Study On Karstic Dual-conductivity Modeling Of Groundwater Flow And Solute Transport

Qiligou is the main water supply source located in South Xuzhou. Karst groundwater inthis area, with area of120km2, is severely contaminated by CCl4. Since2001, we conductedlocal environment and hydrogeological investigation, explored local physical geography,measured field data, performed tracing experiments, and studied transport of CCl4based onduel-conductivity medium theory under the supporting of National Science Foundation ofChina. Amount of80research wells were studied, among which water level and CCl4concentration of35－40wells were measured four times per year, CCl4concentration of21wells were measured every month. Based on3000field data in10years, local hydrogeologyinformation, and drilling data, dual structure of unconsolidated formation and karst aquiferwas established. Transport model of CCl4and dual-conductivity medium flow model werebuilt, as well as, future contamination by CCl4was forecasted. The research is meaningful andvaluable both for theory development and engineering application. The results are showedbelow:(1) Karst aquifer medium characteristic in Xuzhou was investigated and divided into4types. The reaseach area is located in the south of city, which has two kinds of aquifermedium characteristics: Quaternary porous medium and Carbonate fracture medium. Inaddition, the supply way and affecting factors was analyzed to construct karst groundwatercircle system.(2) Karst and karst water transport in studied area has significant heterogeneity,excepting the common features of karst porosity, there still exist developed cave, dissolvedpores, cracks, and other conduit media. According to54drilling data and core dissolutioninformation, karst development was investigated, especially for the cracks and large caves’distribution in vertical direction. Double-development characteristic of karst aquifer mediumwas studied via field-scale tracing experiments, and location of karst conduit medium wasidentified as a main migration pathway for both groundwater and CCl4.(3) According to the DRASTIC model on groundwater vulnerable assessment, sevenfactors were used to evaluate weight, calculate and grade the contaminated divisions. Theresults show that, the entire research area was being in high vulnerable condition; more than60%of the area was easy to be contaminated. Unfortunately, CCl4contamination source, westinsecticide factory, was located in the high susceptible contamination area.(4) The loose layer soil samples were collected to conducted lab-scale infiltration experiments. By using Hydrus-1D software, we verified transport of CCl4in loose layer andsimulated CCl4concentration in karst aquifer from contamination source and bypass looselayer, according drilling data of contamination area. The results showed the average CCl4leaking concentration at the bottom of loose layer was2773.6μg/L, which was the initialconcentration in karst aquifer.(5) Dual-conductivity mathematical model was built based on dual-conductivity theoryand water level data for more than40wells from April2004to April2012; hydro-exchangeparameter was introduced, and dual-medium was divided by ModelMuse software;dual-medium flow model was built according to model identification period, from April2004to April2005, as well as model verification period, from April2005to April2012. The errorwas between0.27m－0.43m, which indicated the reliability of the model by comparing withwater level in typical wells.(6) Research area CCl4transport model was established based on continuous monitordata for40general wells and21regular wells from August2004to April2012. The relativeerror was between11.4%－16.7%for CCl4concentration, which indicated the reliability ofthe model by comparing with real data. CCl4concentration in2015,2018,2022, and2025wasforecasted, which showed contamination concentration will decline over time, contaminationplume will decrease, and the southern typical well shows the highest decay rate of CCl4,followed by northern and middle ones. The contamination plumes in the middle and northernarea which will be disappeared by2022and2025due to the closing of pesticide factory andwater regulation and control. The forecasting results will provide the technical guide for watersource area pollution control and water resource planning.