaction of cation exchange and adsorption occurred between roofwater andzeolites in filter tank, resulted in the increase of Na+. And this made rainwater hydrochemicaltype changed from HCO₃—Ca·SO₄to HCO₃—Na·Ca·Mg. Mixing action existed betweenroofwater and groundwater, which had some impacts on water quality, but in view of thesmall recharge amount, there was no change in groundwater hydrochemical type. Roofwateris erosive, so groundwater with it largely would strengthen mineral dissolution of the aquifer,and resulted in an increasing mineral solution-trend.（4） Changes in environmental factors, such as temperature, pH and P_CO2would producedramatic and various impacts on fracture-karst aquifer water-rock interaction. Whentemperature increased, the saturation indices of calcite, aragonite and dolomite also increased,which resulted in precipitation. But the saturation indices of gypsum and CO₂（g） decreasedwhen temperature increased, which indicted that higher temperature will promote itsdissolution. Calcite, aragonite and dolomite were more sensitive than plaster and CO₂（g） ontemperature. Saturation indices of calcite, aragonite and dolomite increased with the pH,indicated that acid condition was in favor of their dissolution. Moreover, pH has little effecton gypsum. The pH responses of minerals were larger than temperature. And mineralssaturation index were all decreased as P_CO2increased, which suggested that increase the partial pressure of carbon dioxide will speed up the dissolution of the minerals.