The effect of multiple-layer pumping on groundwater extraction

Multiple-layer pumping wells are popular in application to aquifers in Taiwan due to their low cost in installation. Conventionally groundwater discharge is estimated by distributing the total discharge proportionally using transmissibility in each layer. Since the packing filter media is a well permeable material and the filter pack was not exactly coincided with the aquifer formation, the discharge in each layer is actually different from the value calculated by transmissibility. To understand discharge in each aquifer in multiple-layer pumping, TOUGH2 was used for numerical modeling. We first build a conceptual model to explore the effects of pumping time, position of pumping source, and size of well screen. Then we take Taiwan Water Company's No.7 water pumping well in Yun-lin area as an example to demonstrate. Our results show that in the early time of pumping, discharge is larger near the pumping source; when the pumping time increases, ratio of each aquifer discharge reaches a constant value. The value is different for different configuration. The major factors control the discharge of aquifer are the position of pumping source and location of well screen. Ratio of discharge and transmissibility in each aquifer are not the same. In the area connected to well screen and filter pack and aquifer closer to pumping source discharge is larger. The results provide a general idea for discharge in each aquifer of multiple-layer pumping and can be used for responsibility discrimination for the cause of subsidence by different groundwater users.