Subsurface salinity distribution and evolution in low-permeability coastal areas after land reclamation: Field investigation

Abstract

Coastal land reclamation can modify the original hydrological settings that control water salinity distribution. For low-permeability coastal lands reclaimed for agricultural use, there is an urgent need to understand the salinity distribution and evolution mechanism for a sustainable exploitation. In this study, electrical resistivity tomography (ERT) measurements at different temporal and spatial scales are combined with water stable isotope and hydrochemical analyses to investigate the distribution and evolution rules of underground salinity in a coastal reclamation zone of eastern China. The results show that the phreatic aquifer is dominated by highly saline water due to the flooding of seawater before reclamation. After being isolated from marine conditions, water salinity in the near-surface zone becomes highly variable caused by dilution effect of rainwater and salt accumulation effect driven by evaporation. Due to the low-permeability muddy flat geohydrological settings and intense evaporation, natural groundwater desalination is extremely slow and only temporarily occurs in the near-surface zone. After long-term reclamation for agricultural use, the leaching effect of freshwater input from not only precipitation but also crop farming dominates and accelerates the desalination of the upper phreatic aquifer, leading to a desalting rate of 1.6 ± 2.4 %/yr and an obvious downward migration of saline groundwater. Deeper confined aquifers are dominated by brackish or even fresh groundwater, which is minimally contaminated by upper saline water. Findings of this study provide a representative example of the salinity distribution and evolution characteristics in coastal agricultural lands reclaimed from low-permeability intertidal muddy flats.