Research on the division technology of karst groundwater source protection areas based on numerical simulation

Zoucheng City is located in the southwest of Shandong Province with a total area of 1，616 km2.Taking a karst water source in Zoucheng City as an example, the application of numerical simulation methods in the division of karst groundwater protection zoning and the effect of hydrogeological parameters on the classification results of water source protection zones were studied. The water source is located in the middle of the Guoliji monoclinic karst water system, with Wangyun river in the south. Groundwater is abstracted from the fractured karst aquifer comprising the Middle and the Lower Ordovician carbonate rocks, with the single well yield of more than 3，000 m3?d-1. There are two abstraction well fields in the water source, of which the first well field consists of 6 production wells and 4 production wells in second one. The karst groundwater runs from the southwest to the northeast, and mainly receives the leakage recharge of the overlying porous aquifer and the lateral runoff recharge. Using GMS software, a three-dimensional unsteady groundwater numerical model was constructed. Using particle tracing technique, the protection area of the karst water source area were divided by tracing the position and the migration trajectory of tracer particles after 100 days, 1，000 days and 25 years. Taking the change of the protection area after changing the parameters as the index to measure the sensitivity of the parameters, the sensitivity coefficient of each parameter was calculated and the influence of the hydrogeological parameters on the division scope of the protection area was studied. The numerical simulation shows that the maximum migration distance of the tracer particles after 100 d is 44.91 m, the maximum migration distance after 1，000 d is 301.85 m, and the maximum migration distance after 25 a is 1，523.27 m. According to this result, the area of the primary protection zone is 1.27×104m2, the area of the secondary protection zone is 0.42 km2, and the area of the quasi-protection zone is 3.47 km2. The sensitivity analysis result shows that when the parameter change range is within 20%, the sensitivity coefficient of vertical permeability coefficient VK1 of the first layer can reach 2.63×10-3, and the sensitivity coefficient of vertical permeability coefficient VK2 of the second floor can reach 3.64×10-3. The sensitivity of the vertical permeability coefficient is significantly higher than other parameters, indicating that the vertical permeability coefficient has the greatest impact on the division of the protection area. Therefore, when applying the numerical simulation method to divide the protection areas of karst groundwater sources, we should pay special attention to the accuracy and rationality of the vertical permeability coefficient of each aquifer in the model.