Groundwater–seawater interactions off the coast of southern Taiwan: Evidence from environmental isotopes

Abstract

We have conducted detailed hydrochemical surveys off the Pingtung Plain coast, SW Taiwan, using environmental isotope compositions (δ D, δ 18O, δ 13C DIC, tritium, and Δ 14C) as the main tracers to investigate groundwater–seawater interactions in the region. The δ 18O values of seawater in this area show significant variability with heavy isotope enrichment relatively to groundwater. The δ D and δ 18O data also confirmed that submarine groundwater discharge (SGD) originated from meteoric water and were mixed with seawater. Due to an overdraft of groundwater, seawater has intruded inwardly through the offshore outcrops for both the unconfined and confined aquifers at a depth of 300 m. However, δ 18O-depleted layer is found along deep parts of Kaoping Canyon. This result provides evidence for SGD in this region. This phenomenon is also reflected in depleted δ 13C DIC and lower Δ 14C values in deep canyon water than in adjacent seawater, and indicating that the discharge zone extends over a considerable distance to the offshore area (∼25 km). Based on these three parameters (δ 18O, δ 13C DIC and Δ 14C), we used two end-member mixing model to estimate that fresh submarine groundwater constitute. The submarine fresh groundwater contributed about 3%, and the estimated submarine fresh groundwater discharge was calculated as 4.5 × 10 9 m 3 per year. This result is comparable with groundwater annual loss in the Pingtung Plain. As documented by the isotopic composition of the collected samples, detailed results show that SGD in the Pingtung coastal zone raises a potential environmental concern and provides implication for the management of groundwater resource in the region.