HYDROCHEMISTRY CHARACTERISTICS OF INTERTIDAL POREWATER: A CASE STUDY IN A MANGROVE SWAMP IN DAYA BAY, CHINA

A typical shore-perpendicular transect in a tidal mangrove swamp was selected to investigate the interactions between surface water and porewater. This transect is composed of three subzones including mangrove zone, tidal creek zone and bare mudflat zone. Samples of intertidal porewater, inland fresh groundwater, adjacent river water and seawater were collected for measuring concentrations of major ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, SO₄^2-, HCO₃^- and Cl^-), nutrients (including dissolved inorganic phosphorous and reactive silicate) and heavy metals (including Cu, Zn, As, Hg, Cd, Pb and Cr). The salinity, pH, and vertical hydraulic conductivity of well samples were measured in the field. Relative submerged time and the percentage of Cl^- in water samples were used to calculate the hydroperiod and seawater fraction, respectively. The piper diagram indicated that the intertidal porewater is composed of a mixture of seawater and inland groundwater. The ions Cl^- and Na⁺ are approximately conservative ions. Significant losses of other ions were observed, including loss of K⁺ due to uptake by mangroves, loss of SO₄^2- due to sulfate removal, and losses of Mg and Ca due to precipitation. The rank of mixing degree of seawater among three subzones is: mangrove zone > creek zone > mudflat zone. The mangrove zone has the smallest hydraulic conductivity and hydroperiod. Positive correlation between hydraulic conductivity and DIP concentration in the intertidal porewater was found. Heavy metals concentrations are higher in mangrove zone and creek zone than in mudflat zone.

Acknowledgments: This research was supported by the National Basic Research Program of China (“973” Program, Grant No. 2015CB452902) and the Key Program of National Natural Science Foundation of China (Grant No. 41430641). We would like to thank Gang Li, Meng Zhang, Tiao Guo, Liang Zhong, Kanghan Zhou, and Shuaishuai Liu for their field work.