The Construction of Multi-Channel Wells and Analysis of Related Hydraulic Tests to Explore the Physical Mechanism behind Land Subsidence in Changhua and Yunlin, Taiwan.

Abstract

To enhance the density and precision of groundwater level observations, this study installed two 300-meter multi-channel wells (MCW) in XiHu (Changhua) and YuanChang (Yunlin). These wells can concurrently monitor water levels in 15 layers within the protective tubing. Fifteen sets of 1-inch groundwater observation wells, varying in depths, were placed in the protective pipe. The groundwater level observation equipment utilized two sets with different principles—electronic and optical fiber—to record water levels. The objective was to compare the accuracy and stability of the optical fiber water level measuring equipment. Despite the potential simplification of well construction processes for fiber-optic water pressure gauges, a pressure issue still needs to be addressed. Therefore, the electronic water level meter remains the more reliable option between the two. Pumping tests were conducted at two sites to observe the flow characteristics of each layer in the multi-channel well and the depth changes of the magnetic ring in the multi-level compaction monitoring well (MLCW). During pumping in XiHu #1, the water level in the shallow layer rose due to additional groundwater recharge caused by pumping. The rise in water level in the sub-deep layers is attributed to the gravel composition of the stratum at this depth, facilitating better groundwater flow.The YuanChang standard observation well is divided into five depths. The pumping test results indicate that the aquifer at each depth is less influenced by the upper and lower layers. Water level changes in the MCW, affected by the pumping test, can be categorized into three groups: MCW#1~#5, MCW#6~#12, and MCW#13~#15. The depth changes of the magnetic ring in the MLCW were analyzed during water pumping and recovery. The changing trend of the magnetic ring in the XiHu pumping test site aligns with the pumping and recovery process. The magnetic rings of YuanChang, specifically NO.1 to NO.5 comprising unconfined aquifer layers, exhibit minimal rebound during the recovery stage of Yuanchang #1. The trends in magnetic rings NO.18 to NO.26 align with water pumping and recovery, indicating characteristics typical of a confined aquifer. The water pumping and recovery in YuanChang#3 and YuanChang#4 display a hysteresis effect on the changing trend of magnetic rings NO.1 to NO.7. The MCWs established in this study, along with the pumping test results, can provide more accurate on-site survey data. Coupled with the comparison of MLCW observation results, this can serve as a demonstration site for studying stratigraphic subsidence, significantly contributing to improving scientific research demonstrations.