Gravity estimation of groundwater mass balance of sandy aquifers in the land subsidence-hit region of Yunlin County, Taiwan

Land subsidence will decrease the safety factor of bridges and structures. Many highways and railways constructed decades ago are experiencing damage due to continual land subsidence in several alluvial fans of Taiwan. Groundwater over-pumping for industrial and agricultural uses leads to severe (>3 cm/year) land subsidence in the middle to distal Choushui River Alluvial Fan (CRAF), the largest alluvial fan in Taiwan. The Taiwan High Speed Rail passes through CRAF and land subsidence is now a major concern. Replenishing groundwater with artificial recharge lakes is a potential solution to mitigate land subsidence impacts. Using gravimetry, we examined two undetermined regional unconfined aquifers (RUAs) in the land subsidence-hit region that could host potential artificial recharge lakes to replenish groundwater. We established seven absolute gravity sites and measured time-lapsed gravity values in Yunlin in southern CRAF in 2021, including five sites in the subsidence-hit region and over two unconfined aquifers in the proximal fan. A consistent pattern of residual gravity changes associated with water storage changes at all the gravity sites confirms the recharge potential of the two RUAs in the land subsidence-hit region. Here we estimated groundwater storage changes by residual gravity changes around gravity sites without using prior hydrology information such as groundwater levels and storage coefficients. Of all the gravity sites in the land subsidence-hit region, the most significant March-to-September residual gravity change (26.6 μgal) and vertical displacement (−4.2 cm) were observed at Siutan elementary school (STES). The estimated groundwater storage change around STES is significantly large to increase the water balance in Yunlin, despite the site's severe land subsidence in 2021. We used electrical resistivity imaging (ERI) to aid the identification of the RUA near STES and discussed a potential joint gravimetry-ERI study of the RUAs for subsidence-mitigation engineering works such as constructions of recharge lakes.