Deep submarine groundwater discharge indicated by tracers of oxygen, strontium isotopes and barium content in the Pingtung coastal zone, southern Taiwan

We have conducted detailed hydrographic surveys off the Pingtung Plain coast, SW Taiwan, using environmental isotope and trace element compositions (δ18O, Δ87Sr, and Ba) as the main tracers to investigate submarine groundwater discharge (SGD) in the region. The δ18O values of seawater in this area show significant variability with heavy isotope enrichment relatively to groundwater, and two δ18O-depleted layers are found at depths of 400-700 m and 1200 m along deep parts of Kaoping Canyon. Assuming two end-member mixing model, we estimate that fresh submarine groundwater may constitute a few percent of the total SGD in the canyon. This is reflected in lower salinity, higher Δ87Sr values and Ba concentrations in deep canyon water than in adjacent seawater. Pore water samples collected from the canyon floor have light δ18O values and low chloride concentrations, consistent with the overlying δ18O-depleted water. Elsewhere, seabed pore waters have normal seawater compositions. The results of this study indicate that the deep SGD in the Pingtung Plain coastal zone is mainly fed by recirculating seawater with some admixture of fresh groundwater in canyon area (down to ~ 1200 m). Areas with notable SGD are located a long distance from the shoreline (~ 25 km), and the ‘deep’ SGD may be widespread off southern Taiwan. This discharge must be considered as a significant source of trace elements and other chemical constituents to the coastal ocean.