Aquifer structure of karst areas derived from borehole pumping and tracer tests

Karst aquifers have strong heterogeneity and anisotropy. The investigation and understanding of their spatial structure are the important foundation for research of groundwater flow and contaminant migration. However, hydrogeology surveys are very difficult to conduct in karst areas due to their special geologic structure. To tackle this problem, this work conducted pumping tests and multiple tracer experiments (sodium fluorescein and rhodamine, fluorescent whitening agent) at the same. Using the Theis standard curve comparison, we analyzed pumping test data and calculate the permeability tensors of test sites. The structure properties of aquifers were determined by the results of the tracer tests. The calculated coefficient of transmissibility in the main direction of permeability tensor in the study area are: 0.297 m2·h-1 and 0.028 m2·h-1, specific yield is 1.19e-04 and predominant direction is east by south 18.47°. Tracer recovery rates in the tracer tests under the pumping condition for three kinds of traces are: fluorescein sodium 97.98%, rhodamine B 47.34%, and fluorescent whitening agent 39.54%. The average flow velocities are 0.378 m2·h-1, 0.3 m2·h-1 and 0.14 m2·h-1, respectively. They reflect the karst pores and fractures are small in the study area, tracer migration is slow and needs long time. The BTC curve of fluorescein sodium and fluorescent whitening agent is of unimode type, the BTC curve of rhodamine B is multi-peak, showing the development of single dominant fractures and relatively uniform fractures in the study area. At the same time, the direction of maximum recovery of the tracer is in accordance with the main direction of the permeability tensor from pumping tests. The results can provide a method and basis for precise description of karst aquifer characteristics and the determination of physical parameters of hydrological models.