Estimation of land subsidence caused by loss of smectite-interlayer water in shallow aquifer systems

Traditionally, land subsidence that results from groundwater over-pumping has often been described by the theory of consolidation. The mechanism of land subsidence due to the dehydration of clay minerals is not well addressed. A model of the “hydration state of smectite”, and a “solid solution model of smectite dehydration”, incorporating a thermodynamic solid solution model and laboratory results concerning clay-water systems of swelling pressure, hydration state and basal spacing in smectite interlayer, are employed to examine the effect of the release of water from the smectite interlayer on land subsidence in the coastal area of the Chou-Shui River alluvial fan and the Yun Lin offshore industrial infrastructure complex in Taiwan. The results indicate that 9.56–22.80% of the total cumulative land subsidence to a depth of 300 m is consistent with smectite dehydration following the over-pumping of groundwater. This dehydration-related land subsidence occurred to a depth of 0–60 m, with subsidence due to smectite dehydration accounting for 6.20–13.32% of the primary consolidation. Additionally, the total amount of subsidence resulting from both smectite dehydration and primary consolidation is consistent with the subsidence observed in the field. This study reveals that smectite dehydration appears to be important in assessing and predicting land subsidence in shallow aquifer systems.