Tidal responses of groundwater level and salinity in a silty mangrove swamp of different topographic characteristics

Abstract

Tides are one of the main factors affecting coastal aquifers, and thus they are important in supporting material transport and carbon circulation. In coastal mangrove swamps, the groundwater responses to tidal variations become more complicated owing to the complex topography, soil types, and the interactions of groundwater with fauna and flora. Perhaps because of this, quantitative relationships between the groundwater dynamics and the tidal actions of coastal mangrove swamps remain unclear. There is still a lack of studies that consider the integrated tidal effects on subsurface aquifers of different soil zones and topographic types. From January to December 2018, we simultaneously monitored the fluctuations of the piezometric head and salinity at the active root zone (shallow zone) and the low-permeability silty zone (middle zone) in the riverine forest and fringe forest and on the mudflat of a coastal mangrove swamp. Tidal signals were extracted from high-resolution time-series groundwater data using a Bayesian statistics modeling method-based code (Baytap08). Different time lags between tidal elevation fluctuations and the fluctuations of the piezometric head and salinity were detected. At the inland riverine forest and seaward mudflat, the upper aquifer quickly dried at low tides, whereas that of the fringe forest in between remained wet all the time. Although the groundwater fluctuations, especially the salinity fluctuations, were highly attenuated owing to capillary effects in the middle zone, the shallow zone showed high dependence on the tidal variations. Most previous studies have focused either on the macropore function (animal burrows and decayed roots) of mangrove swamps or on the capillary attenuation of coastal aquifers. Our results indicate that we need to consider both when analyzing the tidal responses of groundwater in mangrove swamps. The insights obtained from this study may help track the movement of water and materials in the coastal aquifers of mangrove swamps and provide references for better management of these valuable ecosystems and other similar coastal aquifers with complicated structures.